Environmental Chemistry Letters How application of agricultural waste can enhance soil health in soils acidified by tea cultivation? a review --Manuscript Draft-- Manuscript Number: ECLE-D-21-00537R1 Full Title: How application of agricultural waste can enhance soil health in soils acidified by tea cultivation? a review Article Type: Reviews Keywords: Agricultural waste; Soil acidification; Biochar; Organic manure; Soil health ·Tea plantations Corresponding Author: Lesueur Didier, Ph.D CIRAD Hanoi, Ha Noi VIET NAM Corresponding Author Secondary Information: Corresponding Author's Institution: CIRAD Corresponding Author's Secondary Institution: First Author: Viet San Le, PhD student First Author Secondary Information: Order of Authors: Viet San Le, PhD student Laetitia Herrmann, PhD Lee Hudek, PhD Nguyen Thi Binh, Master Lambert Bräu, PhD Lesueur Didier, Ph.D Order of Authors Secondary Information: Funding Information: Abstract: Tea is one of the world’s most consumed drinks and an important crop of many developing countries. As tea plants can retain their productive life span for decades, intensive tea cultivation has negative impacts on soil health properties and the environment. Globally, soil acidification in tea plantations has become a severe issue, threatening soil health, tea production and the environment. However, the ways in which soil acidification affects soil health, tea productivity and the environment, and suitable methods to control this issue have not been critically reviewed. Here, we review the mechanisms of tea soil acidification and its consequences; the potential of common agricultural wastes for ameliorating soil acidity and enhancing soil health and crop productivity, as well as reducing environmental pollution under tea cultivation. We show that intensive application of chemical nitrogen is the main cause of soil acidification in tea plantations, while tea plants also play a part in accelerating tea soil acidity. Agricultural waste and products derived from these resources have a great potential to correct soil acidity, enhance soil health and tea productivity and quality. These soil amendments also introduce risks such as heavy metals and/or pathogens as well as production and application costs that require consideration. Suggested Reviewers: Wenyan Han, Dr Chinese Academy of Agricultural Sciences Tea Research Institute hanwy@tricaas.com Dr Han published several relevant papers on soil acidification in tea plantations L Wang, Dr Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation Nanjing Institute of Environmental Sciences pingfanshangren@126.com Dr Wang published relevant paper about the utilization of agricultural wastes such as biochar for increasing low soil pH in tea plantations Jianyun Ruan, Dr Institute of Genetics and Developmental Biology Center for Agricultural Resources Research jruan@tricaas.com Dr Ruan is very much interested on the effects of organic substitution for synthetic N fertilizer on soil bacterial diversity and community composition in tea plantations Response to Reviewers: Environmental Chemistry Letters ECLE- D- 21-00537 Responses to the Editor’s comments We are sincerely indebted to the Editor for giving comments on our manuscript. We have revised our manuscript following all the comments and we hope that this revised version will fit with the editor’s expectations. Editor’s comments 1. The introduction is too long and needs to be shortened down to 1-2 paragraphs Answer: Thanks for the Editor’s comment. The introduction has now been rewritten and the length has been reduced by around a half. 2. Possibility to discuss the ocean and other acidifications for attracting a wider range of readers. Answer: We have now added a new sub-section 2.1 to briefly discuss the status and mechanism of ocean and soil acidification and how they relate to agricultural activities. 3. The figures need to be accurately presented (texts, legend, white space, Y and X axis…) Answer: We have now carefully revised all the figures attached in the manuscript. We hope that they will fit with the editor’s expectations. 4. Write at the end of each article section a conclusion of about 1-2 sentences to summarize the major points of the section and its significance. Answer: Thanks for the Editor’s advice. We now reviewed the whole manuscript and added summary/ conclusion to many sections/subsections where we believe that they are accurate. 5. Finding relevant articles published by Environmental Chemistry Letters (5-10 articles, using keywords and search from the journal home page) and consider citing them as the references. I believe we have published articles concerning soil and its degradation, tea as well as acidification. Answer: Thanks for the Editor’s advice. We have been carefully checked and cited 8 articles published by ECL as references intext and highlighted these references in the bibliography. They are as follow: 1. Akhil D, Lakshmi D, Kartik A, Vo D-VN, Arun J, Gopinath KP (2021) Production, characterization, activation and environmental applications of engineered biochar: a review. Environ Chem Lett 19: 2261-2297. http://doi.org/10.1007/s10311-020-01167-7 2. Gunarathne V, Ashiq A, Ramanayaka S, Wijekoon P, Vithanage M (2019) Biochar from municipal solid waste for resource recovery and pollution remediation. Environ Chem Lett 17: 1225-1235. https://doi.org/10.1007/s10311-019-00866-0 3. Ochedi FO, Yu J, Yu H, Liu Y, Hussain A (2021) Carbon dioxide capture using liquid absorption methods: a review. Environ Chem Lett 19: 77-109. https://doi.org/10.1007/s10311-020-01093-8 4. Patra BR, Mukherjee A, Nanda S, Dalai AK (2021) Biochar production, activation Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation and adsorptive applications: a review. Environ Chem Lett 19: 2237-2259. https://doi.org/10.1007/s10311-020-01165-9 5. Rana A, Rana S, Kumar S (2021) Phytotherapy with active tea constituents: a review. Environ Chem Lett 19: 2031- 2041. https://doi.org/10.1007/s10311-020-01154- y 6. Saliu T, Oladoja N (2021) Nutrient recovery from wastewater and reuse in agriculture: a review. Environ Chem Lett 19: 2299–2316. https://doi.org/10.1007/s10311-020-01159-7 7. Sánchez A, Artola A, Font X, Gea T, Barrena R, Gabriel D, Sánchez-Monedero MÁ, Roig A, Cayuela ML, Mondini C (2015) Greenhouse gas emissions from organic waste composting. Environ Chem Lett 13: 223-238. https://doi.org/10.1007/s10311-015-0507- 5 8. Sharma H, Dhir A (2021) Capture of carbon dioxide using solid carbonaceous and non-carbonaceous adsorbents: A review. Environ Chem Lett 19: 851-873. https://doi.org/10.1007/s10311-020-01118-2 Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation Manuscript Click here to access/download;Manuscript;ECLE- D- 21-00537 R1.docx Click here to view linked References 1 2 3 4 How application of agricultural waste can enhance soil health in soils acidified by tea cultivation: a 5 6 7 review 8 9 Viet San Le 1,2,5*, Laetitia Herrmann1,5, Lee Hudek1, Nguyen Thi Binh6, Lambert Bräu1 and Didier 10 11 Lesueur1,3,4,5* 12 13 14 1 15 School of Life and Environmental Sciences, Faculty of Science, Engineering and Built Environment– 16 17 Deakin University, Melbourne, VIC 3125, Australia 18 19 2 The Northern Mountainous Agriculture and Forestry Science Institute (NOMAFSI), Phu Tho, Vietnam 20 21 3 Centre de Coopération Internationale en Recherche Agronomique pour le Développent (CIRAD), UMR 22 23 24 Eco&Sols, Hanoi, Vietnam 25 4 26 Eco&Sols, University of Montpellier (UMR), CIRAD, Institut National de la Recherche Agronomique 27 28 (INRAE), Institut de Recherche pour le Développent (IRD), Montpellier SupAgro,34060 Montpellier, 29 30 France 31 32 5 33 Alliance of Bioversity International and International Center for Tropical Agriculture (CIAT), Asia hub, 34 35 Common Microbial Biotechnology Platform (CMBP), Hanoi, Vietnam 36 37 6 Independent Researcher 38 39 * Corresponding author email: d.lesueur@cgiar.org 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 1 2 3 Application of agricultural waste to enhance soil health in soils acidified by tea cultivation: a review 4 5 6 7 8 Abstract 9 10 Tea is one of the world’s most consumed beverages and an important crop of many developing countries. 11 12 13 As tea plants can retain their productive life span for decades, intensive tea cultivation has negative 14 15 impacts on soil health properties and the environment. While soil acidification in tea plantations is 16 17 globally acknowledged to be a severe issue, threatening soil health, tea production and the environment, 18 19 the ways in which soil acidification affects soil health, tea productivity and the environment, and suitable 20 21 22 methods to control this issue have not been critically reviewed. Here, we review the mechanisms of tea 23 24 soil acidification and its consequences; the potential of common agricultural wastes for ameliorating soil 25 26 acidity and enhancing soil health and crop productivity, as well as reducing environmental pollution under 27 28 tea cultivation. We show that intensive application of chemical nitrogen is the main cause of soil 29 30 acidification in tea plantations, while tea plants also play a part in accelerating tea soil acidity. 31 32 33 Agricultural waste and products derived from these resources have a great potential to correct soil acidity, 34 35 enhance soil health and tea productivity and quality. These soil amendments also introduce risks such as 36 37 heavy metals and/or pathogens as well as production and application costs that require consideration. 38 39 40 41 42 Keywords: Agricultural waste · Soil acidification · Biochar · Organic manure · Soil health ·Tea 43 44 45 plantations 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 1 2 3 1. Introduction 4 5 6 7 Soil acidification has been a major threat to soil health and environmental sustainability in various 8 9 10 agricultural systems and regions (Dai et al. 2017; Li et al. 2016; Yan et al. 2020), and occurs in many tea 11 12 growing countries, such as China (Lin et al. 2019; Ni et al. 2018; Zou et al. 2014), India (Bandyopadhyay et 13 14 al. 2014), Japan (Oh et al. 2006), Sri Lanka, Rwanda (Mupenzi et al. 2011), and Vietnam (Huu Chien et al. 15 16 2019). In China, the leading global tea producer and exporter, greater soil acidification occurred in tea 17 18 19 plantations compared to other cash and cereal cropping systems, with 46% of tea plantations nationwide 20 21 reporting soil pH below 4.5 (Yan et al. 2020). The reduction of soil pH in tea plantations will have impacts of 22 23 soil characteristics by changing soil chemical processes, resulting in soil nutrient losses and imbalance, and 24 25 increasing occurrence of Al and Mn toxicity (Alekseeva et al. 2011; Ni et al. 2018; Yan et al. 2018). In 26 27 28 addition, soil acidification significantly degrades the diversity and functionality of soil organisms (Goswami 29 30 et al. 2017; Li et al. 2017). While soil acidification occurs naturally in tea plantations and increases with 31 32 increasing tea plant age and plant density, intensive application of mineral nitrogen (N) is the main cause of 33 34 the issue (Li et al. 2016; Yan et al. 2018). 35 36 37 38 39 The use of agricultural organic waste products to ameliorate soil acidification has been recognized in 40 41 Agriculture systems worldwide (Cai et al. 2015; Cornelissen et al. 2018; Dai et al. 2017). By definition, 42 43 agricultural wastes or agriculture by-products are the unwanted residues generated from agriculture activities, 44 45 such as crop residues, animal manure, forest waste, vegetable matter and weeds (Dai et al. 2018; Ramírez- 46 47 48 García et al. 2019). Animal wastes, green manures and products derived from these wastes such as biochars 49 50 and compost are generally alkaline in nature and have high pH buffering capacity which can neutralize soil 51 52 acidification (Cai et al. 2021; Rayne and Aula 2020). Also, the presence of basic cations such as Mg2+ and 53 54 Ca2+, and organic anions in these materials contribute to increased soil pH (Cai et al. 2021; Tang et al. 2013). 55 56 57 In addition to increasing soil pH, agricultural wastes have long been known to enhance soil health, including 58 59 soil physical, chemical and biological properties (Bhatt et al. 2019; Cai et al. 2021; Rayne and Aula 2020). 60 61 Globally, an estimated of 1 billion tons of agricultural wastes per year is generated, which China, USA and 62 63 64 65 1 2 3 India being the largest agricultural waste producing nations worldwide (Fig. 2) (Clauser et al. 2021; Obi et al. 4 5 2016), and this figure has been projected to increase rapidly because of the growing demand of agricultural 6 7 products (Dai et al. 2018; Wei et al. 2020). Thus, the utilization of agricultural wastes as soil amendments 8 9 10 could be a win-win strategy, which can benefit not only soil health but also reduce the pressure of using fossil 11 12 fuels, mitigate serious environmental problems and human health threats (Bijarchiyan et al. 2020; Mpatani et 13 14 al. 2021). 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 Fig. 1 Total production volumes of manures and crop residues in the world’s largest agricultural waste 46 47 generating countries from 2010-2018. Manures and crop residues were measured by kilotons of N content 48 49 and nutrients, respectively. Of these countries, China, India, Vietnam, Indonesia and Argentina have been 50 51 52 also the top global tea producers in the same period. Data was based on FAO (2021). 53 54 55 56 Studies on the utilization of agricultural wastes and its components to alleviate soil acidification caused by 57 58 tea cultivation have been well-reported in China, but poorly implemented in other parts of the world. Among 59 60 61 these soil amendments, biochar application is considered as the most effective way to counter low soil pH, 62 63 64 65 1 2 3 resulting in subsequent benefits to soil health and tea productivity (Wang et al. 2018; Wang et al. 2014; Yan 4 5 et al. 2021). Several studies have also reported the positive impacts of organic manures on acidification of tea 6 7 soil (Lin et al. 2019; Qiu et al. 2014), while the benefit of plant residues varied significantly. Recent reviews 8 9 10 have highlighted the potentials of biochar in mitigating soil acidification (Dai et al. 2017), and the effects of 11 12 organic manure on soil health (Bhatt et al. 2019; Rayne and Aula 2020). However, to our best knowledge, 13 14 there has not been any reviews published that specifically focus on the mechanisms and consequences of 15 16 acidification in tea plantation soils, the advantages and drawbacks of using agricultural wastes and other 17 18 19 relevant options in alleviating soil acidification as a result of long-term tea cultivation. This review provides a 20 21 comprehensive overview of mechanisms and consequence of soil acidification by tea cultivation, the 22 23 utilization of agricultural wastes and its products on mitigating soil acidification and enhancing soil health 24 25 properties under tea plantations. 26 27 28 29 30 2. Soil acidification by tea cultivation and its consequences 31 32 2.1. Ocean and soil acidification 33 34 35 36 Ocean and soil acidification have been widely reported as the most critical issues, affecting the sustainability 37 38 39 of numerous ecosystems and regions around the world (Ochedi et al. 2021; Yan et al. 2020). Ocean acidity 40 41 has increased by ~25% since 1860s, and the soil pH values of 50% of total arable land worldwide are below 42 43 5.5 (Dai et al. 2017; Hall et al. 2020). Ocean acidification appears due to rising atmospheric carbon dioxide 44 45 (CO2) concentrations and absorption by seawater, which subsequently leads to a fall of pH and carbonate ion 46 47 48 concentrations in surface seawater (Agostini et al. 2018; Sharma and Dhir 2021). Ocean takes up around 25% 49 50 of global anthropogenic CO2, making it the largest atmospheric CO2 absorbent on Earth (Hauck and Völker 51 52 2015). Among the CO2 emission sources, agriculture directly contributes around 14% of the total amount 53 54 globally, and this proportion is likely to be exceeded in the future (Ayyildiz et al. 2021). Intensive agriculture 55 56 57 and land use practices have been also the main causes of global soil acidification, particularly inappropriate 58 59 uses of ammonium-based fertilizers (Cai et al. 2015; Dai et al. 2017). Additionally, soil nutrient leaching, 60 61 62 63 64 65 1 2 3 product removal, acidic parent materials, acid deposition and host plants are all likely to be significant factors 4 5 resulting in soil pH reduction (Tang et al. 2013, Yan et al. 2020). 