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Journal of Environmental Management xxx (2014) 1

Contents lists available at ScienceDirect

Journal of Environmental Management
journal homepage: www.elsevier.com/locate/jenvman

Highlights
 The global soil organic carbon (SOC) sequestration potential of agricultural land projected for the coming 87 years was predicted to
Q1
range between 31 and 64 Gt.
 This is equal to 1.9e3.9% of the average SRES-A2 projected 87-year anthropogenic emissions.
 SOC sequestration would peak 2032e33, and about 30 years later reduce by half.
 SOC sequestration is not a C wedge that could contribute increasingly to mitigating climate change.

http://dx.doi.org/10.1016/j.jenvman.2014.05.017
0301-4797/© 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Sommer, R., Bossio, D., Dynamics and climate change mitigation potential of soil organic carbon sequestration,
Journal of Environmental Management (2014), http://dx.doi.org/10.1016/j.jenvman.2014.05.017

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Journal of Environmental Management xxx (2014) 1e5

Contents lists available at ScienceDirect

Journal of Environmental Management
journal homepage: www.elsevier.com/locate/jenvman

Short communication

Dynamics and climate change mitigation potential of soil organic
carbon sequestration
Q4

Rolf Sommer*, Deborah Bossio
International Center for Tropical Agriculture (CIAT), ICIPE Duduville Campus, Kasarani, P.O. Box 823-00621, Nairobi, Kenya

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 9 December 2013
Received in revised form
12 May 2014
Accepted 20 May 2014
Available online xxx

When assessing soil organic carbon (SOC) sequestration and its climate change (CC) mitigation potential
at global scale, the dynamic nature of soil carbon storage and interventions to foster it should be taken
into account. Firstly, adoption of SOC-sequestration measures will take time, and reasonably such
schemes could only be implemented gradually at large-scale. Secondly, if soils are managed as carbon
sinks, then SOC will increase only over a limited time, up to the point when a new SOC equilibrium is
reached. This paper combines these two processes and predicts potential SOC sequestration dynamics in
agricultural land at global scale and the corresponding CC mitigation potential. Assuming that global
governments would agree on a worldwide effort to gradually change land use practices towards turning
agricultural soils into carbon sinks starting 2014, the projected 87-year (2014e2100) global SOC
sequestration potential of agricultural land ranged between 31 and 64 Gt. This is equal to 1.9e3.9% of the
SRES-A2 projected 87-year anthropogenic emissions. SOC sequestration would peak 2032e33, at that
time reaching 4.3e8.9% of the projected annual SRES-A2 emission. About 30 years later the sequestration
rate would have reduced by half. Thus, SOC sequestration is not a C wedge that could contribute
increasingly to mitigating CC. Rather, the mitigation potential is limited, contributing very little to solving
the climate problem of the coming decades. However, we deliberately did not elaborate on the importance of maintaining or increasing SOC for sustaining soil health, agro-ecosystem functioning and productivity; an issue of global significance that deserves proper consideration irrespectively of any
potential additional sequestration of SOC.
© 2014 Elsevier Ltd. All rights reserved.

Keywords:
Carbon sequestration
Anthropogenic CO2 emissions
C wedge
SRES-A2

1. Introduction
Anthropogenic carbon dioxide (CO2) emissions into the atmosphere have increased significantly over the last 20 years, from
6.3 Gt carbon (C) in 1994 to 8.7 Gt C in 2009 (Boden et al., 2013), i.e.
by 39% over this period. Each year an additional approximate 2 Gt C
is set free as CO2 in response to land use change, including deforestation and burning of forests or degradation of soils and loss of
soil organic carbon (SOC). CO2 is the foremost greenhouse gas
(GHG), its enrichment in the atmosphere triggering an increase in
atmospheric temperature and thus global climate change (CC).
Ways and mechanisms are needed more than ever to mitigate CC by
either reducing GHG emissions or e.g. by capture or sequestration
of C in aboveground biomass or soils.
Soil C sequestration e often with a focus on agricultural soils e
has been repeatedly proposed as a promising way out of the

