CIAT Research brief, December 2019 1 Over the past decades, numerous crop-soil models have been developed to represent dynamic processes in cropland systems, including soil organic carbon (SOC) dynamics (Campbell and Paustian, 2015). These models use mathematical equations that determine carbon allocation in the vegetation and biomass and soils to represent biogeochemical processes, such as photosynthesis, respiration and decomposition. Furthermore, a range of crop management practices are represented in most of the models, enabling an assessment of their impacts on SOC in agricultural systems. Although models were initially developed for research purposes, they are increasingly becoming important in many aspects of environmental policies (Manlay et al., 2007). Extensively tested models provide effective tools that can be used in identifying sustainable land management practices across different agro- ecological conditions. Compared to field experiments, which are time and resource consuming, models are more effective for making predictions and understanding crop and SOC dynamics on large scales and different time scales. However, the choice of the model depends on the ability of the model to simulate key processes in the region of interest. We conducted a survey to identify the features of the commonly used crop-soil models in order to inform the choices for application in sub-Saharan Africa. The survey was administered online to the model developers. In addition, we also conducted a literature search to assess the usage of the different models in different parts of sub-Saharan. In this brief, we provide a basic summary of the information from the survey and literature review. Introduction © G eo rg in a S m ith /C IA T Crop and soil organic matter simulation models – A brief review of their basic features and application in sub-Saharan Africa Sylvia Sarah Nyawira CIAT Research brief, December 20192 Summary of the basic features The models considered for the survey included point and landscape level that have been applied in different in regions in the globe for simulating crop and soil organic carbon dynamics. The 9 models that participated in the survey are: DayCent/Century, RothC, DNDC, DAISY, MONICA, CropSyst, APSIM, DSSAT, and STICS. Below a brief description of each of the models. i. DayCent is a biogeochemical model that simulates crop growth, SOC dynamics, carbon and trace gas fluxes in croplands as well as forests, grasslands, and savannah ecosystems (Del Grosso et al., 2002; Parton et al., 1998). It is the daily time step version of the Century model. The model allows for an assessment of a wide range of agronomic management practices (e.g., tillage, fertilization, irrigation, crop harvest and manure addition). ii. MONICA is the latest generation of HERMES (Kersebaum, 1995, 2007) model versions that simulates crop growth, water and nitrogen uptake, and the SOC dynamics in the soil (Nendel et al., 2011). iii. DAISY is a mechanistic model that represents physical and biological processes in agricultural fields (Hansen, 2002). It simulates water, energy, carbon and nitrogen cycles in the vegetation and soils agricultural systems. iv. RothC is a model for turnover of carbon in non- water-logged topsoil (Coleman, 2014). It allows for the assessment of the effects of soil type, temperature, soil moisture and plant cover on the turnover processes. The model does not represent crop growth and is hence not used in modelling crop dynamics. v. STICS simulates plant growth, water, carbon and nitrogen fluxes in annual crops, perennial grasses or trees (Brisson et al., 2003, 1998). vi. DSSAT (Decision Support System for Agrotechnology Transfer) is a software application program that comprises crop simulation models for over 42 crops, which simulate growth, development and yield as a function of the soil-plant-atmosphere dynamics. DSSAT simulates water, nitrogen and carbon cycles for these crops, and can be used to assess the effects of climate change impacts and different management decisions (Jones et al., 2003). vii. CropSyst is a cropping systems simulation model developed as an analytical tool to study the effects