6 7 8 9 10 2.2. Soil acidification in tea plantations 11 12 Tea plant 13 14 Tea (Camellia synesis Kotze) is one of the oldest and most popular beverages in the world, and is an 15 16 important crop being cultivated in around 50 countries (Gebrewold 2018). Global tea production in 2019 was 17 18 19 more than 9.2 million tons, valued at approximately $US55.3 billion (Fig. 2) (Allied Market Research 2020; 20 21 Food and Agriculture Oragnization (FAO) 2021). 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 Fig. 2 Map of the 20 world’s largest tea producing nations in 2019. China was the largest tea producer 49 50 51 worldwide in 2019, followed by India, Kenya, Sri Lanka and Vietnam. Most of the global tea producers are 52 53 in Asia and Africa continents. The top 20 global tea producing countries contributed to around 70% of total 54 55 global tea production volume in the same year. Data was retrieved from FAO (2021). 56 57 58 Tea plants are native to the Asia continent, but they can adapt to a wide range of soil and climatic conditions 59 60 (Rana et al. 2021; Yan et al. 2018; Yao et al. 2012). This perennial crop requires acidic soils for optimum 61 62 63 64 65 1 2 3 growth and productivity, with the optimal soil pH for tea plants being between 4.5- 6, and the plant 4 5 themselves are capable of acidifying soil (Fig. 3) (Gebrewold 2018; Li et al. 2016). Being a woody perennial, 6 7 tea plants can remain their productivity for decades, and thus have long-term interactions with soil organisms 8 9 10 and physicochemical processes, affecting soil health and plant productivity (Arafat et al. 2020; Yan et al. 11 12 2020). 13 14 15 Soil acidity by tea cultivation practices 16 17 18 Soil acidification in tea plantations results predominantly from inappropriate management practices, 19 20 particularly the intensive overuse of mineral N (Li et al. 2016; Yan et al. 2018). Tea growers apply N to 21 22 ensure high tea productivity and as a replacement for soil nutrient loss. In Japan, tea fields are amended with 23 24 more than 1000 kg/ha of N fertilizers per annum (Abe et al. 2015; Zou et al. 2014) and a majority of tea 25 26 27 farmers in China apply a large amount of nitrogen to ensure high tea yield and maintain soil fertility (Yan et 28 29 al. 2018). A recent study has shown that nitrogen fertilizer application rate can even reach 1200 kg/ha in 30 31 Chinese tea plantations (Wu et al. 2016). Soil pH significantly reduces when N fertilizers such as ammonium 32 33 nitrate and urea is applied above 50kg/ha/year, and increased N added rate will accelerate soil acidification 34 35 36 (Tian and Niu 2015). Moreover, heavy N application results in greater decrease of subsoil pH compared with 37 38 that of the topsoil (Ni et al. 2018). When fertilizers are applied at 2700 kg/ha, only 18,3% of applied nitrogen 39 40 were absorbed by tea plants and of that, about 52% of nitrogen were stored in the soil, and 30% were lost 41 42 through runoff, polluting surrounding watercourses and soils (Chen and Lin 2016; Xie et al. 2021). 43 44 45 46 47 The main mechanisms of soil acidification resulting from inappropriate management practices in tea 48 49 50 cultivation are shown in Fig. 3. When NH + 4 -N fertilizer is applied, tea plants directly take up the nutrient and 51 52 tea roots subsequently excrete an equivalent proton into the rhizosphere, causing the concentration of 53 54 hydrogen ions to increase. NH +4 nitrification leads to a net production of 2 mol H+ for each mol of NH +4 55 56 applied, contributing to the decrease of soil pH (Hui et al. 2010; Li et al. 2016; Yan et al. 2020). Cai et al. 57 58 59 (2015) estimated that an application rate of 300kg/ha/year of N fertilizers could produce 21.4 kmol 60 + 61 H /ha/year by the nitrification processes. N fertilizer application in the long term also promoted the 62 63 64 65 1 2 3 accumulation of exchangeable Al3+ including hydrolysis, which further generated H+ and aggravated the 4 5 acidification of tea plantation soils (Zhang et al. 2020). Finally, increasing tea plant age and planting density 6 7 also result in an increase of organic and carbonic acids induced by tea roots into the rhizosphere, which 8 9 10 facilitate soil acidification (Hui et al. 2010). Tea plantation soil is not acidified at planting densities of 5000 11 12 plants/ ha (Li et al. 2016). 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 Fig. 3 The main mechanical causes of soil acidification by tea cultivation. Heavy addition of N fertilizers is 42 43 the main reason causing soil acidification, and the accumulation of organic and carbonic acids released by tea 44 45 roots also play a part in acidifying tea plantation soils. 46 47 48 49 50 Soil acidification by tea plants 51 52 53 Acidification of soils may naturally occur in soils cultivated with tea – even without any imposed N proton 54 55 additions, and this issue becomes more challenging with increasing tea plantations (Arafat et al. 2017; Han et 56 57 58 al. 2007; Li et al. 2016). In tea plantations, soil pH in the topsoil naturally decreased by 0.071 units per 59 60 annum, and the values following 13, 34 and 54 years of tea cultivation were 1,1; 1,62 and 2,07 units 61 62 63 64 65 1 2 3 respectively (Hui et al. 2010; Ni et al. 2018). The acidification rate observed in the cultivated soil layers (0- 4 5 10cm) could reach 4.40 kmol H+/ha/year during the 0-13 years of tea cultivation period (Hui et al. 2010). 6 7 Organic acids secreted by tea roots such as malic acid, citric acid, and oxalic acid are the main proton source 8 9 10 for soil acidification in the tea tree- soil systems (Fig. 3) (Yan et al. 2018). Tea roots also excrete carbonic 11 12 acids and polyphenols which can aggravate soil acidification, affect soil nutrient release and subsequent 13 14 element uptake (Ni et al. 2018; Wang et al. 2013). Additionally, the accumulation of chemical compounds 15 16 such as epigallocatechin gallate, epigallocatechin, epicatechin gallate, catechin, and epicatechin, found in the 17 18 19 tea residues also negatively affect soil pH and soil health properties (Arafat et al. 2020). Thus in summary, 20 21 intensive application of N fertilizers is the main cause of soil acidity under tea plantations, and the 22 23 accumulation of acid excreted by tea plants promotes the acidification. 24 25 26 27 28 29 2.3. Consequences of acidification in tea plantation soils 30 31 Soil chemical parameters 32 33 34 Soil acidification negatively affect chemical processes and properties of tea plantation soils (Fig. 4). One of 35 36 the most serious challenges of soil acidification under tea cultivation can be the reduction and imbalance of 37 38 39 nutrient base cations, including Ca 2+, Mg2+, Na+ and K+ (Alekseeva et al. 2011; Ni et al. 2018; Zhang et al. 40 41 2020). Under heavy N application, released protons (H +) may replace the soil exchange base cations, which 42 43 may have leached with the NO - 3 as accompanied cations due to the charge balance in soil solutions (Cusack et 44 45 al. 2016; Ni et al. 2018). Moreover, a significant increase of Al3+ and Mn2+ has been widely recorded in acidic 46 47 48 tea plantation soils, which could lead to Al and Mn toxicity (Alekseeva et al. 2011; Hui et al. 2010). Under 49 50 acidic soil conditions, mineral Al solubilizes into trivalent Al 3+, which is highly toxic to animals, plants and 51 52 microorganisms (Zioła-Frankowska and Frankowski 2018). Gruba and Mulder (2015) indicated that the 53 54 concentration of exchangeable Al maximizes in soils with a pHH2O ≈ 4.2. Similarly, with decreasing soil pH, 55 56 the amount of exchangeable Mn2+ increases in the soil solution (Millaleo et al. 2010). High concentration of 57 58 Al3+59 can inhibit the expansion, elongation, and division of root cells, reducing water and nutrient uptake by 60 61 the root systems (Wang et al. 2015). Similarly, high levels of Mn2+ in soil is one of the main factors causing 62 63 64 65 1 2 3 nutrient imbalances, especially with divalent cations such as Mg2+, Zn2+ and Ca2+ (Venkatesan et al. 2010). 4 5 Soil acidification can also promote the dissolution of minerals and movement of Fe in the profile, resulting in 6 7 reduction of ferrimagnetic mineral content (Alekseeva et al. 2011). Increased Al and Mn toxicity have been 8 9 10 considered as the most serious consequences of soil acidification by tea cultivation regarding soil chemical 11 12 property. 13 14 15 16 17 18 Soil biological parameters 19 20 21 Soil pH is a crucial factor affecting soil organisms (Li et al. 2018; Neina 2019). Mulder et al. (2005) indicated 22 23 that soil acidification has close inverse relationship with bacterial, fungal, nematode and arthropod 24 25 abundance. Long-term soil acidification is responsible for reduction of soil microorganisms, which are 26 27 regulating the reduction in soil pH by both ecological and evolutionary mechanisms because of the 28 29 30 environmental changes (Zhang et al. 2015). In tea plantations, a low soil pH (pH<4) could lead to a loss of up 31 32 to 70% of important soil biota (Han et al. 2007). Likewise, soil fauna communities were significantly higher 33 34 in the soil with pH 7.0 (21 classes) compared to acidic soil with pH 2.5 (11 classes) and pH 3.5 (14 classes). 35 36 In this study, in terms of total individuals, the figures were 3710 (pH 7.0); 759 (pH 3.5) and 645 (pH 2.5) 37 38 39 (Wei et al. 2017). Severe soil acidification also leads to significant decreases in soil enzymatic activities, 40 41 microbial activities, and microbial biomass (Li et al. 2017; Zhang et al. 2015). Arafat et al. (2019) found a 42 43 close association between the decline of some beneficial fungus such as Mortierella elongatula and 44 45 Mortierella alpina and a low soil pH caused by long-term tea monoculture. Soil acidification also enhances 46 47 48 the environment for growth of some soil- borne pathogen diseases. For instance, when soil pH reduced from 49 50 5.07 to below 3.5 as a result of 35 years of continuous tea monoculture, the abundance of some pathogenic 51 52 bacterial species including Fusarium oxysporum, Fusarium solani, and Microidium phyllanthi, which are 53 54 responsible for diseases in tea plants such as root rot and die back, was significantly increased (Arafat et al. 55 56 2019). Investigating the relationship between soil acidity and bacterial wilt disease, Li et al. (2017) found that 57 58 59 the proportion of soil affected by bacterial wilt much higher when the soil pH lower than 5.5, and 60 61 significantly less as the soil pH increases. Likewise, the highest population of Xiphinema chambersi was 62 63 64 65 1 2 3 found in soil with a pH 4.5, and the figure decreased when soil pH increased from 4.5 to 6.4 (Chen et al. 4 5 2012). Thus, soil acidification by tea cultivation could not only impact soil beneficial microbial diversity, but 6 7 also promote the development of some potentially pathogenic microbes (Fig. 4). 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 Fig. 4 A summary of the main consequences of soil acidification caused by tea cultivation in the aspects of 38 39 soil chemical and biological properties, tea growth and quality, soil management cost and the environmental 40 41 risks. 42 43 44 45 46 47 Tea productivity and quality 48 49 50 Although tea plants prefer acidic soil for optimal growth and productivity, severe soil acidity negatively 51 52 effects plant performance and quality (Fig. 4). When the soil pH is lower than 4.0, tea plant growth is 53 54 inhibited, affecting both the quality and quantity of tea production (Li et al. 2016; Yan et al. 2020). Heavy N 55 56 57 addition also significantly decreases the Polyphenol/free amino acid ratio and affects other tea quality 58 59 indicators by altering the relative content of chemical constituents (Qiao et al. 2018). High concentrations of 60 61 Mn2+ negatively affects tea quality indicators such as amino acid composition and reduces the chlorophyll 62 63 64 65 1 2 3 and carotenoid content of tea leaves (Venkatesan et al. 2010). Free Al3+ at a concentration of more than 1 mM 4 5 retards tea growth, while the concentration at 10 mM leads to defoliation of tea plants (Fung et al. 2008). 6 7 8 9 10 11 Management cost and environmental risks 12 13 14 Despite the limited study on the management and other associated costs of soil acidification in the tea 15 16 farming industry, research conducted on negative impacts of soil acidification on other agricultural sectors 17 18 has highlighted the issues this causes. For instance, the annual loss of agricultural production due to soil 19 20 21 acidification in New South Wales, Australia was around $387 million (Li 2020). Likewise, soil acidification 22 23 resulted in an estimated economic value decrease of $US214,000 per hectare (ha) in the forest industry in 24 25 America (Caputo et al. 2016). Lime has been considered as the most effective ameliorant to control acidic 26 27 soils, but it is still too costly for farmers in many countries, due mainly to its transportation costs (Cai et al. 28 29 30 2015; Tang et al. 2013). In tea plantation soils, acidification also occurs at the subsoil layers (100-120cm), 31 32 thus deep incorporation of lime and other alternatives could be very expensive or even impractical due to the 33 34 costs of suitable machinery (Li et al. 2016; Tang et al. 2013). Tea soil acidification can also promote the 35 36 accumulation of chemical elements such as arsenic (As), mercury (Hg), lead (Pb), chromium (Cr), cadmium 37 38 39 (Cd) and nickel (Ni) in the soil and tea leaves, increasing the human health and environmental risks of heavy 40 41 metals (Bayraklı and Dengiz 2020; Zhang et al. 2020). It has been reported that more than 75% of soil Cd, 42 43 Hg, Pb and Zn under acidic tea plantations exceeded uncultivated background concentrations, possibly due to 44 45 the acidic environment promoted weathering pedogenic process releasing heavy metals (Tao et al. 2021). 46 47 48 49 50 3. Possible agricultural wastes for correcting tea soil acidification and enhancing soil health 51 52 3.1. Agricultural wastes for soil acidification and soil health 53 54 Agricultural wastes such as organic manures have been considered as a significant resource for agriculture for 55 56 over hundred years (Rayne and Aula 2020), and since the downsides of agrochemical intensification on 57 58 59 human beings and the ecosystem have become the global issue, the potential role of these alternate materials 60 61 is being scrutinised increasingly closely (Chen et al. 2018; De Corato 2020). Most of agricultural wastes are 62 63 64 65 1 2 3 widely available, cheap, biodegradable and rich in organic matter and nutrient and thus can be recycled as 4 5 fertilizers or soil amendments (Kaur 2020; Onwosi et al. 2017; Saliu and Oladoja 2021). The nutrient 6 7 compositions of agricultural wastes and products derived from these resources varies greatly and depend on 8 9 10 multiple factors, such as their original sources, animal diets, waste storage and management, as well as 11 12 production procedures (Amoah-Antwi et al. 2020; Dai et al. 2017; Rayne and Aula 2020). Common 13 14 agricultural by-product and their components applied to agricultural soils as fertilizers and amendments are 15 16 illustrated in Fig. 5. 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 Fig. 5 A simplified illustration demonstrated the common types of agricultural wastes and products using 47 48 these wastes as main feedstocks, how they could be produced and used to mitigate soil acidification and 49 50 improve soil health, crop growth and quality. 