* Corresponding author. Tel.: þ254 20 863 2811.
E-mail address: r.sommer@cgiar.org (R. Sommer).

dilemma (Lal, 2002, 2011; Smith et al., 2008). The argument quite
often relies on the magnitude of C stored in soils as well as the vast
land area coverage of soils. Agricultural soils occupy 37% of the
earth's surface. The C found in the upper 1 m of soils is estimated to
about 2000e2500 Gt, whereas about 60% of this is organic (SOC)
and about 40% inorganic (Janzen, 2004; Sommer and De Pauw,
2011). Thus, the amount of C in soils is for instance approximately
three times higher than the amount of C bound in the aboveground
biomass, and at least 230 times higher than the 2009-global
anthropogenic CO2 emissions. So, the argumentation is that small
positive changes in the global SOC pool could have a major impact,
or in other words, soils could be major sinks of the GHG carbon
dioxide. This argument is captured within the carbon wedge
framework of Pacala and Socolow (2004) in which ‘adoption of
conservation tillage in all agricultural soils worldwide’, is included
as a component of the carbon wedge ‘natural sinks’.
However, the capacity of soils to store C is limited and organic C
contents of soils in pristine ecosystems are largely at equilibrium,
i.e. the release of CO2 by Soil Organic Matter (SOM) decomposition

http://dx.doi.org/10.1016/j.jenvman.2014.05.017
0301-4797/© 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Sommer, R., Bossio, D., Dynamics and climate change mitigation potential of soil organic carbon sequestration,
Journal of Environmental Management (2014), http://dx.doi.org/10.1016/j.jenvman.2014.05.017

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Table 1
SOC sequestration parameters (four-parameter sigmoid function; Eq. (1)) for scenario 1 and 2; for the cumulative C-sequestration calculations, a soil profile depth of 25 cm and
a soil bulk density equal to 1.3 g/cm3 was assumed.
Land use

Scenario

a (e)

b (e)

t0 (yr)

Cumulative C-sequestration after 78 years (Mg/ha)

Notation in Fig. 1

Arable land and permanent crops

2
1
2
1

1.202
0.697
0.697
0.350

9.8
11.5
11.5
11.5

7
4
4
3

26.2
13.3
13.3
6.4

High
Medium
Medium
Low

Permanent meadows & pastures

and new formation of SOM by organic matter input from plant
debris balance each other. This means, increasing SOC e bound in
SOM e beyond the natural capacity of the agro-ecosystem may be
possible, but requires a continuous input of increased levels of
organic matter. Even if such are available, such levels of SOC may be
difficult or costly to maintain given the labile nature of newly added
SOM. It is rather soils which SOM has been depleted over the last
centuries by continuous land use that have a potential for SOC
sequestration.
Furthermore, an addition of organic matter to the soil at quantities available/achievable in-situ in most cases will result in only a
slow increase of SOC over time; the rate slowing down and ceasing
once the SOC level reaches the aforementioned (new) equilibrium.
It is reasonable to assume that SOC-sequestration has its limits and
that not all soils may be turned into notable SOC-sinks.
Finally, it will take time to adopt measures to increase the SOC
content of soils, i.e. realistically not all soils can be turned into SOC
sinks tomorrow. Suitable measures to increase the SOC content of
soils are described elsewhere (such as in Smith et al., 2008). In this
study we assume that soils could be turned into a C sink when
properly managed, even though there is evidence that a measure
that successfully increases the SOC content in one soil was less
successful in another (see for instance Murphy et al., 2011).
In this paper we focus on illustrating the dynamic nature of SOC
sequestration to describe its climate change mitigation potential on
agricultural lands at global scale. To do so we developed an optimistic and a pessimistic scenario based on estimates of adoption of
SOC-sequestration measures and subsequently the increase in SOC
over time. This paper provides a simplified attempt to address the
issue at global scale.
2. Methods
Three data sets were used for this study:
1) The latest (2009) data on anthropogenic CO2 emissions from
fossil-fuel burning, cement production, and gas flaring provided
by the Carbon Dioxide Information Analysis Center (http://cdiac.
ornl.gov/; Boden et al., 2013).
2) Prediction about future anthropogenic CO2 emissions published
by IPCC in their Special Report on Emissions Scenarios (SRES),
namely the qualitative storyline family A2 (bold line in Fig. 3 of
IPCC, 2000).
3) Data from FAO about agricultural area distinguishing arable land
and permanent crops and permanent meadows and pastures
(FAOSTAT, 2013). These are the two major types of agricultural
land distinguished by FAO suitable for SOC sequestration by
means of improved agronomic land use practices. Forest area is
another major land type classified by FAO, but is not considered
further in this study. Permanent meadows and pastures is the
land used permanently (five years or more) to grow herbaceous
forage crops, either cultivated or growing wild (wild prairie or
grazing land). The arable area also includes temporary meadows
and pastures and fallow land. It excludes, however, abandoned
land resulting from shifting cultivation.