of climate, soils, and management on cropping systems productivity and the environment (Stöckle et al., 2003). viii. APSIM is a comprehensive model developed to simulate biophysical to simulate biophysical processes in agricultural systems. APSIM includes modules that simulate soil processes including water balance, N and P transformations, soil pH, erosion and a full range of management controls in diverse range of crops (Keating et al., 2003). ix. DNDC simulates C and N in agro-ecosystems and predicts crop growth, SOC dynamics and N leaching and trace gases emissions (Li et al., 2012). Table 1 provides a summary of the features of the model including, the time step, number of layers and carbon pools, the extent of application and the simulated nutrients and greenhouse gases. The responses show that most of the models can simulate both crop and soil organic carbon (SOC) dynamics, with the exception of RothC which simulates only SOC dynamics. Apart from RothC, all the reviewed models include a layered soil profile to represent water dynamics in the soil. Most of the models use a tipping bucket approach for simulating soil hydrologic cycle and water redistribution, with the exception of CropSyst and Daisy that use the Richard’s equation. Although the tipping bucket model has been shown to work well in representing soil water holding properties, it has less accuracy in estimating soil moisture distribution in the soil profile (Shelia et al., 2018). Majority of the models can simulate CO2 and N20 fluxes with a few also simulating CH4 fluxes. Apart from RothC, all the SOM dynamics are able to simulate the most common agronomic management practices (i.e. tillage, fertilization, manuring, and crop rotation) (Table 2). More details on the representation of soil, plant ecophysiology, management and greenhouse gases and their weaknesses can be found in Brilli et al., 2017. CIAT Research brief, December 2019 3 Table 1: Overview of the basic features of the surveyed models DayCent RothC DNDC DSSAT CropSyst MONICA STICS Daisy APSIM Scope Crop and SOC modeling SOC modeling Crop and SOC modeling Crop and SOC modeling Crop and SOC modeling Crop and SOC modeling Crop and SOC modeling Crop and SOC modeling Crop and SOC modeling Timestep Daily Monthly Daily Daily Daily Daily Daily Hourly Daily Extent of application Point-scale, regional and global Point-scale, regional and global Point-scale and regional Point-scale, regional and global Point- scale and regional Point-scale and regional Point-scale Point-scale Point-scale, regional and global No. of SOC pools 3 5 4 3 3 3 2 3 3 Name of pools Active, Slow, passive Decomposable Plant Material (DPM), Resistant Plant Material (RPM), Microbial Biomass (BIO) and Humified Organic Matter (HUM) Litter, microbes, Humads, Passive Active, Slow, Passive Active, Passive, Slow Added Organic Matter (AOM), Soil Microbial Bio-mass (SMB), Native Soil Organic Matter (SOM) Active and Inactive fractions Added organic matter (AOM), Soil Microbial Biomass (SMB), and Native Soil Organic Matters (SOM) Microbial biomass (BIOM), Humus (HUM), Inert organic matter (IOM) Layered profile Yes No Yes Yes Yes Yes Yes Yes Yes Maximum no. of soil layers 14 1 Variable across soils 20 17 20 5 User specified 6 Maximum depth of SOC simulation 20 cm 30 cm User specified All layers All layers 40 cm User specified User specified User specified Equation governing decomposi- tion 1st order kinetics 1st order kinetics 1st order kinetics 1st order kinetics 1st order kinetics 1st order kinetics 1st order kinetics 1st order kinetics 1st order kinetics Soil water redistribu- tion equation Tipping bucket Tipping bucket Tipping bucket Tipping bucket Richard’s equation Tipping bucket Tipping bucket Richard’s equation Tipping bucket and Richard’s equation Water erosion module No No Yes Yes Yes No Yes No Yes Simulated gas fluxes CO2 , N20, CH4 CO2 CO2 , N20 CO2 , N20 CO2 , N20 CO2 , N20 CO2 , N20 CO2 , N20 CO2 , N20 Simulates soil nutrients Nitrogen, Phosphorous, Sulphur Carbon Nitrogen, Phosphorous, Nitrogen, Phosphorous, Nitrogen Nitrogen, Sulphur Nitrogen Nitrogen Nitrogen and Phosphorous Mulch layer affects water dynamics and heat