51 52 53 54 55 There are various types of agricultural organic wastes applied to croplands, but they can be divided into two 56 57 different groups based on their origins and common uses (Fig. 5). Organic manures include animal wastes 58 59 from livestock and poultry industries, and green manures are mainly leguminous and forage crops (Maitra et 60 61 al. 2018; Rayne and Aula 2020). Globally, animal waste has been predominantly attributed to manures from 62 63 64 65 1 2 3 livestock and in 2018, contributed around 35 million tons of N applied to croplands globally, compared to 4 5 more than 13 million tons from poultry (FAO 2021). Organic manures can be applied to soils or used as main 6 7 materials for compost production, the natural biological processes of decomposing organic wastes involving 8 9 10 numerous microbial species (Azim et al. 2018; Bhatt et al. 2019; Sánchez et al. 2015). Compared to manures 11 12 and compost, plant straws and other organic biomass such as wood chips and tree pruning residues are not 13 14 often applied directly to soils as fertilizers, but can also be incorporated as mulches, mainly for enhancing soil 15 16 structure and water retention (Amoah-Antwi et al. 2020; Siedt et al. 2020). Alternatively, using agricultural 17 18 19 by-products to produce biochar has been also an increasingly accepted way of recycling wastes. Biochar 20 21 could be best described as a “soil conditioner”, a rich carbon product produced by thermochemical 22 23 decomposition of organic matter under low oxygen environment and high temperature, normally from 300- 24 25 7000C (Peng et al. 2018; Verheijen et al. 2010). Feedstocks for biochar production consist of various biomass 26 27 28 types, including municipal wastes and agro-industrial residues, and the feedstock types are important factors 29 30 affecting biochar properties (Amoah-Antwi et al. 2020; Gunarathne et al. 2019; Guo et al. 2020). Details of 31 32 elemental properties of some common agricultural wastes, compost and biochar are summarised in Table 1. 33 34 35 36 The various agricultural wastes have differing effects on alleviating soil acidification. Organic compost and 37 38 39 biochar produced from organic manures and plant residues are naturally alkaline and have a higher pH value 40 41 compared to that in the acid soils, so the addition of these organic amendments can increase soil pH to some 42 43 extent (Cornelissen et al. 2018; Shi et al. 2019). Additionally, organic manure and its components naturally 44 45 contain some basic cations such as Mg2+, Ca2+, Na2+ and K+, which can form carbonates or oxides and then 46 47 48 subsequently react with the H + in the acidic soils and lead to the acid neutralization (Dai et al. 2017; Rayne 49 50 and Aula 2020). In contrast, some studies showed that the decomposition of some mulching materials such as 51 52 woody chips, crop straw and pine bark could generate organic and carbonic acids, which facilitate soil acidity 53 54 (Arafat et al. 2020; Zhao et al. 2018). Nevertheless, numerous studies have reported the neutral to positive 55 56 57 effects of mulching practices on soil acidification (Cu and Thu 2014b; Ni et al. 2016; Sadek et al. 2019; Vijay 58 59 60 61 62 63 64 65 1 2 3 2014). 4 5 6 7 With regards to soil physical aspects, plant residues, organic fertilizers and biochar applications can benefit 8 9 10 soil hydrothermal environment, soil structure and water holding capacity (Kader et al. 2017; Siedt et al. 2020; 11 12 Wang et al. 2020). In terms of soil chemical properties, adding organic fertilizers and biochar significantly 13 14 improve soil organic matter, soil macronutrients and micronutrients, reduce Al and Mn toxicity risks and 15 16 nutrient leaching (Ding et al. 2020; Gong et al. 2020; Patra et al. 2021; Siedt et al. 2020; Zhongqi et al. 2016). 17 18 19 Recently, a number of studies have reported the positive impacts of agricultural residue practices on soil 20 21 organism abundance and functional diversity, such as the applications of organic mulches (Xiang et al. 2021; 22 23 Zhang et al. 2020b), biochar and compost (Amoah-Antwi et al. 2020; Liu et al. 2021) and organic manures 24 25 (Rayne and Aula 2020; Su et al. 2021). Despite the preference in using synthetic fertilizers, agricultural 26 27 28 wastes and products derived from these resources are being used intensively as soil amendments and 29 30 fertilizers, to partially or fully substitute for chemical fertilizers (Amoah-Antwi et al. 2020; Lin et al. 2019; 31 32 Shaji et al. 2021). However, since the nutrient compositions and efficacy of agricultural wastes and its 33 34 products varied significantly (Table 1), they cannot be applied in a homogenous manner (Dai et al. 2017; 35 36 Rayne and Aula 2020). Therefore, having a good understanding of characters of agricultural wastes and its 37 38 39 components would be important to increase their application efficiency and reduce the pollutant risks to 40 41 ecosystems (Amoah-Antwi et al. 2020; Ayilara et al. 2020; Cai et al. 2021). 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Table 1 Nutrient composition of some main types of agricultural wastes and its based products used as soil amendments in tea cultivation and croplands. 20 Type of waste Nutrient composition Reference 21 22 1. Animal manure N P K Na Fe Cu Mn Zn Total C 23 Horse 20.7 7.6 41.4 7.58 729 22 110 167 43.3 Moreno-Caselles et al. (2002); Chong et 24 al. (2019) 25 26 Cow 18.6 7.89 17.6 5.38 3527 20 111 79 43.88 Mendonça Costa et al. (2015); Moreno- 27 Caselles et al. (2002) 28 29 Calf 17.5 9.6 35.1 24.6 2839 40 225 233 - Moreno-Caselles et al. (2002) 30 Pig 21.7 14.4 8.9 2.34 1559 170 328 427 - Moreno-Caselles et al. (2002) 31 Sheep 18.7 5.67 34.3 6.94 3786 21 137 159 41.84 Mendonça Costa et al. (2015); Moreno- 32 33 Caselles et al. (2002) 34 Goat 22.2 8.1 59.2 16.9 1729 31 170 202 - Moreno-Caselles et al. (2002) 35 Rabbit 17.9 9.2 18.2 5.07 2623 61 225 453 - Moreno-Caselles et al. (2002) 36 37 Chicken 31.4 13.2 24.7 4.85 154 40 237 304 34 Moreno-Caselles et al. (2002); 38 Ravindran and Mnkeni (2016) 39 40 Turkey 39.7 10.9 24.5 3.97 172 45 327 336 39.7 Moreno-Caselles et al. (2002) ; Calbrix 41 et al. (2007) 42 Ostrich 16.5 7.7 10.7 4.64 1303 56 257 200 - Moreno-Caselles et al. (2002) 43 44 Earthworm 17.3 11.9 7.8 2.34 6503 78 335 348 - Moreno-Caselles et al. (2002) 45 Note: N, P, K (g/kg, dry weight); Na, Fe, Cu, Mn, Zn (mg/kg, dry matter); Total C (%, dry weight). 46 47 48 2. Plant residues N P K C Ca Mg pH C:N Ash 49 ratio content 50 51 Wheat straw 55 9 42 43.9 22.61 2.88 5.1 124.4 23.2 Jalali and Ranjbar (2009); Torma et al. 52 53 Potatoes 59 6 61 - - - 6.1 22.0 20.4 (2018); Wang et al. (2009) 54 Maize straw 39 3 19 42.14 6.40 4.60 - - 48.8 55 56 Oat straw 55 8 58 36.35 - - - 54.25 Torma et al. (2018); Zhao et al. (2018) 57 Rye 45 8 24 - - - - - - Torma et al. (2018) 58 Barley 43 7 40 - - - - - 7.14 Torma et al. (2018); Plazonić et al. 59 60 Triticale 54 8 28 - - - - - 5.27 (2016) 61 Pea straw 112 14 74 43.56 17.32 6.51 - - 61.6 Torma et al. (2018); Wang et al. (2009) 62 63 64 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Soybean straw 132 14 72 44.06 18.24 17.86 44.06 72.0 20 Sugar beet 20 2 13 - - - - - - 21 Torma et al. (2018) Mustard 91 21 127 - - - - - - 22 23 Sunflower 108 15 218 - 5.3 81.4 10.4 Jalali and Ranjbar (2009); Torma et al. 24 Rape 107 15 218 - - - 5.1 65.5 5.4 (2018) 25 Rice straw 0.5- 0.8a 26 0.07- 1.16- 41.25 7.03 3.96 - - 33.6 Ayinla et al. (2016); Chivenge et al. 27 0.12a 1.66a (2020) 28 Note: N content, P, K (kg/ ha); OM, C (%); Ca, Mg (cmol (+)/kg); Ash content: (%; dry weight); a (%). 29 30 31 Tea and wood N P K Dry C Ca Mg C:N Ash 32 residues matter ratio content 33 Tea pruned foliage 252 30 72 7.2 2.9 - - 11 - 34 35 Tea pruned twigs 85 10 21 3.6 1.4 - - 17 - 36 Primary wood 101 28 2 4.2 1.8 - - 42 - Kamau (2008) 37 Secondary wood 44 13 13 4.2 1.8 - - 40 - 38 39 Acacia bark 133.4 2.6 8.4 8.9 - 76.5 1.2 - 2.1 Taflick et al. (2015); Van Bich et al. 40 (2018) 41 42 Eucalyptus biomass 307.5 28.8 249.3 - - - 455.7 131.7 15.4 Reina et al. (2016); Resquin et al. (2020) 43 Note: N, P, K, Ca, Mg (kg/ ha, dry weight); C (t/ ha). 44 45 3. Biochar N P K Ca Mg Total pH C:N Ash 46 C ratio content 47 48 49 Rice straw biochar 19.8 2.0 24 8.8 5.7 56 8.7 - 39 50 at 400 oC Naeem et al. (2017) 51 Wheat straw 19.4 3.8 33 10.3 9.6 62 7.8 - 36 52 53 biochar at 400 oC 54 Pine woodchip 0.7 <0.001 2.1 10.1 2.7 244.5c 8.7 366 - Brantley et al. (2015) 55 biochar at 500 oC 56 57 Rice biochar 0.92 a 3.23a 2.48a 875.2 578.9 46.4 11.0 - 34.6 58 at 500 oC 59 Bamboo biochar at 0.58a 1.85a 1.01a 560.3 320.6 77.3 11.3 - 5.8 Yan et al. (2021) 60 o 61 750-800 C 62 63 64 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Peanut biochar at 2.6a - 22.0b 47.4b 45.6b 19 55.1 9.2 21.5 228.4 b Wang et al. (2014a) 20 300 oC 21 Vermicompost 8.7 <0.1 1.3 26.3 - 181c 8.09 20.9 8.09 Adhikary (2012) 22 23 24 Note: Total N, P, K Ca, Mg, (g/kg); Total C (%); Ash content (%); a (%), b (cmol (+)/kg), c (g/kg). 25 26 4. Compost N P K Ca OC pH C:N OM Moisture 27 28 ratio 29 Chicken manure 13.19 12.5 20.00 - 325.3 7.92 26.06 72.56 29.9 Li et al. (2021) 30 compost 31 32 Pig manure compost 29.82 15.13 8.16 - - 8.37 - 73.01 78.89 Li et al. (2012) 33 Buffalo manure 1.3 - - - - 7.3 14 - - Doan et al. (2014); Ngo et al. (2011) 34 35 compost 36 Cow manure 21.3 10,4 21.7 23.7 - 9.6 - 56.96 29.1 Gil et al. (2008) 37 compost 38 39 Note: N, P, K, Ca (g/kg); OC, OM and moisture (%). 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 1 2 3 3.2. Organic fertilizer and organic tea management practices 4 5 6 Applying animal manure to tea plantation soils could be an effective solution not only for ameliorating soil 7 8 acidification, improve soil health of tea plantations but also as a waste management tool. Manures from various 9 10 animals such as sheep, pig, cow and chicken used as organic fertilizers or compost for tea gardens significantly 11 12 increased pH of acid soils, compared to their chemical nutrient counterparts (Cai et al. 2015; Gu et al. 2019; Ji et 13 14 15 al. 2018; Lin et al. 2019; Qiu et al. 2014). For example, Gu et al. (2019) indicated that long-term applications of 16 17 animal manure resulted in a significant increase of soil pH (5.36), compared to that in non- fertilizer (4.71) and 18 19 chemical fertilizer practices (4.31). Likewise, application of pig manure over 18 years increased soil pH by 1.1 20 21 units (Cai et al. 2015). Additionally, the replacement of chemical fertilizer by organic fertilizer in organic and 22 23 24 agroecological tea cultivation has also had positive impacts on soil pH and other soil health indicators (Li et al. 25 26 2014; Viet San et al. 2021; Yan et al. 2020). Analyzing more than 2000 tea soil samples collected from 27 28 conventional and organic tea plantations, Yan et al. (2020) concluded that conventional tea cultivation which 29 30 employ heavy application of synthetic fertilizers caused severe soil acidification, while organic tea management 31 32 approach did not result in significant soil acidification. Similarly, our recent study shown that agroecological tea 33 34 35 management practices with chicken and buffalo manures as main nutrient supplies significantly improved soil 36 37 pH compared to conventional tea cultivation which employs intensive chemical NPK (unpublished data). As 38 39 outlined above, the mitigation of acidification of tea plantation soils by organic substance addition could be by 40 41 alkaline matter and basic cations from added organic fertilizers, which can neutralize the soil acidity (Ji et al. 42 43 44 2018). Moreover, other chemical processes involving manure supplementation such as organic anion 45 46 decarboxylation and organic N ammonification may play a part in reducing soil acidity (Xiao et al. 2013; Xu et 47 48 al. 2006). Organic fertilizer can also support soil buffering action, thus reducing soil acidification (Chen et al. 49 50 2009). More examples of positive effects of organic manure and compost usage on soil acidification are 51 52 53 indicated in Fig. 6 and Table 2. 54 55 56 57 58 59 60 61 62 63 64 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 Fig. 6 Effects of different fertilizer type applications on soil pH under tea cultivation. Organic fertilization 30 31 32 consistently resulted in grater soil pH in comparison with chemical fertilizer and non-fertilizer practices. Heavy 33 34 uses of synthetic fertilizers also led to highest reduction of soil pH, compared to other fertilization approaches. 35 36 Adapted from Lin et al. (2019); Cai et al. (2015); Ji et al. (2018) Gu et al. (2019); Qiu et al. (2014); He et al. 37 38 (2019). (*) the data for non-fertilizer management practice not available. 39 40 41 42 43 Apart from ameliorating soil acidification, recycling organic amendments as the partial or full substitutes for 44 45 46 chemical fertilizers can bring about a range of benefits for other aspects of tea plantation soil health and the 47 48 environment. Organic fertilizer applications consistently improved soil OM, soil OC, soil exchangeable cations 49 50 such as Ca2+, Mg2+, Na+ and K+, and nutrient availability, while reducing risks of Al toxicity, heavy metal 51 52 accumulation, greenhouse gas emissions and nutrient run off such as N and P (Table 2) (Cai et al. 2015; He et al. 53 54 55 2019; Ji et al. 2018; Lin et al. 2019; Qiu et al. 2014). Sustainable effects of adopting organic soil amendments in 56 57 tea plantation soils on biological soil health has been also clearly indicated. Organic materials such as sheep, 58 59 cow, chicken manures or compost significantly improved soil fauna communities, soil microbial diversity and 60 61 functional structures (Gui et al. 2021; Li et al. 2014; Lin et al. 2019; Zhang et al. 2020a). Organic fertilizers are 62 63 64 65 1 2 3 naturally rich in nutrients contain more organic matter compared to chemical compound, thus the replacement of 4 5 6 organic amendments provide more organic matter in the soils (Wu et al. 2020; Xie et al. 2019). Richer soil 7 8 organic contents will attract soil fauna and facilitate the activities of soil microbial communities in converting 9 10 soil nutrients, which ultimately increase soil nutrient of tea plantation soils (Fan et al. 2017; Xie et al. 2019; Xie 11 12 et al. 2021). These positive changes in turn, will result in increasing soil organism diversity and community 13 14 15 structure (Gu et al. 2019; Wu et al. 2020). 16 17 18 19 20 There do exist some concerns for recycling animal manures and organic compost which need further 21 22 consideration. Firstly, organic fertilizer such as rapeseed cake had inconsistent effect on soil pH (Xie et al. 2019; 23 24 Xie et al. 2021). This discrepancy may result from the dissimilarity of chemical composition of the product and 25 26 27 other conditions such as soil type, application rate and management practices (Gu et al. 2019; Wu et al. 2020). 28 29 Secondly, it has been reported that organic manure cannot ameliorate deep-soil acidification in tea plantations 30 31 (Li et al. 2016). In this case, biochar or a combined utilization of manure and biochar may be an effective 32 33 solution to not only mitigate soil acidification but also enhance soil health and tea productivity (Dai et al. 2017; 34 35 He et al. 2019). Thirdly, long- term application of animal manure and compost to manage acidic tea soils and 36 37 38 restore soil health could led to the risks of heavy metal accumulation and manure- borne pathogen contamination 39 40 (Cai et al. 2021; Li et al. 2020). For heavy metal contamination, Ji et al. (2018) indicated that 10 - year 41 42 application of pig manure did not result in increase of most heavy metals, and Lin et al. (2019) found that sheep 43 44 manure and rape cake application reduced levels of Cd, Pb and As in soils as well as in tea leaves. To date 45 46 47 however, the relationship between animal manure, compost and pathogenic diseases of tea plants has been 48 49 poorly understood. Thus, an integrated approach including appropriate application rates, reducing chemical 50 51 inputs and concentrations of heavy metals in animal feed could be all necessary to minimize the environmental 52 53 risks from using these organic materials as soil amendments and increase their efficacy (Cai et al. 2021; Ji et al. 54 55 56 2018). 57 58 59 60 61 62 63 64 65 1 2 3 3.3. Biochar amendment 4 5 6 Among the ameliorants of soil acidification, biochars could be one of the most effective options as it can also 7 8 improve soil quality, plant productivity and contribute to a reduction in greenhouse gas emissions (Akhil et al. 9 10 2021; Siedt et al. 2020; Zhang et al. 2018). In tea farming, biochars produced from plant residue such as rice, 11 12 wheat straw and bamboo residues have been commonly incorporated as soil amendment (Chen et al. 2021; Ji et 13 14 15 al. 2020b; Wang et al. 2018). Depending on biochar types and application rates, soil condition, tea management 16 17 practices and the application duration, the liming effect of biochars varied significantly, (Wang et al. 2014a; Yan 18 19 et al. 2021). As demonstrating in Fig. 7, applying biochars at rates of from 1% to 5% of soil dry weight can 20 21 significantly increase soil pH from 0.2 to more than 1 units within a few months (Ji et al. 2020a; Oo et al. 2018; 22 23 24 Wang et al. 2018; Zheng et al. 2019). Studies conducted in tea plantations also demonstrated the positive 25 26 outcomes of biochar utilization for correcting soil acidification caused by tea cultivation (Table 2) (He et al. 27 28 2019; Ji et al. 2020b; Yang et al. 2021). 