The data set on anthropogenic CO2 emissions served as the
baseline for putting SOC sequestration potentials into the global
picture, i.e. to evaluate its relative magnitude. The agricultural area
was used for scenario analysis, where it was assumed that global
governments would agree on a worldwide effort to change land use
practices towards turning agricultural soils into carbon sinks
starting 2014. With this assumption, at least two major subsequent
questions arise:
1) What is the annual per hectare sequestration potential of arable
land and pastures?
2) How quickly can a worldwide effort put in place, and what is the
percentage of total agricultural area that can be reached after,
for instance, 20 years of a worldwide effort to sequester SOC?
Both questions where tackled by the following assumptions.
The increase in %-SOC in response to whatever sequestration
measures was described with a four-parameter sigmoid function of
the form:

SOC ¼ SOC0 þ

a
1 þ eÀ

tÀt0
b

(1)

where SOC0 is the initial soil organic carbon content (%), a and b are
empirical constants and t the time expressed in years. t0 is the year
where the slope of the curve is largest, i.e. the annual sequestration
rate highest. To convert %-SOC into Mg C sequestered per hectare, it
was assumed that an increase in SOC would occur homogenously in
the upper 25 cm of the soil having a bulk density of 1.3 g/cm3.
The annual increase in area of arable land and permanent crops,
or permanent meadows and pastures that would be included into a
SOC sequestration plan was expressed by a simple exponential
decay function of the form:

Ay ¼ kAðtÀ1Þ

(2)

where A is the added area per year (t) and k is the (area) rate
constant, allowing the added area to diminish from year to year as
adoption of a sequestration schemes progress and less area could
be easily added.
A pessimistic SOC sequestration scenario (1) and an optimistic
scenario (2) was formulated (Table 1; Fig. 1). It was assumed that
arable land and permanent crops could achieve a higher sequestration rate than permanent meadows and pastures. This owing the
fact that the latter land use area includes a considerable fraction of
rangelands (wild prairie or grazing land) with reported potential
sequestration rates considerably lower than under land with
annual crops. Likewise, it was assumed that pasture areas are more
difficult to reach out to for implementing SOC sequestration
schemes, and thus the rate of adoption e expressed in the
increasing land under such scheme e was also assumed to be lower
than for arable land; starting off at 3% in the first year and reaching
40% of the total area after 20 years, as opposed to 60% for arable
land (starting with 5%; Fig. 2).