balance in soil Yes No Yes Yes Yes Yes Yes Yes Yes CIAT Research brief, December 20194 Table 2: Overview of the crop management practices included the in surveyed models Management DayCent RothC DNDC DSSAT CropSyst MONICA STICS DAISY APSIM Crop harvest ✓ ✓ ✓ ✓ ✓ ✓ ✓ Tillage ✓ ✗ ✓ ✓ ✓ ✓ ✓ ✓ Fertilization ✓ ✗ ✓ ✓ ✓ ✓ ✓ ✓ Irrigation ✓ ✗ ✓ ✓ ✓ ✓ ✓ ✓ Manuring ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ Crop rotation ✓ ✗ ✓ ✓ ✓ ✓ ✓ ✓ Intercropping ✗ ✗ ✓ ✓ ✗ ✗ ✓ ✓ Agroforestry ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ Cover cropping ✓ ✗ ✓ ✓ ✓ ✓ ✓ ✓ Pesticides application ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✓ Application of the models in sub-Saharan Africa Table 3: A summary of the reviewed modeling studies in sub-Saharan Africa including the model, country/region of study, purpose of study and the reference. MODEL COUNTRY/ REGION CROP PURPOSE OF STUDY REFERENCE Century Sudan Millet and sorghum Estimating SOC in different land use including cropland Ardö and Olsson, 2003 Century South Africa Sugarcane Validation, calibration and predicting SOC in sugarcane systems under different management Galvados et al 2009 Century and RothC Nigeria and Sudan Millet and groundnuts Assessing the effects of improved agricultural management practices on SOC Farage et al., 2007 Century and RothC Kenya Maize and maize-bean rotation Model evaluation using long-term experiments Kamoni et al., 2007 RothC Niger Millet Validation of SOC dynamics using long- term experiments Nakamura et al., 2011 DSSAT Ghana Maize and groundnuts Economic analysis on management practices that enhance SOC sequestration González-Estrada et al., 2008 Published modelling studies in sub-Saharan Africa were reviewed to assess the type of studies crops, purpose of the study and the applied models. The review indicates that APSIM and DSSAT are the most widely used crop- soil mods in SSA with most of the studies being in West and Southern Africa region and only a few studies in East Africa (Table 3). Majority of the APSIM and DSSAT studies focussed mainly of crop production (i.e. yield) and a lot of emphasis has also laid on assessing and quantifying the impacts of climate change and management on yields. There are a few studies on SOC, but the key focus has been on model evaluation. While agronomic management impacts on SOC are widely studied in other regions in the globe, there are only a few studies within SSA. Out of the 40 reviewed studies, majority were on point-scale modeling with only 2 being on a landscape level. CIAT Research brief, December 2019 5 MODEL COUNTRY/ REGION CROP PURPOSE OF STUDY REFERENCE DSSAT Kenya and Uganda Maize and maize-bean rotation Validation of DSSAT using two long- term experiments and projection of SOC changes Musinguzi et al., 2014 DSSAT Burkina Faso Cotton, sorghum, peanut and maize Model evaluation with SOC and yields using long-term experiment data Soler et al., 2011 DSSAT Cote d'Ivoire Maize Assessing the impacts of conservation agriculture on yields Worou et al., 2019 DSSAT Niger Maize Assessing the contribution of weather, crop and soil to uncertainties in simulated yields Jones et al 2012 DSSAT Ghana Maize Calibrating and validating simulated grain and biomass yields in the model McCarthy et al 2012 APSIM Malawi Maize and maize double legume intercrops Model calibration, validation and assessing the sustainability of double- legume rotations Smith et al., 2016 APSIM Ghana Sorghum Modelling the impacts of nutrient and residue management on yield and SOC MacCarthy et al., 2009 APSIM Eastern and Southern Africa - Ethiopia, Kenya, Tanzania, Malawi, Mozambique and Zimbambe Maize Charactering maize production in different climate conditions Seyoum et al., 2017 APSIM Kenya Maize/ Agroforestry systems Simulating the impacts of shading on maize Dilla et al., 2018 APSIM Nigeria Maize Evaluation of APSIM using yield data from different maize cultivars Yamusa and Akinseye, 2018 APSIM Ghana Maize Assessing the impacts of climate change and climate variability on maize yields Fosu-Mensah et al., 2019 APSIM Malawi Maize and maize-pigeon pea Model evaluation and assessing the impacts of climate change on yields