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 Fig. 7 Effects of biochar application rate on pH of tea plantation soils. Data collated from recent publications: 55 56 57 Chen et al. (2021); Ji et al. (2020a); Oo et al. (2018); Wang J et al. (2018); Wang L et al. (2014); Wang Y et al. 58 59 (2014) and Zheng et al. (2019). 60 61 62 63 64 65 1 2 3 Biochar ameliorates soil acidification by its natural alkalinity, high pH value and pH buffering capacity. Biochar 4 5 6 generally has an alkaline pH value, thus soil amended with this product can become less acidic (Table 1). For 7 8 instance, a meta- analysis by Dai et al. (2017) indicated that biochar applications significantly increased soil pH 9 10 by up to 2 units, and in most cases, the pH of biochars is greater than 7.0, which is at least 1.5 units higher than 11 12 the pH in acid soils. Moreover, mineral constituents of biochar including basic cations such as Ca, Mg, K, Na 13 14 15 and alkaline oxides that originated from feedstocks can mitigate soil exchangeable acidity (mainly H + and Al3+) 16 17 in the soil and ultimately increase soil pH (Dai et al. 2017; Patra et al. 2021; Yuan et al. 2011). In addition, soil 18 19 pH buffering capacity is an important factor contributing to biochar amelioration of soil. Shi et al. (2019) 20 21 illustrated that rice straw and peanut straw biochar application increased pH buffering capacity by 22% and 32% 22 23 24 respectively. It has been verified that the increases in CEC of the soil by biochar incorporation, driven by 25 26 protonation- deprotonation processes, was the main mechanism of increasing soil pH buffering capacity (Shi et 27 28 al. 2017; Xu et al. 2012). Biochar application also suppressed soil nitrification by limiting the availability of NH3 29 30 or NH +4 for oxidation because of the surface adsorption or increased emissions of NH3 due to enhanced soil pH 31 32 (Wang et al. 2018; Yang et al. 2015). This in turn generally reduces the proton (H+) released into soil and 33 34 35 ultimately increase soil pH (Shi et al. 2019). 36 37 38 39 40 41 Biochar addition also enhanced soil quality indicators, tea growth and productivity, as well as reduced the 42 43 44 environmental risks from pollution by heavy metals and greenhouse gases such as CO2, N2O and NO (Chen et al. 45 46 2021; Ji et al. 2020a; Yan et al. 2021). Consistently, biochar incorporation in soil improved soil OC, soil nutrient 47 48 availability including Ca, Na, Mg, P and K contents, soil total N and C (Yan et al. 2018; Wang 2014; Zheng et 49 50 al. 2019). While the impact of biochar on soil fauna has been poorly investigated, this carbon-rich material has 51 52 significant effects on enhancing soil microbial diversity and community structure (Table 2) (Ji et al. 2020a; Yang 53 54 55 et al. 2021; Zheng et al. 2019). Biochar itself is a source of nutrients, including microminerals, trace elements, 56 57 ash and so on, so its application also supplies essential agronomic benefits to farmers (Rawat et al. 2019). More 58 59 importantly, biochar can absorb fertilizers and slowly release these into the soil, which helps to not only retain 60 61 the nutrient availability in the soil but also reduce fertilizer leaching and drainage, which then contribute to 62 63 64 65 1 2 3 environmental pollution (Rawat et al. 2019). Since soil pH and nutrient status has a close correlation with soil 4 5 6 microorganism, the changes in soil chemical and physical properties as a result of biochar application could be 7 8 the key driven factor for the alteration of soil biological properties (Cheng et al. 2019; Yang et al. 2021). 9 10 11 12 13 14 Several downsides of biochar incorporation need to be considered to improve its effectiveness and reduce the 15 16 detrimental effects on the environment. Biochar has been considered as the most expensive soil management 17 18 19 solution, particularly for large-scale use in agriculture (Siedt et al. 2020). Since the application rate of biochar 20 21 normally ranges from 10 to 150 tons/ha and controlling strongly acid soils may require large quantity of biochar, 22 23 which leads to an increased costs for energy inputs, feedstocks, transportation and incorporation (Dai et al. 24 25 2017). Furthermore, most studies on biochar application for managing soil acidification in tea farming to date 26 27 28 have been conducted in controlled conditions in China, suggesting that further research either in long-term field 29 30 conditions or in other tea producing areas would be needed. Overall, biochars indicate a great potential in 31 32 ameliorating soil acidification and improving tea plantation soil health, however, more comprehensive and 33 34 reliable evidence should be provided to validate these advantages. 35 36 37 38 39 40 3.4. Plant residues as organic mulching practices 41 42 Organic mulching practices employing plant residues and other agricultural wastes have received limited 43 44 45 attention to date. Some studies conducted on tea fields indicated that mulching materials such as Fern 46 47 (Gleichenia linearis) and tea pruning materials can alleviate soil acidity (Cu and Thu 2014a; b). Other materials 48 49 such as crop straws and legume residues also had positive effects on increasing pH of tea plantation soils, either 50 51 in field or laboratory trial conditions (Table 2) (Wang et al. 2009; Xianchen et al. 2020). In contrast, there have 52 53 54 been a number of investigations revealing the negative impacts of organic mulching on soil pH from other 55 56 cropping systems. Otero-Jiménez et al. (2021) found that rice straw mulch and rice straw burning significantly 57 58 reduced soil pH by 0.55 and 0.19 units respectively, and the application of wheat straw mulching reduced soil 59 60 pH by 0.11 units (Mehmood et al. 2014). Finally, some studies have demonstrated that plant residues have no 61 62 63 64 65 1 2 3 significant effects on soil pH (Iqbal et al. 2020; Ni et al. 2016). Positive effects of crop residues in increasing soil 4 5 6 pH could be mainly due to the decarboxylation of organic anions, which can neutralize soil exchangeable H + and 7 8 Al3+, and also reduce the toxicity of Al species to plant roots (Dai et al. 2017). Declines in soil pH following 9 10 application plant residue mulches could be attributed to the release of H+ from nitrification of NH +4 , which is 11 12 produced during the mineralization of organic N in the residues (Dai et al. 2017). Decomposition of crop 13 14 15 residues may also produce some organic and carbonic acids, potentially causing soil acidity (Arafat et al. 2020). 16 17 18 19 20 The potential of crop residue mulching in enhancing other soil health indicators have been widely recognized. 21 22 23 Plant residues improve soil moisture content, soil structure and regulate soil temperature, support soil microbial 24 25 activities and improve soil nutrient availability, as well as suppress weeds and reduce soil erosion, all of which 26 27 contribute to enhance soil health and crop productivity (Chatterjee et al. 2017; Kader et al. 2017; Ngosong et al. 28 29 2019). These benefits have also been demonstrated in tea cultivation systems. Covering the surface of tea 30 31 32 plantation soils with rice straw and tea pruning residues significantly reduced soil temperature variation, soil 33 34 compactness and soil bulk density, while increasing soil water retention and soil moisture (Cu and Thu 2014b; 35 36 Xianchen et al. 2020). Organic mulches can also enhance soil nutrient availability (Ca2+ and Mg2+, available N, 37 38 P, K) soil OM content but reduce soil Al+ concentration (Cu and Thu 2014a; Wang et al. 2009; Xianchen et al. 39 40 2020). Enrichment of soil microbial diversity and community structure as a result of mulching material addition 41 42 43 have been reported in these studies (Cu and Thu 2014a; b) (Table 2). Organic mulch cover creates favorable 44 45 moisture and thermo regimes in soils by controlling surface evaporation rates and alter soil temperatures, by 46 47 reducing temperature in the summer and raising it in the winter (Kader et al. 2017). Under appropriate soil 48 49 microclimatic conditions, plant litter can decompose and add nutrients to soils. Plant residues and other organic 50 51 52 mulch materials generally contain higher level of nutrients compared with inorganic mulch materials, but the 53 54 influence of organic mulching application on soil nutrients has been also determined by other factors such as soil 55 56 characteristics, climatic conditions (Iqbal et al. 2020; Kader et al. 2017). In addition, soil physicochemical 57 58 conditions including soil moisture, soil temperature and soil nutrients play a crucial part in governing soil 59 60 61 organisms (Kader et al. 2017; Onwuka and Mang 2018; Tan et al. 2018). For example, Brockett et al. (2012) 62 63 64 65 1 2 3 concluded that soil moisture is the major factor affecting the community structure of soil microbes as well as 4 5 6 enzyme activities. Examples of plant residue mulching and the summary of beneficial impacts of organic 7 8 mulching, organic fertilizer and biochar applications in tea plantation soils are shown in Fig.8. 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 Fig. 8 Plant residues (rice straw, Acacia bark and woodchips) and organic manure (poultry manures) 35 36 applications in tea plantations (a) and summaries of the beneficial effects of some soil amendments derived from 37 38 agricultural wastes on soil properties of tea plantations (b). Photo was taken in Thai Nguyen province, Northern 39 40 Vietnam by the author. 41 42 43 44 45 46 However, some of mulching materials such as crop straws generally decompose quickly, thus need to be 47 48 49 frequently incorporated for long-term use. This may require extra labour and investments, preventing farmers 50 51 from adopting them in the long run (Amoah-Antwi et al. 2020; Dai et al. 2017). Extensive use of plant residues 52 53 such as tea pruned litters to mulch tea soils could also lead to a decrease of soil pH and the accumulation of 54 55 active allelochemicals, which can cause soil sickness and tea growth deterioration (Arafat et al. 2020). Too much 56 57 58 organic mulch could also result in other issues such as excess moisture and nitrogen, pests and anaerobic 59 60 conditions, damaging the plant root and negatively affecting its growth and productivity (Iqbal et al. 2020; Kader 61 62 63 64 65 1 2 3 et al. 2017). Overall, organic mulching employing plant residues is an effective soil management tool to improve 4 5 6 soil physicochemical properties, but its role in controlling tea soil acidity needs further investigations. 7 8 9 10 11 12 3.5. Intercropping and agroforestry 13 14 Tea plants intercropped with loquat, waxberry and citrus significantly improves soil pH, organic matter, N, P and 15 16 K availability, tea quality indicators, and reduces soil heavy metal concentrations compared with monoculture 17 18 tea gardens, regardless of sampling seasons (Wen et al. 2019). Similarly, Xianchen et al. (2020) found that inter- 19 20 21 planting of Vulpia myuros at the density of 22.5kg/seeds/ha in tea plantations significantly increased soil 22 23 nutrients (OM, available N, P, K), soil water holding capacity while reducing soil temperature fluctuations and 24 25 soil compactness at all observed soil depths (0-10 and 10-20cm). In terms of soil organism, intercropping 26 27 adoption in tea cultivation enriched soil enzyme activity and regulated tea pests (Xianchen et al. 2020; Zhang et 28 29 al. 2017) (Table 2). In addition, tea – Ginkgo tree (Ginkgo biloba L.) agroforestry significantly increased soil pH 30 31 32 (5,86 vs 5.21), soil organic carbon (17.92 vs 16.38 and total N (1,91 vs 1.79) compared with single tea 33 34 plantations (Tian et al. 2013). The increase of soil pH in the Ginkgo – tea agroforestry is likely due to the 35 36 alkaline matter formed during the decomposition of Ginkgo tree residues which neutralizes soil acidity (Tian et 37 38 al. 2013). Intercropping and agroforestry might increase overall ecosystem productivity and nutrient retention by 39 40 41 increasing species diversity, increase soil organic matter by plant residues, attribute to the decomposition of fine 42 43 roots in the deep mineral layers and surface leaves of trees (Brooker et al. 2015; Cong et al. 2015; Dollinger and 44 45 Jose 2018). Among these impacts, organic matter enrichment could play a key role, containing basic cations and 46 47 contributing to increasing the supply of important nutrients (Cardinael et al. 2020; Dollinger and Jose 2018). 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Table 2 Summaries of current studies of organic fertilizers, biochar, plant residues and other relevant options on mitigating soil acidification and 19 20 improving soil health, tea plant growth, and reducing environment risks. 21 22 Material/ Soil type Experiment type Soil pH effect Other positive and/or negative impacts on soil, Reference 23 Practice Location Application rate/time tea plants and the environment 24 25 Sheep manure + Red soil - Field experiment - Organic fertilizers resulted in - Significant increased soil bacterial abundance, Lin et al. 26 rape cake China - Trial time: 30 years an increase by 0.2 units (4.2 vs total K, while deceased the contents of Cd, As and (2019) 27 4.0) compared to chemical Pb in rhizosphere and tea leaves. 28 29 fertilizers. - Reduced soil total N (0.23 g/kg); total P (1.24 30 g/kg). 31 Pig manure Red soil - Field experiment - Increased by 1.1 units after 18 - Pig manure application reduced exchangeable Cai et al. 32 (Ferralic - Trial time: 18 years years of pig manure application. Al3+ and significantly increased soil exchangeable (2015) 33 Cambisol) Ca2+, Mg2+, Na+ and K+. 34 China 35 36 Cow manure + Haplic Acrisol - Field experiment - Soil pH value with chicken and - Organic fertilizer application increased soil Gu et al. 37 Pig manure Chia - Manure: 1000- pig manure practices were 5.36 microbial diversity by 8.59–33.14% and resulted (2019) 38 2.000kg/ha and 5.09 respectively, compared in an improvement of potential ecosystem 39 - Trial time: 1 year to 4.71 of non- fertilization and function compared with synthesized fertilizer. 40 4.31 of mineral compound - Increased total P but decreased total N. 41 (NPK) application. 42 43 Pig manure Red soil - Field experiment - 0.66 unit increased by - Significantly increased soil OC, total N, NH4+-N Ji et al. 44 China - Substitution of 25%, application of 100% N substitute contents, available P and K. (2018) 45 50%, 75% and 100% compared to the non- fertilizer - Soil microbial biomass carbon (MBC) and 46 N by organic manure plots microbial biomass nitrogen (MBN), soil bacterial 47 - Trial time: 10 years - 1.23 units higher compared to diversity and community structure were improved 48 49 the pH value of synthetic significantly. 50 fertilizer use. 51 52 Cattle manure Planosols - Field experiment - Organic fertilizer and biochar - Cattle manure and biochar applications reduced Han et al. 53 (Clay loam) - Manure + biochar, application resulted in greater NO emission. (2021) 54 China 20.000 kg/ha soil pH compared to chemical - Adding cattle manure as a partial substitute for 55 -Trial time: 2 years fertilizer. biochar reduced NO emission, and sorely biochar 56 application reduced N2O emission by 14%. 57 58 59 60 61 62 63 64 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Chicken manure China - Field experiment - Chicken manure application - Significantly increased soil OM, total N and P; Qiu et al. 19 - 11.400kg/ha resulted in the highest soil pH available N, P and K. (2014) 20 - Trial time: 5 years (5.67), compared to non- - Organic manure uses promoted bacterial 21 fertilization (5.64) and mineral diversity, while that was reduced by chemical 22 compound (NPK) (5.40). fertilizer application. 