Please cite this article in press as: Sommer, R., Bossio, D., Dynamics and climate change mitigation potential of soil organic carbon sequestration,
Journal of Environmental Management (2014), http://dx.doi.org/10.1016/j.jenvman.2014.05.017

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C-sequestration (Mg/ha/yr)

R. Sommer, D. Bossio / Journal of Environmental Management xxx (2014) 1e5

0.8

1.8

0.4

1.4

0

3

SOC (%)

1
0

20

40

60

Year
Seq. rate, low

Seq. rate, medium

Seq. rate, high

SOC low

SOC, medium

SOC, high

Fig. 1. Assumed increase in %-SOC (right y-axis) and corresponding annual SOC
sequestration (Mg/ha; left y-axis); y0 (Eq. (1)) in the shown cases is 0.60%, 0.71% and
0.85% for the high medium and low graphs, respectively; note that the values for y0 are
exemplarily only and do not affect SOC sequestration rates.

100%

Arable land and
permanent crops

75%

% of total area

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60%
50%

Permanent meadows
and pastures

40%

Fig. 3. Temporal dynamics (yearly time-step) of SOC sequestration (left y-axis) and its
potential mitigation potential expressed in percent of projected SRES-A2 emissions
(right y-axis) under pessimistic (top) and optimistic (bottom) assumptions.

25%

0%
2014

2034

2054

2074

2094

Year
Fig. 2. Assumed increase in area included in a SOC sequestration campaign expressed
in percent of the total area.

The corresponding area rate constants (Eq. (2)) were 0.959 and
0.949 for pasture and arable land, respectively. The latter (area)
settings were assumed to apply for both scenarios, to avoid confounding effects, for an easier interpretation of results. Projections
were made for the coming 87 years, i.e. 2014e2100.
3. Results and discussion
The average annual C sequestration rates over the first 50 years
of the low, medium and high figures (Fig. 1) would be 0.125, 0.258
and 0.515 Mg/ha, respectively. However, it is obvious that, especially for the medium and high rates, providing averages fails to
describe the notable sequestration dynamics. Using averages also
increases the risk of laymen misinterpreting figures assuming they
were steady, not diminishing and ceasing after some decades.
Both scenarios resulted in annual SOC sequestration rates e
arable land and pasture combined e that start at a modest level
(scenario 1: 57 Mt/yr; scenario 2: 109 Mt/yr) in 2014 and climb to a
maximum value (663 and 137 Mt/yr) in 2035 (Fig. 3). During the
subsequent years, SOC sequestration decreased again, as less and

less new land would be included into the sequestration plan, and
those soils already part of it would sequester less and less C. At peak
sequestration, arable land contributed approximately 60% and
pasture land about 40% to the total annual sum.
In comparison to the projected future SRES-A2 emissions, SOC
sequestration optimally (scenario 2) could mop up/mitigate up to
8.9% of the projected emissions (white dotted line in Fig. 3) in the
year 2033. For the pessimistic scenario (1) this would be only
maximal 4.3% in 2032. The 87-year SRES-A2 related mitigation
potential was 1.9 and 3.9% for scenario 1 and 2, respectively.
To be able to estimate their national importance, global
sequestration figures were disentangled by country based on the
percentage share of agricultural land (see table in supplementary
material). Instead of relating mitigation potentials to projected
future emissions, here we relate them against 2009 actual C
emissions from fossil-fuel burning, cement production, and gas
flaring (Boden et al., 2013).
The four largest anthropogenic C emitters, China (2172 Mt C in
2009, equal to 26.3% of the world total), USA (1445 Mt, 17.5%), India
(540 Mt, 6.5%) and Russia (429 Mt, 5.2%), would also have the
highest SOC sequestration potentials ranging between 1.8 and
3.1 Gt C (scenario 1) and 3.7 and 6.4 Gt C (scenario 2) over the
considered 87-year period. Only Australia (109 Mt, 1.3%) and Brazil
(100 Mt, 1.2%) e world rank 16 and 17 in terms of 2009 anthropogenic emissions e could sequester SOC of similar magnitudes.
Averaging across these years, the mitigation potential of the top-4
emitters would not surpass 10% of countries individual 2009 national emissions even under optimistic assumptions (scenario 2).
The national mitigation potential of Australia and Brazil, on the
other hand, would be notable even assuming pessimistic SOC