Ollengburger Mary 2012iversity APSIM Ethiopia Sorghum Assessing impacts of climate change and climate variability on production Gebrekiros and Araya, 2015 APSIM Ghana Maize Assessing the effects of seasonal climate variability on efficiency of mineral fertilizer MacCarthy et al., 2015 APSIM Tanzania Maize Assessing the impacts of improved management practices on maize yields under current and future climate Tumbo et al., 2012 APSIM Eastern and Southern Africa - Ethiopia, Kenya, Tanzania, Malawi, Mozambique and Zimbambe Sorghum Assessing the impacts of increased temperatures on sorghum yields Turner and Rao, 2013 CIAT Research brief, December 20196 MODEL COUNTRY/ REGION CROP PURPOSE OF STUDY REFERENCE APSIM West Africa Sorghum Assessing the impacts of climate change on yields Sultan et al., 2014 APSIM West Africa Sorghum Assessing the options for climate change adaptation Guan et al., 2017 APSIM Southern Africa Sorghum Quantifying the response of maize yield to projected climate change and key management practices (i.e. planting date, cultivar, fertilizer use) Rurinda et al., 2015 APSIM Zimbabwe Maize Model calibration and simulating yield response to reduced tillage and mulching Mupangwa et al., 2011 APSIM Niger Millet Assessing impacts of nitrogen management on yields Akponikpè et al., 2010 APSIM South Africa Maize Assessing impacts of no-till on water fluxes and maize productivity Mupangwa and Jewitt, 2011 APSIM Kenya Maize Evaluation of the model with data on nitrogen and residue man-agement Kisaka et al., 2016 APSIM South Africa Sorghum- cowpea intercrop Evaluation of growth, yield and crop water use Chimonyo et al., 2016 APSIM Zimbabwe Maize Model calibration for maize yield and N mineralization and simulat-ing the effects of tillage, fertilization management and planting dates on yields and N mineralization Masvaya et al., 2018 APSIM Zimbabwe Maize-Mucuna rotation Comparing conventional farmer practices with improved practices comprising of manure application and rotations with cover crop (i.e. Mucuna) Masikati et al., 2014 APSIM Ghana Maize Simulating the long term influence of nitrogen and phosphorous on maize yield Fosu-Mensah et al., 2012 APSIM Malawi Maize Assessing the effective use of nitrogen and phosphorous with rainfall variations Kamanga et al., 2014 DSSAT and APSIM Southern Ethiopia Maize Assessing maize growth and yield under present and future climate Araya et al., 2015 DSSAT and APSIM West Africa Sorghum Assessing the performance of the models in simulating sorghum germplasm in different climate and soil conditions Akinseye et al 2014 CropSyst Burkina Faso Millet Simulating yields across different climatic conditions Badini et al., 1997 CropSyst Cameroon Maize, sorghum, groundnut, and soybean Yield validation Tingem et al., 2009 CropSyst Kenya Maize and Maize-Tephrosia rotation Simulating nitrogen dynamics and nitrous oxide emissions in a long- term trial under integrated soil fertility management Sommer et al., 2016 STICS Mali Sorghum Simulating crop developments Folliard et al., 2004 CIAT Research brief, December 2019 7 Conclusion The objective of this research brief was to summarize the main features of some of the most widely used crop models and assess their application in sub-Saharan Africa (SSA). The conducted survey indicates that most models can simulate crop growth and SOC dynamics with the exception of RothC, which simulates on SOC dynamics. Except for agroforestry and intercropping, the other common agronomic management practices are well represented in most of the models. Out of the 9 surveyed models, APSIM and DSSAT are the most widely used in the region. For SOC, the emphasis has been on calibrating and validating the models with only a few study with model application in understanding the drivers of SOC dynamics and the impacts of agronomic management practices. Although this review may not be exhaustive, it shows that despite the use of models gaining momentum in SSA the focus has been on point-scale modelling. Furthermore, modelling studies in the East Africa region still remain scarce compared to West and Southern Africa. 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