23 24 Rapeseed cake Yellow brown - Field experiment - Rape seed cake (6.207 kg/ha) - Soil OM, available P and K increased by 31.4%, Xie et al. 25 China - 1.904, 3.928, 6.207 decreased soil pH by 0.19 units 26.2%, and 21.7%, respectively (2019) 26 kg/ha while with chemical fertilizer - Increased restoration of NH4- N, NO3-N, total P 27 - Trial time: 1 year was 0.33 units. and K contents in soil while reduced the 28 substances in runoff water. 29 30 Cow manure Brown loamy -Field experiment - Data not provided - Significantly increased the relative abundance of Zhang et al. 31 China - 20 tons/ha Proteobacteria and Bacteroidetes species and (2020a) 32 - Trial time: 6 months enhanced the diversity of bacterial communities. 33 Rapeseed cake Acid yellow - Field experiment - Significantly increased soil pH - Increased total OM and preserved soil C and N Xie et al. 34 brown - 1.708, 4.270, 6.831 by 2.19 – 4.29% compared to pools of the tea plantations (2021) 35 China and 8.539 kg/ha/year chemical compound treatments. - Reduced the nitrogen inputs (NH - N and NO - 36 4 3 37 - 8 months N) in the tea plantation runoff. 38 Pig, chicken and Alfisol - Field trial - Soil pH for pig, chicken and - Increased soil OC, total N while reducing N2O He et al. 39 cattle manure China - Trial time: 1 year cattle manure compost uses and NO emissions. (2019) 40 compost were 4.56, 4.48 and 4.57 - Organic fertilizer has no influence on tea yield, 41 respectively, compared to 4.44 but that was increased by chicken manure and 42 of non-fertilizer and 4.31 of biochar combined application. 43 chemical fertilizer practices. 44 45 Organic Ferralsol - Field trial - Organic tea management with - Increased soil OM, soil N and C/N ratio. Li et al. 46 management China - Chinese Pennisetum: organic fertilizer uses resulted in - Enhanced species diversity, species richness and (2014) 47 (Chinese 4.000kg/ha; rape cake: greater soil pH compared to trophic diversity of nematodes in the soil. 48 Pennisetum, rape 3.000kg/ha; farmyard: conventional tea management; 49 cake and 2.000kg/ha/year but lower compared to natural 50 farmyard manure) - Trial time: 6 years tea plantations. 51 52 Organic Ultisols - Field experiment - Soil pH has an inconsistent - Increased soil microbial C by 164.4% and soil Gui et al. 53 management China - 4.500- 9.000 correlation with tea management microbial N by 482.9% on average. (2021) 54 (rape cake, kg/ha/year methods. - Total OC, N and available P increased 55 compost and - Trial time: around significantly in organically managed tea 56 commercial 10 years plantation soils, but Ca and Mg availability 57 organic decreased in comparison with conventional 58 fertilizers) management. 59 60 61 62 63 64 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Agroecological Ferralic - Field experiment - Increased soil pH by 0.35 units - Significantly improved soil OM, colonization 19 management Acrisols - 6.000- 8.000 on average, compared to and intensity of arbuscular mycorrhizal fungi Unpublished 20 (chicken and cow Vietnam kg/ha/year conventional tea plantations. (AMF). data 21 manure as main - Trial time: 5-10 years - Reduced soil total N. 22 nutrient supplies) 23 24 Organic Red soil - Field experiment - Soil pH increased by 0.91 units - Increased total OC, available P, NH4- N and Tan et al. 25 management China - Management compared to conventional tea NO3- N but total P and N were lower than that in (2019) 26 (cow and pig duration: 14 years plantations, and 0.06 units the non- polluted tea management). 27 manure, compared with the tea - Improved soil microbial diversity, increased the 28 commercial plantations employed a abundances of beneficial soil microbes, and 29 organic fertilizer) combined application of organic altered the interaction network structure compared 30 and chemical fertilizers (non- with conventional and pollution- free management 31 polluted management practices). practices. 32 33 Organic Bangladesh - Field research - Soil pH of organically - Increased total OM and nutrient availability (K, Sultana et al. 34 management managed tea plantation was 5.1, Ca, Mg, P, Zn and S) (2014) 35 compared to 4.2 of - Significantly increased tea yield and economic 36 conventionally managed tea efficiency. 37 plantation. 38 Organic Laterites - Field research - Soil pH was significantly - Organic tea management increased soil P Wu et al. 39 management China - 6.000kg/ha/year, dry lower compared to that in availability, enhance soil microbial communities (2020) 40 (Sheep manure) matter longan orchard, both in the (bacteria, fungi, actinomycetes and AMF) 41 - Management time: 3 surface (5.05 vs 5.32) and 10- compared to conventional tea management. 42 43 years 20cm depth (5.04 vs 5.24). - Conversion of longan to tea plantation 44 - No significant difference significantly reduced soil fertility. 45 compared to conventional tea 46 management plantations. 47 Rice straw Oxisols - Laboratory - Soil pH was 4.4; 4.2 and 3.9 - Nitrification would be detrimental to the N Wang et al. 48 biochar China incubation for 5%, 2% and 1% of biochar uptake of tea, while NO3-N produced from (2018) 49 - 1%, 2% and 5% of applications respectively) nitrification could be lost by leaching, runoff and 50 51 the dry soil weight - Soil pH significantly increased denitrification. 52 (w/w) by biochar application, but that - Tea soil pH should be maintained at higher value 53 - Trial time: 21 days was lower compared to lime than the optimum pH for nitrification (⁓5.1) 54 (CaO) application. 55 Rice husk biochar China - Laboratory - Application of biochar at 2 and - The incorporation of fast pyrolysis rice husk led Wang et al. 56 at 550 0C incubation 4% significantly increased soil to a significant increase of soil total C, N, (2014) 57 - 0.5%, 1%, 2% (w/w) pH (3.52 and 3.63 respectively). extractable Ca, Na, Mg and K contents, while 58 - 60 days available Al and Pb were reduced. 59 60 Rice, wheat and Ultisol - Laboratory - Soil pH increased in all - Significantly increased soil exchangeable cations Wang et al. 61 peanut residue China incubation biochar application treatments, but reducing soil exchangeable Al and acidity (2014) 62 biochar at - 1%, 2% (w/w) and the highest soil pH value 63 64 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 300 0C - Trial time: 65 days was observed in peanut biochar, - Increasing biochar application rate has no further 19 followed by wheat and rice effect on soil pH. 20 residue biochar. - Reduced acidity produced from N cycle. 21 Rice straw Loamy clay - Glasshouse trial - pH increased by 0.9 units by - Increased plant nutrients (P, K and Mg Yan et al. 22 biochar at 550 0C; China - 2% and 5% (w/w) bamboo biochar application, 1 concentrations), while reducing Mn and Cu (2021) 23 24 Bamboo straw - Trial time: 1 year unit (from 4.30- 5.30) by rice concentrations. 25 biochar at 750- biochar use at the rate of 5%. - Significantly improved tea growth characters 26 800 0C - Increasing biochar additional compared to conventional tea management 27 rate resulted in greater soil pH without biochar. 28 increase. - Rice and bamboo biochar has no significantly 29 different effect on tea growth and tea soil 30 nutrients. 31 Tea pruning Red- yellow - Laboratory - Biochar amendment - Tea pruning residue use as mulch significantly Oo et al. 32 33 residue biochar at Japan incubation significantly increased soil pH at increased soil total N, C, and also N2O and CO2 (2018) 34 500- 600 0C - 4% (w/w) the surface (0-5 cm, 0.23 units) emissions. 35 - Trial time: 90 days and 5- 10 cm soil layer (0.73 - Converting tea pruning residue to biochar 36 units). amendment and its incorporation significantly 37 mitigate N2O emission by up to 74.2%, but 38 increased CO2 emission. 39 Bamboo residue Inceptisols - Glasshouse trial - Soil pH increased by 0.31 units - Reduced NH4+ -N leaching by up to 91.9%; Chen et al. 40 biochar at 500 0C - 3% and 6% (w/w) with application rate of 3%, 0.75 NO3- -N by a maximum of 66.9% and total N by (2021) 41 - Trial time: 180 days units with incorporation rate at up to 72.8%. 42 43 6%. - Enhanced soil nutrient retention (N by up to 44 23.9%). 45 - Improved soil microbial biomass and enzyme 46 activity. 47 Wheat straw Plinthosols - Laboratory - Soil pH increased 1.09 units - Biochar amendment increased the abundance of Ji et al. 48 biochar at 450 0C China incubation compared to non-fertilizer ammonia oxidizing bacteria and Nitrous oxide (2020a) 49 - 4% (w/w) practices, but lower compared to reductase genes. 50 - Trial time: 35 days the combined application of - Increased soil C/N ratio and decreased N2O 51 52 biochar and N fertilizer (5.2 vs emission in acidic soil. 53 5.4). - Biochar could increase N2O emission in alkaline 54 soils 55 Legume and non- Utisols - Laboratory - Soil pH immediately increased - Increased soil dissolved OC but reduced Zheng et al. 56 legume biomass China incubation by around 0.4 units after biochar inorganic N. (2019) 57 at 500 0C - 1% (w/w) addition, then remained stably. - Suppressed N2O emission by around 40% 58 -Trial time: 30 days - Legume biochar has greater - Significantly altered fungal community 59 impact on increasing soil pH structure, relative abundance of Ascomycota 60 61 compared to that of non-legume community, but has no significant effect on 62 biochar. bacterial community. 63 64 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Wheat straw Plinthosols - Field experiment - Significantly increased soil pH - Biochar application decreased N2O and NO Ji et al. 19 biochar at 450 0C China - 20.000kg/ha by 0.2 units. emissions from acidic tea soils. (2020b) 20 - Trial time: 2 years - Denitrification was mainly responsible for 21 producing N2O in acidic soil. 22 - Nitrification and denitrification processes were 23 both facilitated by biochar addition. 24 25 Wheat straw Alfisol - Field experiment - Increased soil pH by 0.68 units - Biochar applications reduced N2O and NO He et al. 26 biochar at 450 0C China -7.500 kg/ha compared to conventional emission factor by 1.82 and 1.38 respectively, (2019) 27 - Trial time: 1 year chemical N, and by 0.55 units compared to chemical N use. 28 compared with non-fertilizer - Biochar combined with manure chicken applied 29 treatment. to tea soils could mitigate N gas emissions and 30 increase tea productivity. 31 Mushroom Ultisols - Field experiment - Biochar application at a rate of - Biochar application enhanced plant beneficial Yang et al. 32 33 residue biochar at China - 1.350 kg/ha and 1.350 kg/ha increased soil pH by fungal genera such as Chloridium, Clavulina, (2021) 34 500 0C 2.390kg/ha 0.1 units after one year, while Amylocorticium, Rhodosporidiobolus and 35 - Trial time: 1 year the figure for the higher rate bacterial genera such as, Mizugakiibacter, 36 (2.390kg/ha, biochar + based Rhodanobacter and Pedobacter. 37 chemical fertilizer) was 0.27 - Increased tea yield and yield components, tea 38 units. quality indicators such as amino acids and water 39 extract contents. 40 Rice straw - China - Field experiment - Increased soil pH by 0.13 units - Reduced soil temperature variation and having a Xianchen et 41 42 - 7cm thick compared to non- mulching significant cooling effect in the deep soil layer al. (2020) 43 - Trial time: 8 months practice. -Significantly improved soil water retention while 44 reducing soil compactness. 45 - Significantly increased soil OM, available N, P, 46 K and total N. 47 Plant residue ash Alfisol - Laboratory - Plant residue ash significantly - Reduced soil Al exchangeable concentrations. Wang et al. 48 (canola, wheat China incubation increased soil pH (by 0.3 units (2009) 49 rice, corn, - 20g ash/ 350g soil on average). 50 51 soybean - Trial time: 60 days - Leguminous residues had more 52 peanut…) significant effects in raising soil 53 pH than the non-legumes. 54 Fern (Gleichenia Acrisols - Field experiment - Application rate of 15 and 25 - Significantly increased soil basic cations (Ca2+ Cu and Thu 55 linearis) Vietnam - 0, 15, 25, 35 and 45 tons/ ha significantly increased and Mg 2+) while reducing soil Al3+ (2014a) 56 tons/ha (fresh weight) soil pH at the 3 years of - Improved soil moisture, soil bulk density and 57 - Trial time: 3 years experiment, while the rates of 35 humus substances, and enhanced soil microbial 58 and 45 tons/ha had inconsistent activities. 59 60 effect on soil pH. - Application rate at 25tons/ha of fern is 61 recommended. 62 63 64 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Tea pruned Acrisols - Field experiment - Tea residue mulches - Increased soil moisture, soil OM content and Cu and Thu 19 residues Vietnam - 30 tons/ha significantly increased soil pH reduced soil bulk density. (2014b) 20 - Trial time: 3 years (by 0.3 units after 1 year; 1.1 - Significantly increased total number of soil 21 units after 3 years) compared to bacterial, fungi and actinomyces. 22 no- mulching practice. - The influences of tea pruned residues on soil 23 properties reduced rapidly after 3 application 24 25 years. 26 Peanut hull Brown soil - Field experiment - Soil pH slightly increased - Significantly increased soil moisture contents, Zhang et al. 27 China - 10cm thick (0.04 units) compared to non- OM, total N and K, available N but reduced total (2020b) 28 mulch treatments. P, available P and K. 29 - Increased fungal community diversity in 0– 30 20 cm soils and that of bacterial communities in 31 20–40 cm soils. 32 33 Intercropping China - Field experiment - Increased soil pH by 0.06 units - Significantly increased soil OM, soil available Xianchen et 34 with - 7cm thick compared to tea monoculture. N, P, K and total N, and soil enzyme activity. al. (2020) 35 Vulpia myuros - Trial time: 8 months - Optimized topsoil temperature, increased soil 36 water holding capacity while reducing soil 37 compactness. 38 Intercropping Acidic - Greenhouse trial - Data not provided - Decreased the population of tea green Zhang et al. 39 with aromatic Histosols - Trial time: 2 years leafhoppers while increasing the natural enemies (2017) 40 plants (Cassia China of tea pests such as spiders, lacewings, and 41 42 tora, Medicago parasitoids. 43 sativa, Leonurus 44 artemisia, 45 and Mentha 46 haplocalyx) 47 Intercropping Yellow soil - Field experiment - Soil pH at three soil depths (0- -Increased soil OM, available P and K while Wen et al. 48 with fruit trees China - Trial time: 30 years 10, 10-20 and 20-30cm) reducing heavy metal (Cr, Cd, As, Hg, and Pb) (2019) 49 (loquat, waxberry significantly increased by - Improved tea quality indicators such as amino 50 51 and citrus) intercropping practices, acid and catechin. 52 compared to that in mono tea 53 plantations. 54 Agroforestry (tea- China - Field experiment - Increased soil pH at all - Significantly increased soil OC, OM and total N Tian et al. 55 Gingko tree - Growing distance: observed soil depths (by 0.65 contents, soil microbial biomass, and enzyme (2013) 56 (Ginkgo biloba 10 x 10m and 6 x 6m units at 0-10cm layer, 0.15 at activity. 57 L)). - Trial time: 11 years 10- 20cm layer and 0.35 at 20- - Enhanced soil productivity and sustainability. 58 30 cm layer). 59 60 61 62 63 64 65 1 2 3 4. Conclusion and perspectives 4 5 Soil acidification is becoming an increasingly severe problem in many tea growing countries, resulting in serious 6 7 impacts on soil chemical properties, tea productivity and quality and the environment. To date however, how 8 9 10 low pH affects tea soil biological and physical properties as well as its management cost have been poorly 11 12 explored. Agriculture wastes and products have demonstrated a great potential to mitigate soil acidification by 13 14 tea cultivation and improve tea soil health. Being naturally alkaline with high pH value and buffering capacity, 15 16 these materials could supply alkaline matter and essential elements to neutralize soil acidity and alter soil 17 18 19 properties, positively influencing soil nutrient availability, enrich soil organisms and ultimately improve tea 20 21 yield and quality indicators. While promising, their expanded uses would need further understanding to improve 22 23 their application efficacy while reducing any potential negative consequences on the environment. In addition, 24 25 the risks of introduction of heavy metal and pathogens from animal manures, compost and biochar applications 26 27 28 have been widely reported (Alegbeleye and Sant'Ana 2020; Dai et al. 2017), but how they could affect soil and 29 30 tea plants have not been clearly understood. Moreover, most of reports on effective impacts of biochar for 31 32 correcting soil acidification have been the outcomes of laboratory or glasshouse studies, thus the results need to 33 34 be validated in field conditions (Dai et al. 2017). Finally, the majority of studies on utilizing agricultural wastes 35 36 in tea cultivation to date have been implemented in China, with specific but limited soil characteristics, climate 37 38 39 conditions and tea management practices. It has been clearly indicated that differences in such conditions could 40 41 significantly affect the effectiveness of these soil acidification ameliorants (Gu et al. 2019; Siedt et al. 2020; Wu 42 43 et al. 2020). This research gap highlights the need and opportunities for further investigations in other systems to 44 45 provide comprehensive knowledge and reliability in recycling these soil amendments. 