Please cite this article in press as: Sommer, R., Bossio, D., Dynamics and climate change mitigation potential of soil organic carbon sequestration,
Journal of Environmental Management (2014), http://dx.doi.org/10.1016/j.jenvman.2014.05.017

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sequestration rates (scenario 1), under which the two countries
could still mop up 20e22% of their emissions. Optimistically, these
percentages would go up to 41e45%. Most of the large-area, lowemitting countries (lower part of table in supplementary material)
would be able to become CO2 neutral or even net C sinks. Nevertheless, the global CC mitigation potential of SOC sequestration
would still be modest, with on average 4.4 and 8.9% under scenario
1 and 2, respectively. This is approximately two-times the aforementioned mitigation potential of future projected (SRES-A2)
emissions. It is subject of further debate which figures are more
plausible.
Our 2009 emission related average figures, though based on
rather simple assumptions, compare well with an earlier study
carried out by Paustian et al. (2004; Table 2). Our disaggregated
estimates also compare well with a more detailed study from
Australia by Lam et al. (2013) who calculate that the SOC mitigation
potential on agricultural lands is modest and will markedly
diminish over time to near 0 in most cases after 30 years.
The low, medium and high C sequestration rates were assumed
to peak 3e7 years after the initialization of measures to increase
SOC. This is in line with a range of observations, for instance when
conventional tillage practices were given up in favor of reduced or
zero-tillage (West and Post, 2002; Sommer and Ryan, 2009). The
four-parameter sigmoid function allowed us to accommodate this
fact. For the overall SOC sequestration, however, identifying the
appropriate peak year was not that critical, and changing this by ±3
years would not have any notable influence.
We assumed that SOC sequestration would start leveling off
significantly after about 20e40 years (compare Fig. 1), which corresponds to observations published elsewhere (Lal et al., 1998;
Campbell et al., 1996; West and Post, 2002).
The global peak annual sequestration potential of scenario 2 of
our study (year 2036: 1.37 Gt) is close to the maximum (SRES-A2
linked scenario) biophysical potential of 1.62 Gt/yr (¼ 5950 Mt CO2
eq./yr) potentially achievable by 2030 according to Smith et al.
(2008). The figures compare better, when taking into account that
in their study not all but only approximately 89% of this potential
was due to SOC sequestration (i.e. 1.44 Mt/yr). However, the most
noteworthy difference between our study and that of Smith et al.
(2008) is that they are inconclusive about the temporal dynamics
of GHG mitigation by SOC sequestration. Even though they mention
the limited ecological capacity of sequestration (a new C equilibrium is reached over time), this is not further considered. Thus, it is
not surprising that our annual average, scenario-2 SOC sequestration rate (0.74 Mt/yr) is only about half of that of Smith et al. (2008).
Our average annual sequestration rates are within the range of
those of Smith et al. (2008) for practices that rely on a continuation
of the current cropping practice, but in an improved way (including
improved agronomy, nutrient, and tillage and residue management,

Table 2
Estimates of potential SOC sequestration due to improved land management globally and in selected countries as compared to total carbon emissions.
Region

Total C
emission

Global
Global
Global
Europe
USA
Australia
Central Asia

8.27
9.1
7.91
1.2
2
0.154
0.107

Potential SOC
sequestration

(Gt/yr)