46 47 48 Funding 49 50 This work was supported by Deakin University and Alliance of Bioversity International and International Center for 51 52 Tropical Agriculture (CIAT), Asia Hub, Common Microbial Biotechnology Platform (CMBP), Hanoi, Vietnam. 53 54 55 56 Conflicts of Interest 57 58 The authors declare no conflict of interest. 59 60 61 62 63 64 65 1 2 3 References 4 5 6 Abe SS, Hashi I, Masunaga T, Yamamoto S, Honna T, Wakatsuki T (2015) Soil Profile Alteration in a Brown 7 Forest Soil under High-Input Tea Cultivation. Plant Prod Sci 9: 457-461. 8 9 http://doi.org/10.1626/pps.9.457 10 11 Adhikary S (2012) Vermicompost, the story of organic gold: A review. Agricultural Sciences 03: 905-917. 12 13 http://doi.org/10.4236/as.2012.37110 14 15 Agostini S, Harvey BP, Wada S, Kon K, Milazzo M, Inaba K, Hall-Spencer JM (2018) Ocean acidification 16 17 drives community shifts towards simplified non-calcified habitats in a subtropical−temperate transition 18 zone. Sci Rep 8: 11354. http://doi.org/10.1038/s41598-018-29251-7 19 20 Akhil D, Lakshmi D, Kartik A, Vo D-VN, Arun J, Gopinath KP (2021) Production, characterization, activation 21 22 and environmental applications of engineered biochar: a review. Environ Chem Lett 19: 2261-2297. 23 24 http://doi.org/10.1007/s10311-020-01167-7 25 26 Alegbeleye OO, Sant'Ana AS (2020) Manure-borne pathogens as an important source of water contamination: 27 28 An update on the dynamics of pathogen survival/transport as well as practical risk mitigation strategies. 29 Int J Hyg Environ Health 227: 113524. http://doi.org/10.1016/j.ijheh.2020.113524 30 31 Alekseeva T, Alekseev A, Xu RK, Zhao AZ, Kalinin P (2011) Effect of soil acidification induced by a tea 32 33 plantation on chemical and mineralogical properties of Alfisols in eastern China. Environ Geochem 34 35 Health 33: 137-148. http://doi.org/10.1007/s10653-010-9327-5 36 37 Allied Market Research (2020) Tea Market by Type, Distribution Channel and Application: Global Opportunity 38 39 Analysis and Industry Forecast, 2020–2027. https://www.alliedmarketresearch.com/tea-market. 40 41 Accessed 26 May 2021. 42 Amoah-Antwi C, Kwiatkowska-Malina J, Thornton SF, Fenton O, Malina G, Szara E (2020) Restoration of soil 43 44 quality using biochar and brown coal waste: A review. Sci Total Environ 722: 137852. 45 46 https://doi.org/10.1016/j.scitotenv.2020.137852 47 48 Arafat Y, Tayyab M, Khan MU, Chen T, Amjad H, Awais S, Lin X, Lin W, Lin S (2019) Long-term 49 50 monoculture negatively regulates fungal community composition and abundance of tea orchards. 51 52 Agronomy 9: 466. https://doi.org/10.3390/agronomy9080466 53 Arafat Y, Ud Din I, Tayyab M, Jiang Y, Chen T, Cai Z, Zhao H, Lin X, Lin W, Lin S (2020) Soil Sickness in 54 55 Aged Tea Plantation Is Associated With a Shift in Microbial Communities as a Result of Plant 56 57 Polyphenol Accumulation in the Tea Gardens. Front Plant Sci 11: 601. 58 59 https://doi.org/10.3389/fpls.2020.00601 60 61 62 63 64 65 1 2 3 Arafat Y, Wei X, Jiang Y, Chen T, Saqib HSA, Lin S, Lin W (2017) Spatial distribution patterns of root- 4 5 associated bacterial communities mediated by root exudates in different aged ratooning tea monoculture 6 7 systems. Int J Mol Sci 18: 1727. https://doi.org/10.3390/ijms18081727 8 Ayilara MS, Olanrewaju OS, Babalola OO, Odeyemi O (2020) Waste management through composting: 9 10 Challenges and potentials. Sustainability 12: 4456. https://doi.org/10.3390/su12114456. 11 12 Ayinla A, Olayinka BU, Etejere EO (2016) Rice straw: a valuable organic manure for soil amendment in the 13 14 cultivation of groundnut (Arachis hypogaea). Environ Exp Bot 14: 205-211. https://doi.org/ 15 16 10.22364/eeb.14.27 17 18 Ayyildiz M, Erdal G (2021) The relationship between carbon dioxide emission and crop and livestock 19 20 production indexes: a dynamic common correlated effects approach. Environ Sci Pollut Res 28: 597- 21 610. https://doi.org/10.1007/s11356-020-10409-8 22 23 Azim K, Soudi B, Boukhari S, Perissol C, Roussos S, Alami IT (2018) Composting parameters and compost 24 25 quality: a literature review. Org Agric 8: 141-158. https://doi.org/10.1007/s13165-017-0180-z. 26 27 Bandyopadhyay S, Dutta D, Chattopadhyay T, Reza S, Dutta D, Baruah U, Sarkar D, Singh S (2014) 28 29 Characterization and classification of some tea-growing soils of Jorhat district, Assam. Agropedology 30 31 24: 138-145. 32 Bayraklı B, Dengiz O (2020) An evaluation of heavy metal pollution risk in tea cultivation soils of micro- 33 34 catchments using various pollution indexes under humid environmental condition. Rendiconti Lincei 35 36 Scienze Fisiche e Naturali 31: 393-409. https://doi.org/10.1007/s12210-020-00901-1 37 38 Bhatt MK, Labanya R, Joshi HC (2019) Influence of long-term chemical fertilizers and organic manures on soil 39 40 fertility-A review. Univers J Agric Res 7: 177-188. https://doi.org/10.13189/ujar.2019.070502 41 42 Bijarchiyan M, Sahebi H, Mirzamohammadi S (2020) A sustainable biomass network design model for 43 bioenergy production by anaerobic digestion technology: using agricultural residues and livestock 44 45 manure. Energy Sustain Soc 10: 1-17. https://doi.org/10.1186/s13705-020-00252-7 46 47 Brantley KE, Savin MC, Brye KR, Longer DE (2015) Pine woodchip biochar impact on soil nutrient 48 49 concentrations and corn yield in a silt loam in the Mid-Southern US. Agriculture 5: 30-47. 50 51 https://doi.org/10.3390/agriculture5010030 52 53 Brockett BF, Prescott CE, Grayston SJ (2012) Soil moisture is the major factor influencing microbial community 54 55 structure and enzyme activities across seven biogeoclimatic zones in western Canada. Soil Biol Biochem 56 44: 9-20. https://doi.org/10.1016/j.soilbio.2011.09.003 57 58 59 60 61 62 63 64 65 1 2 3 Brooker RW, Bennett AE, Cong WF, Daniell TJ, George TS, Hallett PD, Hawes C, Iannetta PP, Jones HG, 4 5 Karley AJ (2015) Improving intercropping: a synthesis of research in agronomy, plant physiology and 6 7 ecology. New Phytol 206: 107-117. https://doi.org/10.1111/nph.13132 8 Cai Z, Wang B, Xu M, Zhang H, He X, Zhang L, Gao S (2015) Intensified soil acidification from chemical N 9 10 fertilization and prevention by manure in an 18-year field experiment in the red soil of southern China. J 11 12 Soils Sed 15: 260-270. https://doi.org/10.1007/s11368-014-0989-y 13 14 Cai Z, Wang B, Zhang L, Wen S, Xu M, Misselbrook TH, Carswell AM, Gao S (2021) Striking a balance 15 16 between N sources: Mitigating soil acidification and accumulation of phosphorous and heavy metals 17 18 from manure. Sci Total Environ 754: 142189. https://doi.org/10.1016/j.scitotenv.2020.142189 19 20 Calbrix R, Barray S, Chabrerie O, Fourrie L, Laval K (2007) Impact of organic amendments on the dynamics of 21 soil microbial biomass and bacterial communities in cultivated land. Appl Soil Ecol 35: 511-522. 22 23 https://doi.org/10.1016/j.apsoil.2006.10.007 24 25 Caputo J, Beier CM, Sullivan TJ, Lawrence GB (2016) Modeled effects of soil acidification on long-term 26 27 ecological and economic outcomes for managed forests in the Adirondack region (USA). Sci Total 28 29 Environ 565: 401-411. https://doi.org/10.1016/j.scitotenv.2016.04.008 30 31 Cardinael R, Mao Z, Chenu C, Hinsinger P (2020) Belowground functioning of agroforestry systems: Recent 32 advances and perspectives. Plant Soil 453: 1-13. https://doi.org/10.1007/s11104-020-04633-x 33 34 Chatterjee R, Gajjela S, Thirumdasu R (2017) Recycling of organic wastes for sustainable soil health and crop 35 36 growth. Int J Waste Resour 7: 296-292. https://doi.org/10.4172/2252-5211.1000296 37 38 Chen C-F, Lin J-Y (2016) Estimating the gross budget of applied nitrogen and phosphorus in tea plantations. 39 40 Sustain Environ Res 26: 124-130. https://doi.org/10.1016/j.serj.2016.04.007 41 42 Chen C, Xiao B, Yu Y, Gong X (2009) Spatial variability of soil organic matter and pH and the correlation to 43 available nutrients in the tea garden of southern Shaanxi. Journal of Northwest A & F University-Natural 44 45 Science Edition 37: 182-188. 46 47 Chen P, Liu Y, Mo C, Jiang Z, Yang J, Lin J (2021) Microbial mechanism of biochar addition on nitrogen 48 49 leaching and retention in tea soils from different plantation ages. Sci Total Environ 757: 143817. 50 51 https://doi.org/10.1016/j.scitotenv.2020.143817 52 53 Chen S, Sheaffer CC, Wyse DL, Nickel P, Kandel H (2012) Plant-parasitic nematode communities and their 54 55 associations with soil factors in organically farmed fields in Minnesota. J Nematol 44: 361. 56 Chen Y, Camps-Arbestain M, Shen Q, Singh B, Cayuela ML (2018) The long-term role of organic amendments 57 58 in building soil nutrient fertility: a meta-analysis and review. Nutr Cycling Agroecosyst 111: 103-125. 59 60 https://doi.org/10.1007/s10705-017-9903-5 61 62 63 64 65 1 2 3 Cheng J, Lee X, Tang Y, Zhang Q (2019) Long-term effects of biochar amendment on rhizosphere and bulk soil 4 5 microbial communities in a karst region, southwest China. Appl Soil Ecol 140: 126-134. 6 7 https://doi.org/10.1016/j.apsoil.2019.04.017 8 Chivenge P, Rubianes F, Van Chin D, Van Thach T, Khang VT, Romasanta RR, Van Hung N, Van Trinh M 9 10 (2020) Rice Straw Incorporation Influences Nutrient Cycling and Soil Organic Matter. Sustainable Rice 11 12 Straw Management. Springer, Cham 13 14 Chong CT, Mong GR, Ng J-H, Chong WWF, Ani FN, Lam SS, Ong HC (2019) Pyrolysis characteristics and 15 16 kinetic studies of horse manure using thermogravimetric analysis. Energy Convers Manage 180: 1260- 17 18 1267. https://doi.org/10.1016/j.enconman.2018.11.071 19 20 Clauser NM, Felissia FE, Area MC, Vallejos ME (2021) A framework for the design and analysis of integrated 21 multi-product biorefineries from agricultural and forestry wastes. Renew Sust Energ Rev 139: 110687. 22 23 https://doi.org/10.1016/j.rser.2020.110687 24 25 Cong WF, Hoffland E, Li L, Six J, Sun JH, Bao XG, Zhang FS, Van Der Werf W (2015) Intercropping enhances 26 27 soil carbon and nitrogen. Global Change Biol 21: 1715-1726. https://doi.org/10.1111/gcb.12738 28 29 Cornelissen G, Nurida NL, Hale SE, Martinsen V, Silvani L, Mulder J (2018) Fading positive effect of biochar 30 31 on crop yield and soil acidity during five growth seasons in an Indonesian Ultisol. Sci Total Environ 32 634: 561-568. https://doi.org/10.1016/j.scitotenv.2018.03.380 33 34 Cu NX, Thu TTT (2014a) The Effects of Fern (Gleichenia linearis) Mulching on Soil Properties, Humus 35 36 Substance and Microbial Fauna in Soils Growing Tea in Phu Tho Province, Vietnam. Int J Sci Res 3: 37 38 1915-1919. 39 40 Cu NX, Thu TTT (2014b) Effects of tea-pruned mulches and microbial products on the accumulation of organic 41 42 matter and micro biota in soils grown tea in Phu Ho, Phu Tho province, Vietnam. Int J Agric Innov Res 43 3(2): 499-504. 44 45 Cusack DF, Macy J, McDowell WH (2016) Nitrogen additions mobilize soil base cations in two tropical forests. 46 47 Biogeochemistry 128: 67-88. https://doi.org/10.1007/s10533-016-0195-7. 48 49 Dai Y, Sun Q, Wang W, Lu L, Liu M, Li J, Yang S, Sun Y, Zhang K, Xu J (2018) Utilizations of agricultural 50 51 waste as adsorbent for the removal of contaminants: A review. Chemosphere 211: 235-253. 52 53 https://doi.org/10.1016/j.chemosphere.2018.06.179 54 55 Dai Z, Zhang X, Tang C, Muhammad N, Wu J, Brookes PC, Xu J (2017) Potential role of biochars in decreasing 56 soil acidification-a critical review. Sci Total Environ 581: 601-611. 57 58 https://doi.org/10.1016/j.scitotenv.2016.12.169 59 60 61 62 63 64 65 1 2 3 De Corato U (2020) Agricultural waste recycling in horticultural intensive farming systems by on-farm 4 5 composting and compost-based tea application improves soil quality and plant health: A review under 6 7 the perspective of a circular economy. Sci Total Environ: 738: 139840. 8 https://doi.org/10.1016/j.scitotenv.2020.139840 9 10 Ding Z, Kheir AM, Ali MG, Ali OA, Abdelaal AI, Zhou Z, Wang B, Liu B, He Z (2020) The integrated effect of 11 12 salinity, organic amendments, phosphorus fertilizers, and deficit irrigation on soil properties, phosphorus 13 14 fractionation and wheat productivity. Sci Rep 10: 1-13. https://doi.org/10.1038/s41598-020-59650-8 15 16 Doan TT, Bouvier C, Bettarel Y, Bouvier T, Henry-des-Tureaux T, Janeau JL, Lamballe P, Van Nguyen B, 17 18 Jouquet P (2014) Influence of buffalo manure, compost, vermicompost and biochar amendments on 19 20 bacterial and viral communities in soil and adjacent aquatic systems. Appl Soil Ecol 73: 78-86. 21 https://doi.org/10.1016/j.apsoil.2013.08.016 22 23 Dollinger J, Jose S (2018) Agroforestry for soil health. Agrofor Syst 92: 213-219. 24 25 https://doi.org/10.1007/s10457-018-0223-9 26 27 Fan D, Fan K, Zhang D, Zhang M, Wang X (2017) Impact of fertilization on soil polyphenol dynamics and 28 29 carbon accumulation in a tea plantation, Southern China. J Soils Sed 17: 2274-2283. 30 31 https://doi.org/10.1007/s11368-016-1535-x 32 FAO (2021) FAOSTAT. In: FAO. http://www.fao.org/faostat/en/#data/GM. Accessed 22 May 2021 33 34 35 Fung KF, Carr HP, Zhang J, Wong MH (2008) Growth and nutrient uptake of tea under different aluminium 36 37 concentrations. J Sci Food Agric 88: 1582-1591. https://doi.org/10.1002/jsfa.3254 38 39 Gebrewold AZ (2018) Review on integrated nutrient management of tea (Camellia sinensis L.). Cogent Food 40 41 Agric 4: 1-7. https://doi.org/10.1080/23311932.2018.1543536. 42 Gil M, Carballo M, Calvo L (2008) Fertilization of maize with compost from cattle manure supplemented with 43 44 additional mineral nutrients. Waste Manage 28: 1432-1440. 45 46 https://doi.org/10.1016/j.wasman.2007.05.009 47 48 Gong X-J, Qin L, Liu F, Liu D-N, Ma W-W, Zhang T, Liu X, Luo F (2020) Effects of organic manure on soil 49 50 nutrient content: A review. J Appl Ecol 31: 1403-1416. https://doi.org/10.13287/j.1001- 51 52 9332.202004.025 53 Goswami G, Deka P, Das P, Bora SS, Samanta R, Boro RC, Barooah M (2017) Diversity and functional 54 55 properties of acid-tolerant bacteria isolated from tea plantation soil of Assam. 3 Biotech 7: 1-16. 56 57 https://doi.org/10.1007/s13205-017-0864-9 58 59 60 61 62 63 64 65 1 2 3 Gruba P, Mulder J (2015) Tree species affect cation exchange capacity (CEC) and cation binding properties of 4 5 organic matter in acid forest soils. Sci Total Environ 511: 655-662. 6 7 https://doi.org/10.1016/j.scitotenv.2015.01.013 8 Gu S, Hu Q, Cheng Y, Bai L, Liu Z, Xiao W, Gong Z, Wu Y, Feng K, Deng Y (2019) Application of organic 9 10 fertilizer improves microbial community diversity and alters microbial network structure in tea 11 12 (Camellia sinensis) plantation soils. Soil Tillage Res 195: 104356. 13 14 https://doi.org/10.1016/j.still.2019.104356 15 16 Gui H, Fan L, Wang D, Yan P, Li X, Zhang L, Han W (2021) Organic management practices shape the structure 17 18 and associations of soil bacterial communities in tea plantations. Appl Soil Ecol 163: 103975. 19 20 https://doi.org/10.1016/j.apsoil.2021.103975 21 Gunarathne V, Ashiq A, Ramanayaka S, Wijekoon P, Vithanage M (2019) Biochar from municipal solid waste 22 23 for resource recovery and pollution remediation. Environ Chem Lett 17: 1225-1235. 24 25 https://doi.org/10.1007/s10311-019-00866-0 26 27 Guo X-x, Liu H-t, Zhang J (2020) The role of biochar in organic waste composting and soil improvement: A 28 29 review. Waste Manage 102: 884-899. https://doi.org/10.1016/j.wasman.2019.12.003 30 31 Hall ER, Wickes L, Burnett LE, Scott GI, Hernandez D, Yates KK, Barbero L, Reimer JJ, Baalousha M, Mintz J 32 (2020) Acidification in the US Southeast: causes, potential consequences and the role of the Southeast 33 34 Ocean and Coastal Acidification Network. Front Mar Sci 7: 548. 35 36 https://doi.org/10.3389/fmars.2020.00548 37 38 Han W, Kemmitt SJ, Brookes PC (2007) Soil microbial biomass and activity in Chinese tea gardens of varying 39 40 stand age and productivity. Soil Biol Biochem 39: 1468-1478. 41 42 https://doi.org/10.1016/j.soilbio.2006.12.029 43 Han Z, Wang J, Xu P, Li Z, Liu S, Zou J (2021) Differential responses of soil nitrogen‐ oxide emissions to 44 45 organic substitution for synthetic fertilizer and biochar amendment in a subtropical tea plantation. GCB 46 47 Bioenergy. https://doi.org/10.1111/gcbb.12842 48 49 Hauck J, Völker C (2015) Rising atmospheric CO2 leads to large impact of biology on Southern Ocean CO2 50 51 uptake via changes of the Revelle factor. Geophys Res Lett 42: 1459-1464. 52 53 https://doi.org/10.1002/2015GL063070 54 55 He T, Yuan J, Luo J, Wang W, Fan J, Liu D, Ding W (2019) Organic fertilizers have divergent effects on soil 56 N2O emissions. Biol Fertility Soils 55: 685-699. https://doi.org/10.1007/s00374-019-01385-4 57 58 Hui W, Ren-Kou X, Ning W, Xing-Hui L (2010) Soil acidification of alfisols as influenced by tea cultivation in 59 60 eastern China. Pedosphere 20: 799-806. https://doi.org/10.1016/S1002-0160(10)60070-7 61 62 63 64 65 1 2 3 Huu Chien H, Tokuda M, Van Minh D, Kang Y, Iwasaki K, Tanaka S (2019) Soil physicochemical properties in 4 5 a high-quality tea production area of Thai Nguyen province in northern region, Vietnam. Soil Sci Plant 6 7 Nutr 65: 73-81. https://doi.org/10.1080/00380768.2018.1539310 8 Iqbal R, Raza MAS, Valipour M, Saleem MF, Zaheer MS, Ahmad S, Toleikiene M, Haider I, Aslam MU, Nazar 9 10 MA (2020) Potential agricultural and environmental benefits of mulches—a review. Bull Natl Res Cent 11 12 44: 1-16. https://doi.org/10.1186/s42269-020-00290-3 13 14 Jalali M, Ranjbar F (2009) Rates of decomposition and phosphorus release from organic residues related to 15 16 residue composition. J Plant Nutr Soil Sci 172: 353-359. https://doi.org/10.1002/jpln.200800032 17 18 Ji C, Li S, Geng Y, Miao Y, Ding Y, Liu S, Zou J (2020a) Differential responses of soil N2O to biochar depend 19 20 on the predominant microbial pathway. Appl Soil Ecol 145: 103348. 21 https://doi.org/10.1016/j.apsoil.2019.08.010 22 23 Ji C, Li S, Geng Y, Yuan Y, Zhi J, Yu K, Han Z, Wu S, Liu S, Zou J (2020b) Decreased N2O and NO emissions 24 25 associated with stimulated denitrification following biochar amendment in subtropical tea plantations. 26 27 Geoderma 365: 114223. https://doi.org/10.1016/j.geoderma.2020.114223 28 29 Ji L, Wu Z, You Z, Yi X, Ni K, Guo S, Ruan J (2018) Effects of organic substitution for synthetic N fertilizer on 30 31 soil bacterial diversity and community composition: A 10-year field trial in a tea plantation. Agric 32 Ecosyst Environ 268: 124-132. https://doi.org/10.1016/j.agee.2018.09.008 33 34 Kader M, Senge M, Mojid M, Ito K (2017) Recent advances in mulching materials and methods for modifying 35 36 soil environment. Soil Tillage Res 168: 155-166. https://doi.org/10.1016/j.still.2017.01.001 37 38 Kamau DM (2008) Productivity and resource use in ageing tea plantations. Wageningen University, 39 40 Wageningen, The Netherlands 41 42 Kaur T (2020) Vermicomposting: An effective Option for Recycling Organic Wastes. Org Agric. IntechOpen, 43 London 44 45 Li G (2020) Innovative approaches to subsoil liming and management. Grains Research Update. Grains 46 47 Research and Development Corporation, Queensland 48 49 Li J, Chen Q, Li H, Li S, Liu Y, Yang L, Han X (2020) Impacts of different sources of animal manures on 50 51 dissemination of human pathogenic bacteria in agricultural soils. Environ Pollut 266: 115399. 52 53 https://doi.org/10.1016/j.envpol.2020.115399 54 55 Li L, Xu M, Eyakub Ali M, Zhang W, Duan Y, Li D (2018a) Factors affecting soil microbial biomass and 56 functional diversity with the application of organic amendments in three contrasting cropland soils 57 58 during a field experiment. PLoS One 13: e0203812. https://doi.org/10.1371/journal.pone.0203812 59 60 61 62 63 64 65 1 2 3 Li M-X, He X-S, Tang J, Li X, Zhao R, Tao Y-Q, Wang C, Qiu Z-P (2021) Influence of moisture content on 4 5 chicken manure stabilization during microbial agent-enhanced composting. Chemosphere 264: 128549. 6 7 https://doi.org/10.1016/j.chemosphere.2020.128549 8 Li R, Wang JJ, Zhang Z, Shen F, Zhang G, Qin R, Li X, Xiao R (2012) Nutrient transformations during 9 10 composting of pig manure with bentonite. Bioresour Technol 121: 362-368. 11 12 https://doi.org/10.1016/j.biortech.2012.06.065 13 14 Li S, Li H, Yang C, Wang Y, Xue H, Niu Y (2016) Rates of soil acidification in tea plantations and possible 15 16 causes. Agric Ecosyst Environ 233: 60-66. https://doi.org/10.1016/j.agee.2016.08.036 17 18 Li S, Liu Y, Wang J, Yang L, Zhang S, Xu C, Ding W (2017) Soil acidification aggravates the occurrence of 19 20 bacterial wilt in South China. Front Microbiol 8: 703. https://doi.org/10.3389/fmicb.2017.00703 21 Li X, Liu Q, Liu Z, Shi W, Yang D, Tarasco E (2014) Effects of organic and other management practices on soil 22 23 nematode communities in tea plantation: a case study in southern China. J Plant Nutr Soil Sci 177: 604- 24 25 612. https://doi.org/10.1002/jpln.201300610 26 27 Li Y, Li Z, Arafat Y, Lin W, Jiang Y, Weng B, Lin W (2017) Characterizing rhizosphere microbial communities 28 29 in long-term monoculture tea orchards by fatty acid profiles and substrate utilization. Eur J Soil Biol 81: 30 31 48-54. https://doi.org/10.1016/j.ejsobi.2017.06.008 32 Li Y, Li Z, Lin W, Jiang Y, Weng B, Lin W (2018) Effects of biochar and sheep manure on rhizospheric soil 33 34 microbial community in continuous ratooning tea orchards. Journal of Applied Ecology 29: 1273-1282. 35 36 https://doi.org/10.13287/j.1001-9332.201804.036 37 38 Lin W, Lin M, Zhou H, Wu H, Li Z, Lin W (2019) The effects of chemical and organic fertilizer usage on 39 40 rhizosphere soil in tea orchards. PloS one 14: e0217018. https://doi.org/10.1371/journal.pone.0217018. 41 42 Liu D, Ding Z, Ali EF, Kheir AM, Eissa MA, Ibrahim OH (2021) Biochar and compost enhance soil quality and 43 growth of roselle (Hibiscus sabdariffa L.) under saline conditions. Sci Rep 11: 1-11. 44 45 https://doi.org/10.1038/s41598-021-88293-6 46 47 Maitra S, Zaman A, Mandal TK, Palai JB (2018) Green manures in agriculture: A review. J Pharmacogn 48 49 Phytochem 7: 1319-1327. 50 51 Mehmood S, Zamir S, Rasool T, Akbar W (2014) Effect of tillage and mulching on soil fertility and grain yield 52 53 of sorghum. Sci Agric 8: 31-36. https://doi.org/10.15192/PSCP.SA.2014.4.1.3136 54 55 Mendonça Costa LA, Rozatti MAT, Carneiro LJ, Pereira DC, Lorin HEF (2015) Improving the nutrient content 56 of sheep bedding compost by adding cattle manure. J Clean Prod 86: 9-14. 57 58 https://doi.org/10.1016/j.jclepro.2014.08.093 59 60 61 62 63 64 65 1 2 3 Millaleo R, Reyes-Díaz M, Ivanov A, Mora M, Alberdi M (2010) Manganese as essential and toxic element for 4 5 plants: transport, accumulation and resistance mechanisms. J Soil Sci Plant Nutr 10: 470-481. 6 7 http://dx.doi.org/10.4067/S0718-95162010000200008 8 Moreno-Caselles J, Moral R, Perez-Murcia M, Perez-Espinosa A, Rufete B (2002) Nutrient value of animal 9 10 manures in front of environmental hazards. Commun Soil Sci Plant Anal 33: 3023-3032. 11 12 https://doi.org/10.1081/CSS-120014499 13 14 Mpatani FM, Han R, Aryee AA, Kani AN, Li Z, Qu L (2021) Adsorption performance of modified agricultural 15 16 waste materials for removal of emerging micro-contaminant bisphenol A: A comprehensive review. Sci 17 18 Total Environ 780: 146629. https://doi.org/10.1016/j.scitotenv.2021.146629 19 20 21 Mulder C, Van Wijnen HJ, Van Wezel AP (2005) Numerical abundance and biodiversity of below‐ ground 22 23 taxocenes along a pH gradient across The Netherlands. J Biogeogr 32: 1775-1790. 24 https://doi.org/10.1111/j.1365-2699.2005.01321.x 25 26 Mupenzi J, Li L, Ge J, Varenyam A, Habiyaremye G, Theoneste N, Emmanuel K (2011) Assessment of soil 27 28 degradation and chemical compositions in Rwandan tea-growing areas. Geosci Front 2: 599-607. 29 30 https://doi.org/10.1016/j.gsf.2011.05.003 31 32 Naeem MA, Khalid M, Aon M, Abbas G, Tahir M, Amjad M, Murtaza B, Yang A, Akhtar SS (2017) Effect of 33 34 wheat and rice straw biochar produced at different temperatures on maize growth and nutrient dynamics 35 of a calcareous soil. Arch Agron Soil Sci 63: 2048-2061. 36 37 https://doi.org/10.1080/03650340.2017.1325468 38 39 Neina D (2019) The role of soil pH in plant nutrition and soil remediation Applied and Environmental Soil 40 41 Science 2019: 1-9. doi: https://doi.org/10.1155/2019/5794869. 42 43 Ngo PT, Rumpel C, Dignac M-F, Billou D, Duc TT, Jouquet P (2011) Transformation of buffalo manure by 44 45 composting or vermicomposting to rehabilitate degraded tropical soils. Ecol Eng 37: 269-276. 46 47 https://doi.org/10.1016/j.ecoleng.2010.11.011 48 Ngosong C, Okolle JN, Tening AS (2019) Mulching: A sustainable option to improve soil health. Soil Fertility 49 50 Management for Sustainable Development: 231-249. https://doi.org/10.1007/978-981-13-5904-0_11. 51 52 Ni K, Shi Y-z, Yi X-y, Zhang Q-f, Fang L, Ma L-f, Ruan J (2018) Effects of long-term nitrogen application on 53 54 soil acidification and solution chemistry of a tea plantation in China. Agric Ecosyst Environ 252: 74-82. 55 56 https://doi.org/10.1016/j.agee.2017.10.004 57 58 Ni X, Song W, Zhang H, Yang X, Wang L (2016) Effects of mulching on soil properties and growth of tea olive 59 (Osmanthus fragrans). Plos one 11(8): e0158228. https://doi.org/10.1371/journal.pone.0158228 60 61 62 63 64 65 1 2 3 Obi F, Ugwuishiwu B, Nwakaire J (2016) Agricultural waste concept, generation, utilization and management. 4 5 Nigerian Journal of Technology 35: 957–964. 6 7 Ochedi FO, Yu J, Yu H, Liu Y, Hussain A (2021) Carbon dioxide capture using liquid absorption methods: a 8 review. Environ Chem Lett 19: 77-109. https://doi.org/10.1007/s10311-020-01093-8 9 10 Oh K, Kato T, Zhong-Pei L, Fa-Yun L (2006) Environmental problems from tea cultivation in Japan and a 11 12 control measure using calcium cyanamide. Pedosphere 16: 770-777. https://doi.org/10.1016/S1002- 13 14 0160(06)60113-6 15 16 Onwosi CO, Igbokwe VC, Odimba JN, Eke IE, Nwankwoala MO, Iroh IN, Ezeogu LI (2017) Composting 17 18 technology in waste stabilization: On the methods, challenges and future prospects. J Environ Manage 19 20 190: 140-157. https://doi.org/10.1016/j.jenvman.2016.12.051 21 Onwuka B, Mang B (2018) Effects of soil temperature on some soil properties and plant growth. Adv Plants 22 23 Agric Res 8: 34-37. https://doi.org/10.15406/apar.2018.08.00288 24 25 Oo AZ, Sudo S, Win KT, Shibata A, Sano T, Hirono Y (2018) Returning tea pruning residue and its biochar had 26 27 a contrasting effect on soil N2O and CO2 emissions from tea plantation soil. Atmosphere 9: 109. 28 29 https://doi.org/10.3390/atmos9030109 30 31 Otero-Jiménez V, del Pilar Carreño-Carreño J, Barreto-Hernandez E, van Elsas JD, Uribe-Vélez D (2021) 32 Impact of rice straw management strategies on rice rhizosphere microbiomes. Appl Soil Ecol 167: 33 34 104036. https://doi.org/10.1016/j.apsoil.2021.104036 35 36 Patra BR, Mukherjee A, Nanda S, Dalai AK (2021) Biochar production, activation and adsorptive applications: a 37 38 review. Environ Chem Lett 19: 2237-2259. https://doi.org/10.1007/s10311-020-01165-9. 39 40 Peng X, Deng Y, Peng Y, Yue K (2018) Effects of biochar addition on toxic element concentrations in plants: A 41 42 meta-analysis. Sci Total Environ 616: 970-977. https://doi.org/10.1016/j.scitotenv.2017.10.222 43 Plazonić I, Barbarić-Mikočević Ž, Antonović A (2016) Chemical composition of straw as an alternative material 44 45 to wood raw material in fibre isolation. Drvna industrija: Znanstveni časopis za pitanja drvne tehnologije 46 47 67: 119-125. doi: https://doi.org/10.5552/drind.2016.1446 48 49 Qiao C, Xu B, Han Y, Wang J, Wang X, Liu L, Liu W, Wan S, Tan H, Liu Y (2018) Synthetic nitrogen 50 51 fertilizers alter the soil chemistry, production and quality of tea. A meta-analysis. Agron Sustain Dev 38: 52 53 1-10. https://doi.org/10.1007/s13593-017-0485-z 54 55 Qiu S-L, Wang L-M, Huang D-F, Lin X-J (2014) Effects of fertilization regimes on tea yields, soil fertility, and 56 soil microbial diversity. Chil J Agric Res 74: 333-339. https://doi.org/10.4067/S0718- 57 58 58392014000300012 59 60 61 62 63 64 65 1 2 3 Ramírez-García R, Gohil N, Singh V (2019) Recent advances, challenges, and opportunities in bioremediation of 4 5 hazardous materials. In: VC Pandey, K Bauddh (eds) Phytomanagement of Polluted Sites. Elsevier, 6 7 Amsterdam 8 Rana A, Rana S, Kumar S (2021) Phytotherapy with active tea constituents: a review. Environ Chem Lett 19: 9 10 2031- 2041. https://doi.org/10.1007/s10311-020-01154-y 11 12 Ravindran B, Mnkeni P (2016) Bio-optimization of the carbon-to-nitrogen ratio for efficient vermicomposting of 13 14 chicken manure and waste paper using Eisenia fetida. Environ Sci Pollut Res 23: 16965-16976. 15 16 https://doi.org/10.1007/s11356-016-6873-0 17 18 Rawat J, Saxena J, Sanwal P (2019) Biochar: a sustainable approach for improving plant growth and soil 19 20 properties. Biochar-an imperative amendment for soil and the environment. IntechOpen, London 21 Rayne N, Aula L (2020) Livestock Manure and the Impacts on Soil Health: A Review. Soil Syst 4: 64. 22 23 https://doi.org/10.3390/soilsystems4040064 24 25 Reina L, Botto E, Mantero C, Moyna P, Menéndez P (2016) Production of second generation ethanol using 26 27 Eucalyptus dunnii bark residues and ionic liquid pretreatment. Biomass Bioenergy 93: 116-121. 28 29 https://doi.org/10.1016/j.biombioe.2016.06.023 30 31 Resquin F, Navarro-Cerrillo RM, Carrasco-Letelier L, Casnati CR, Bentancor L (2020) Evaluation of the 32 nutrient content in biomass of Eucalyptus species from short rotation plantations in Uruguay. Biomass 33 34 Bioenergy 134: 105502. https://doi.org/10.1016/j.biombioe.2020.105502 35 36 Roychowdhury D, Paul M, Banerjee SK (2014) A review on the effects of biofertilizers and biopesticides on rice 37 38 and tea cultivation and productivity. Int J Eng Sci Technol 2: 96-105 39 40 Sadek II, Youssef MA, Solieman NY, Alyafei MAM (2019) Response of Soil Properties, Growth, Yield and 41 42 Fruit Quality of Cantaloupe Plants (Cucumis melo L.) to Organic Mulch. Merit Res J Agric Sci Soil Sci 43 7: 106-122. https://doi.org/10.5281/zenodo.3463634 44 45 Saliu T, Oladoja N (2021) Nutrient recovery from wastewater and reuse in agriculture: a review. Environ Chem 46 47 Lett 19: 2299–2316. https://doi.org/10.1007/s10311-020-01159-7 48 49 Sánchez A, Artola A, Font X, Gea T, Barrena R, Gabriel D, Sánchez-Monedero MÁ, Roig A, Cayuela ML, 50 51 Mondini C (2015) Greenhouse gas emissions from organic waste composting. Environ Chem Lett 13: 52 53 223-238. https://doi.org/10.1007/s10311-015-0507-5 54 55 Shaji H, Chandran V, Mathew L (2021) Organic fertilizers as a route to controlled release of nutrients. 56 Controlled Release Fertilizers for Sustainable Agriculture. Elsevier, Amsterdam 57 58 Sharma H, Dhir A (2021) Capture of carbon dioxide using solid carbonaceous and non-carbonaceous adsorbents: 59 60 A review. Environ Chem Lett 19: 851-873. https://doi.org/10.1007/s10311-020-01118-2 61 62 63 64 65 1 2 3 Shi R-y, Hong Z-n, Li J-y, Jiang J, Baquy MA-A, Xu R-k, Qian W (2017) Mechanisms for increasing the pH 4 5 buffering capacity of an acidic Ultisol by crop residue-derived biochars. J Agric Food Chem 65: 8111- 6 7 8119. https://doi.org/10.1021/acs.jafc.7b02266 8 Shi R-y, Ni N, Nkoh JN, Li J-y, Xu R-k, Qian W (2019) Beneficial dual role of biochars in inhibiting soil 9 10 acidification resulting from nitrification. Chemosphere 234: 43-51. 11 12 https://doi.org/10.1016/j.chemosphere.2019.06.030 13 14 Siedt M, Schäffer A, Smith KE, Nabel M, Roß-Nickoll M, van Dongen JT (2020) Comparing straw, compost, 15 16 and biochar regarding their suitability as agricultural soil amendments to affect soil structure, nutrient 17 18 leaching, microbial communities, and the fate of pesticides. Sci Total Environ 751: 141607. 19 20 https://doi.org/10.1016/j.scitotenv.2020.141607 21 Su L, Bai T, Qin X, Yu H, Wu G, Zhao Q, Tan L (2021) Organic manure induced soil food web of microbes and 22 23 nematodes drive soil organic matter under jackfruit planting. Appl Soil Ecol 166: 103994. 24 25 https://doi.org/10.1016/j.apsoil.2021.103994 26 27 Sultana J, Siddique M, Kamaruzzaman M, Halim M (2014) Conventional to ecological: Tea plantation soil 28 29 management in Panchagarh District of Bangladesh. J Sci Technol Environ Inform 1: 27-35. 