% Of total
emission

Reference

(%)
0.37e0.74
0.44e0.88
0.44e1.15
0.104
0.288
0.013
0.017

4.4e8.9
5e10
5e15
8.3
14
8.4
16

This study
Paustian et al., 2004
Smith et al., 2008
Smith et al., 2000
Lal et al., 2003
Chan et al., 2008
Sommer and
De Pauw 2011

but omitting set aside, agroforestry, restoration of degraded land and
water management in Table 2 in their study). Distinguishing four
climatic zones, their average sequestration across these zones and
improved management practices was 0.138 Mg C/ha/yr; croplands
with improved tillage and residue management in a warm-moist
climate were assigned the largest high potential annual sequestration rate of 0.49 Mg/ha. Similar figures were given for grasslands
turned into a SOC sink by improved grazing, fertilizer and fire
management (average 0.125, maximum 0.409 Mg/ha/yr). On the
other hand, their division of crop area and grass area deviates from
our (FAO) figures. Depending on the underlying SRES scenario, their
crop area ranged between 2213 and 2430 Mha e significantly
higher than our value, and their grass area between 2213 and
2531 Mha, i.e. significantly lower than our value (compare table in
supplementary material). This deviation, together with the fact that
Smith et al. (2008) included further management practices, such as
agroforestry, restoration of organic soils or degraded lands adding
significantly to their CO2-sink, is responsible for the significant
deviation. Yet, Smith et al. (2008) concluded that their ~1.6 Gt/yr
physical-technical potential annual sequestration rate (~20% of the
total anthropogenic emissions) could never be realized due to
various policy, social, educational and economic constraints, and
that the realistic mitigation potential ranged rather between 5 and
15% of the global anthropogenic emissions of the 1990's (equal to
~7.9 Gt/yr according to Smith et al., 2008).
All in all, there seems to be some agreement among scientists
what averages quantities of global SOC sequestration and its percentage mitigation potential is concerned. Our study emphasizes
the considerable temporal dynamic of SOC sequestration that is
often not well pointed out. Therefore, SOC sequestration is not a
carbon wedge technology, as has been defined by Pacala and
Socolow (2004; see wedge no. 15) or the Carbon Mitigation
Initiative (CMI, 2013), in a sense that, once established, it could
successively sequester increasing amounts of C. Our simple calculations clearly show that SOC sequestration would have a peak and
then over the subsequent 3e6 decades would reduce significantly
and fading out at some point in time.
Clearly, the importance of SOC in the global carbon cycle goes far
beyond filling the SOC sink available in agricultural soils only.
Emissions of CO2 from drain peat/wetlands (Joosten, 2010) and the
release of CO2 and, much more important, methane from thawing
permafrost soils (Gruber et al., 2004; Antony, 2009) could dwarf
any SOC sequestration in agricultural soils.
Furthermore, we deliberately did not elaborate on the importance of maintaining or increasing SOM (and thus SOC) for sustaining soil health, agro-ecosystem functioning and productivity;
an issue of global significance that deserves proper consideration
irrespectively of any potential additional sequestration of SOC.
Finally, the economic viability of SOC sequestration and pros and
cons of its eligibility for Clean Development Mechanism projects
under the Kyoto protocol also merit further attention, which is
however beyond the scope of this study.
4. Conclusion
The 87-year total global SOC sequestration of agricultural land, if
successively turned into a C-sink, according to our estimates ranged
between 32 and 64 Gt. This is equal to 1.9e3.9% of the SRES-A2
projected 87-year anthropogenic emissions, or, if broken down
into equal annual pieces, 4.4e8.9% of the 2009 emissions. However,
our study shows that it is reasonable to assume that SOC sequestration is highly dynamic. According to our two scenarios SOC
sequestration would peak 2032e33, then reaching 4.3e8.9% of the
projected annual SRES-A2 emission. About 30 years later the
sequestration rate would have reduced by half. Thus, SOC

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Journal of Environmental Management (2014), http://dx.doi.org/10.1016/j.jenvman.2014.05.017

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R. Sommer, D. Bossio / Journal of Environmental Management xxx (2014) 1e5

sequestration is not a C wedge that could contribute increasingly to
mitigating CC. Rather, the mitigation potential is limited, providing
only a humble contribution to solving the climate problem of the
coming decades.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jenvman.2014.05.017.
Q3

Uncited reference
Smith et al., 2007.
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