30 31 https://doi.org/10.18801/jstei.010114.03 32 Taflick T, Maich ÉG, Ferreira LD, Bica CID, Rodrigues SRS, Nachtigall SMB (2015) Acacia bark residues as 33 34 filler in polypropylene composites. Polímeros 25: 289-295. https://doi.org/10.1590/0104-1428.1840. 35 36 Tan L, Gu S, Li S, Ren Z, Deng Y, Liu Z, Gong Z, Xiao W, Hu Q (2019) Responses of microbial communities 37 38 and interaction networks to different management practices in tea plantation soils. Sustainability 11: 39 40 4428. https://doi.org/10.3390/su11164428 41 42 Tan X, Shao D, Gu W (2018) Effects of temperature and soil moisture on gross nitrification and denitrification 43 rates of a Chinese lowland paddy field soil. Paddy Water Environ 16: 687-698. 44 45 https://doi.org/10.1007/s10333-018-0660-0 46 47 Tang C, Weligama C, Sale P (2013) Subsurface soil acidification in farming systems: its possible causes and 48 49 management options. Molecular environmental soil science. Springer, Dordrecht 50 51 Tao C, Song Y, Chen Z, Zhao W, Ji J, Shen N, Ayoko GA, Frost RL (2021) Geological load and health risk of 52 53 heavy metals uptake by tea from soil: What are the significant influencing factors? Catena 204: 105419. 54 55 https://doi.org/10.1016/j.catena.2021.105419 56 Tian D, Niu S (2015) A global analysis of soil acidification caused by nitrogen addition. Environ Res Lett 10: 57 58 024019. https://doi.org/10.1088/1748-9326/10/2/024019 59 60 61 62 63 64 65 1 2 3 Tian Y, Cao F, Wang G (2013) Soil microbiological properties and enzyme activity in Ginkgo–tea agroforestry 4 5 compared with monoculture. Agrofor Syst 87: 1201-1210. https://doi.org/10.1007/s10457-013-9630-0. 6 7 Torma S, Vilček J, Lošák T, Kužel S, Martensson A (2018) Residual plant nutrients in crop residues–an 8 important resource. Acta Agric Scand B Soil Plant Sci 68: 358-366. 9 10 https://doi.org/10.1080/09064710.2017.1406134 11 12 Van Bich N, Eyles A, Mendham D, Dong TL, Ratkowsky D, Evans KJ, Hai VD, Thanh HV, Thinh NV, 13 14 Mohammed C (2018) Contribution of harvest residues to nutrient cycling in a tropical Acacia mangium 15 16 Willd. plantation. Forests 9: 577. https://doi.org/10.3390/f9090577 17 18 Venkatesan S, Hemalatha K, Jayaganesh S (2010) Characterization of manganese toxicity and its influence on 19 20 nutrient uptake, antioxidant enzymes and biochemical parameters in tea. Res J Phytochem 4: 248-256. 21 Verheijen F, Jeffery S, Bastos A, Van der Velde M, Diafas I (2010) Biochar application to soils. A critical 22 23 scientific review of effects on soil properties, processes, and functions EUR 24099: 162. 24 25 Viet San L, Lesueur D, Herrmann L, Hudek L, Quyen LN, Brau L (2021) Sustainable tea production through 26 27 agroecological management practices in Vietnam: a review. Environ Sustain: 1-16. 28 29 https://doi.org/10.1007/s42398-021-00182-w 30 31 Vijay K (2014) Effect of different organic mulching materials on soil properties of na'7'aonla (Emblica 32 officinalis Gaertn) under rainfed condition of Shiwalik foothills of Himalayas India. The Bioscan 9: 561- 33 34 564. 35 36 Wang J, Zhang B, Tian Y, Zhang H, Cheng Y, Zhang J (2018) A soil management strategy for ameliorating soil 37 38 acidification and reducing nitrification in tea plantations. Eur J Soil Biol 88: 36-40. 39 40 https://doi.org/10.1016/j.ejsobi.2018.06.001 41 42 Wang L, Butterly C, Wang Y, Herath H, Xi Y, Xiao X (2014) Effect of crop residue biochar on soil acidity 43 amelioration in strongly acidic tea garden soils. Soil Use Manag 30: 119-128. 44 45 https://doi.org/10.1111/sum.12096 46 47 Wang L, Tang J, Xiao B, Yang Y, Yu Y (2013) Enhanced release of fluoride from rhizosphere soil of tea plants 48 49 by organic acids and reduced secretion of organic acids by fluoride supply. Acta Agric Scand B Soil 50 51 Plant Sci 63: 426-432. https://doi.org/10.1080/09064710.2013.795995 52 53 Wang N, Li JY, Xu RK (2009) Use of agricultural by‐ products to study the pH effects in an acid tea garden 54 55 soil. Soil Use Manag 25: 128-132. https://doi.org/10.1111/j.1475-2743.2009.00203.x 56 Wang W, Zhao XQ, Hu ZM, Shao JF, Che J, Chen RF, Dong XY, Shen RF (2015) Aluminium alleviates 57 58 manganese toxicity to rice by decreasing root symplastic Mn uptake and reducing availability to shoots 59 60 of Mn stored in roots. Ann Bot 116: 237-246. https://doi.org/10.1093/aob/mcv090 61 62 63 64 65 1 2 3 Wang Y, Huang Q, Liu C, Ding Y, Liu L, Tian Y, Wu X, Li H, Awasthi MK, Zhao Z (2020) Mulching practices 4 5 alter soil microbial functional diversity and benefit to soil quality in orchards on the Loess Plateau. J 6 7 Environ Manage 271: 110985. https://doi.org/10.1016/j.jenvman.2020.110985 8 Wang Y, Yin R, Liu R (2014) Characterization of biochar from fast pyrolysis and its effect on chemical 9 10 properties of the tea garden soil. J Anal Appl Pyrolysis 110: 375-381. 11 12 https://doi.org/10.1016/j.jaap.2014.10.006 13 14 Wei H, Liu W, Zhang J, Qin Z (2017) Effects of simulated acid rain on soil fauna community composition and 15 16 their ecological niches. Environ Pollut 220: 460-468. doi: https://doi.org/10.1016/j.envpol.2016.09.088 17 18 Wei J, Liang G, Alex J, Zhang T, Ma C (2020) Research progress of energy utilization of agricultural waste in 19 20 China: Bibliometric analysis by citespace. Sustainability 12: 812. https://doi.org/10.3390/su12030812 21 Wen B, Zhang X, Ren S, Duan Y, Zhang Y, Zhu X, Wang Y, Ma Y, Fang W (2019) Characteristics of soil 22 23 nutrients, heavy metals and tea quality in different intercropping patterns. Agrofor Syst 94: 963–974. 24 25 https://doi.org/10.1007/s10457-019-00463-8 26 27 Wu L, Jiang Y, Zhao F, He X, Liu H, Yu K (2020) Increased organic fertilizer application and reduced chemical 28 29 fertilizer application affect the soil properties and bacterial communities of grape rhizosphere soil. Sci 30 31 Rep 10: 9568. https://doi.org/10.1038/s41598-020-66648-9 32 Wu T, Liu W, Wang D, Zou Y, Lin R, Yang Q, Gbokie Jr T, Bughio MA, Li Q, Wang J (2020) Organic 33 34 management improves soil phosphorus availability and microbial properties in a tea plantation after land 35 36 conversion from longan (Dimocarpus longan). Appl Soil Ecol 154: 103642. 37 38 https://doi.org/10.1016/j.apsoil.2020.103642 39 40 Wu Y, Li Y, Fu X, Liu X, Shen J, Wang Y, Wu J (2016) Three-dimensional spatial variability in soil 41 42 microorganisms of nitrification and denitrification at a row-transect scale in a tea field. Soil Biol 43 Biochem 103: 452-463. https://doi.org/10.1016/j.soilbio.2016.09.013 44 45 Xianchen Z, Huiguang J, Xiaochun W, Yeyun L (2020) The effects of different types of mulch on soil properties 46 47 and tea production and quality. J Sci Food Agric 100: 5292-5300. https://doi.org/10.1002/jsfa.10580 48 49 Xiang X, Adams JM, Qiu C, Qin W, Chen J, Jin L, Xu C, Liu J (2021) Nutrient improvement and soil 50 51 acidification inducing contrary effects on bacterial community structure following application of hairy 52 53 vetch (Vicia villosa Roth L.) in Ultisol. Agric Ecosyst Environ 312: 107348. 54 55 https://doi.org/10.1016/j.agee.2021.107348 56 Xiao K, Xu J, Tang C, Zhang J, Brookes PC (2013) Differences in carbon and nitrogen mineralization in soils of 57 58 differing initial pH induced by electrokinesis and receiving crop residue amendments. Soil Biol Biochem 59 60 67: 70-84. https://doi.org/10.1016/j.soilbio.2013.08.012 61 62 63 64 65 1 2 3 Xie S, Feng H, Yang F, Zhao Z, Hu X, Wei C, Liang T, Li H, Geng Y (2019) Does dual reduction in chemical 4 5 fertilizer and pesticides improve nutrient loss and tea yield and quality? A pilot study in a green tea 6 7 garden in Shaoxing, Zhejiang Province, China. Environ Sci Pollut Res 26: 2464-2476. 8 https://doi.org/10.1007/s11356-018-3732-1 9 10 Xie S, Yang F, Feng H, Yu Z, Liu C, Wei C, Liang T (2021) Organic fertilizer reduced carbon and nitrogen in 11 12 runoff and buffered soil acidification in tea plantations: Evidence in nutrient contents and isotope 13 14 fractionations. Sci Total Environ 762: 143059. https://doi.org/10.1016/j.scitotenv.2020.143059 15 16 Xu J, Tang C, Chen ZL (2006) The role of plant residues in pH change of acid soils differing in initial pH. Soil 17 18 Biol Biochem 38: 709-719. https://doi.org/10.1016/j.soilbio.2005.06.022 19 20 Xu R-k, Zhao A-z, Yuan J-h, Jiang J (2012) pH buffering capacity of acid soils from tropical and subtropical 21 regions of China as influenced by incorporation of crop straw biochars. J Soils Sed 12: 494-502. . 22 23 https://doi.org/10.1007/s11368-012-0483-3 24 25 Yan P, Shen C, Fan L, Li X, Zhang L, Zhang L, Han W (2018) Tea planting affects soil acidification and 26 27 nitrogen and phosphorus distribution in soil. Agric Ecosyst Environ 254: 20- 28 29 25https://doi.org/10.1016/j.agee.2017.11.015 30 31 Yan P, Shen C, Zou Z, Fu J, Li X, Zhang L, Zhang L, Han W, Fan L (2021) Biochar stimulates tea growth by 32 improving nutrients in acidic soil. Sci Hortic 283: 110078. https://doi.org/10.1016/j.scienta.2021.110078 33 34 Yan P, Wu L, Wang D, Fu J, Shen C, Li X, Zhang L, Zhang L, Fan L, Wenyan H (2020) Soil acidification in 35 36 Chinese tea plantations. Sci Total Environ 715: 136963. https://doi.org/10.1016/j.scitotenv.2020.136963 37 38 Yang F, Cao X, Gao B, Zhao L, Li F (2015) Short-term effects of rice straw biochar on sorption, emission, and 39 40 transformation of soil NH 4+-N. Environ Sci Pollut Res 22: 9184-9192. https://doi.org/10.1007/s11356- 41 42 014-4067-1 43 Yang W, Li C, Wang S, Zhou B, Mao Y, Rensing C, Xing S (2021) Influence of biochar and biochar-based 44 45 fertilizer on yield, quality of tea and microbial community in an acid tea orchard soil. Appl Soil Ecol 46 47 166: 104005. https://doi.org/10.1016/j.apsoil.2021.104005 48 49 Yao M-Z, Ma C-L, Qiao T-T, Jin J-Q, Chen L (2012) Diversity distribution and population structure of tea 50 51 germplasms in China revealed by EST-SSR markers. Tree Genet Genom 8: 205-220. 52 53 https://doi.org/10.1007/s11295-011-0433-z 54 55 Yuan J-H, Xu R-K, Qian W, Wang R-H (2011) Comparison of the ameliorating effects on an acidic ultisol 56 between four crop straws and their biochars. J Soils Sed 11: 741-750. https://doi.org/10.1007/s11368- 57 58 011-0365-0 59 60 61 62 63 64 65 1 2 3 Zhang C, Liu L, Zhao M, Rong H, Xu Y (2018) The environmental characteristics and applications of biochar. 4 5 Environ Sci Pollut Res 25: 21525-21534. https://doi.org/10.1007/s11356-018-2521-1 6 7 Zhang J, Yang R, Li YC, Peng Y, Wen X, Ni X (2020) Distribution, accumulation, and potential risks of heavy 8 metals in soil and tea leaves from geologically different plantations. Ecotoxicol Environ Saf 195: 9 10 110475. https://doi.org/10.1016/j.ecoenv.2020.110475 11 12 Zhang S, Sun L, Wang Y, Fan K, Xu Q, Li Y, Ma Q, Wang J, Ren W, Ding Z (2020a) Cow manure application 13 14 effectively regulates the soil bacterial community in tea plantation. BMC Microbiol 20: 1-11. 15 16 https://doi.org/10.1186/s12866-020-01871-y 17 18 Zhang S, Wang Y, Sun L, Qiu C, Ding Y, Gu H, Wang L, Wang Z, Ding Z (2020b) Organic mulching positively 19 20 regulates the soil microbial communities and ecosystem functions in tea plantation. BMC Microbiol 20: 21 1-13. https://doi.org/10.1186/s12866-020-01794-8 22 23 Zhang X, Liu W, Zhang G, Jiang L, Han X (2015) Mechanisms of soil acidification reducing bacterial diversity. 24 25 Soil Biol Biochem 81: 275-281. https://doi.org/10.1016/j.soilbio.2014.11.004 26 27 Zhang Z, Zhou C, Xu Y, Huang X, Zhang L, Mu W (2017) Effects of intercropping tea with aromatic plants on 28 29 population dynamics of arthropods in Chinese tea plantations. J Pest Sci 90: 227-237. 30 31 https://doi.org/10.1007/s10340-016-0783-2 32 Zhao J, Wang B, Li Q, Yang H, Xu K (2018) Analysis of soil degradation causes in Phyllostachys edulis forests 33 34 with different mulching years. Forests 9: 149. https://doi.org/10.3390/f9030149 35 36 Zhao Y, Sun F, Yu J, Cai Y, Luo X, Cui Z, Hu Y, Wang X (2018) Co-digestion of oat straw and cow manure 37 38 during anaerobic digestion: Stimulative and inhibitory effects on fermentation. Bioresour Technol 269: 39 40 143-152. https://doi.org/10.1016/j.biortech.2018.08.040 41 42 Zheng N, Yu Y, Shi W, Yao H (2019) Biochar suppresses N2O emissions and alters microbial communities in an 43 acidic tea soil. Environ Sci Pollut Res 26: 35978-35987. https://doi.org/10.1007/s11356-019-06704-8 44 45 Zhongqi H, Pagliari PH, Waldrip HM (2016) Applied and environmental chemistry of animal manure: A review. 46 47 Pedosphere 26: 779-816. https://doi.org/10.1016/S1002-0160(15)60087-X 48 49 Zioła-Frankowska A, Frankowski M (2018) Speciation analysis of aluminium in plant parts of Betula pendula 50 51 and in soil. J Environ Sci 65: 153-161. https://doi.org/10.1016/j.jes.2017.03.021 52 53 Zou Y, Hirono Y, Yanai Y, Hattori S, Toyoda S, Yoshida N (2014) Isotopomer analysis of nitrous oxide 54 55 accumulated in soil cultivated with tea (Camellia sinensis) in Shizuoka, central Japan. Soil Biol Biochem 56 77: 276-291. https://doi.org/10.1016/j.soilbio.2014.06.016 57 58 59 60 61 62 63 64 65 Environmental Chemistry Letters ECLE- D- 21-00537 Responses to the Editor’s comments We are sincerely indebted to the Editor for giving comments on our manuscript. We have revised our manuscript following all the comments and we hope that this revised version will fit with the editor’s expectations. Editor’s comments 1. The introduction is too long and needs to be shortened down to 1-2 paragraphs Answer: Thanks for the Editor’s comment. The introduction has now been rewritten and the length has been reduced by around a half. 2. Possibility to discuss the ocean and other acidifications for attracting a wider range of readers. Answer: We have now added a new sub-section 2.1 to briefly discuss the status and mechanism of ocean and soil acidification and how they relate to agricultural activities. 3. The figures need to be accurately presented (texts, legend, white space, Y and X axis…) Answer: We have now carefully revised all the figures attached in the manuscript. We hope that they will fit with the editor’s expectations. 4. Write at the end of each article section a conclusion of about 1-2 sentences to summarize the major points of the section and its significance. Answer: Thanks for the Editor’s advice. We now reviewed the whole manuscript and added summary/ conclusion to many sections/subsections where we believe that they are accurate. 5. Finding relevant articles published by Environmental Chemistry Letters (5-10 articles, using keywords and search from the journal home page) and consider citing them as the references. I believe we have published articles concerning soil and its degradation, tea as well as acidification. Answer: Thanks for the Editor’s advice. We have been carefully checked and cited 8 articles published by ECL as references intext and highlighted these references in the bibliography. They are as follow: 1. Akhil D, Lakshmi D, Kartik A, Vo D-VN, Arun J, Gopinath KP (2021) Production, characterization, activation and environmental applications of engineered biochar: a review. Environ Chem Lett 19: 2261-2297. http://doi.org/10.1007/s10311-020-01167-7 2. Gunarathne V, Ashiq A, Ramanayaka S, Wijekoon P, Vithanage M (2019) Biochar from municipal solid waste for resource recovery and pollution remediation. Environ Chem Lett 17: 1225-1235. https://doi.org/10.1007/s10311-019-00866-0 3. Ochedi FO, Yu J, Yu H, Liu Y, Hussain A (2021) Carbon dioxide capture using liquid absorption methods: a review. Environ Chem Lett 19: 77-109. https://doi.org/10.1007/s10311-020- 01093-8 4. Patra BR, Mukherjee A, Nanda S, Dalai AK (2021) Biochar production, activation and adsorptive applications: a review. Environ Chem Lett 19: 2237-2259. https://doi.org/10.1007/s10311- 020-01165-9 5. Rana A, Rana S, Kumar S (2021) Phytotherapy with active tea constituents: a review. Environ Chem Lett 19: 2031- 2041. https://doi.org/10.1007/s10311-020-01154-y 6. Saliu T, Oladoja N (2021) Nutrient recovery from wastewater and reuse in agriculture: a review. Environ Chem Lett 19: 2299–2316. https://doi.org/10.1007/s10311-020-01159-7 7. Sánchez A, Artola A, Font X, Gea T, Barrena R, Gabriel D, Sánchez-Monedero MÁ, Roig A, Cayuela ML, Mondini C (2015) Greenhouse gas emissions from organic waste composting. Environ Chem Lett 13: 223-238. https://doi.org/10.1007/s10311-015-0507-5 8. Sharma H, Dhir A (2021) Capture of carbon dioxide using solid carbonaceous and non- carbonaceous adsorbents: A review. Environ Chem Lett 19: 851-873. https://doi.org/10.1007/s10311-020-01118-2