Sommer & Bossio (2014). Dynamics and climate change mitigation potential of soil organic carbon sequestration

Short communication

Dynamics and climate change mitigation potential of soil organic
carbon sequestration

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

Article history:

Received 9 December 2013
Received in revised form
12 May 2014
Accepted 20 May 2014
Available online 12 June 2014

Keywords:

Carbon sequestration
Anthropogenic CO

2

emissions

C wedge
SRES-A2

a b s t r a c t

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 (2014

e

2100) global SOC

sequestration potential of agricultural land ranged between 31 and 64 Gt. This is equal to 1.9

e

3.9% of the

SRES-A2 projected 87-year anthropogenic emissions. SOC sequestration would peak 2032

e

33, at that

time reaching 4.3

e

8.9% 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 impor-
tance of maintaining or increasing SOC for sustaining soil health, agro-ecosystem functioning and pro-
ductivity; an issue of global signi

ﬁ

cance that deserves proper consideration irrespectively of any

potential additional sequestration of SOC.

©

1. Introduction

Anthropogenic carbon dioxide (CO

2

) emissions into the atmo-

sphere have increased signi

ﬁ

cantly 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 CO

2

in response to land use change, including defor-

estation and burning of forests or degradation of soils and loss of
soil organic carbon (SOC). CO

2

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

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 2000

e

2500 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 CO

2

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 CO

2

by Soil Organic Matter (SOM) decomposition

*

Corresponding author. Tel.:

þ

254 20 863 2811.

E-mail address:

r.sommer@cgiar.org

(R. Sommer).

Contents lists available at

ScienceDirect

Journal of Environmental Management

j o u r n a l h o m e p a g e : w w w . e l s e v ie r . c o m / l o c a t e / j e n v m a n

http://dx.doi.org/10.1016/j.jenvman.2014.05.017

0301-4797/

©

Journal of Environmental Management 144 (2014) 83

e

87

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
dif

ﬁ

cult 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 quan-

tities 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 opti-
mistic 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 simpli

ﬁ

ed 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 CO

2

emissions from

fossil-fuel burning, cement production, and gas

ﬂ

aring provided

by the Carbon Dioxide Information Analysis Center (

2) Prediction about future anthropogenic CO

2

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 classi

ﬁ

ed by FAO, but is not considered

further in this study. Permanent meadows and pastures is the
land used permanently (

ﬁ

ve 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 CO

2

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

SOC

0

þ

a

1

þ

e

t

0

b

(1)

where SOC

0

is the initial soil organic carbon content (%),

a

and

b

are

empirical constants and

t

the time expressed in years.

t

0

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/cm

3

.

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:

A

y

¼

kA

ð

t

1

Þ

(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 seques-
tration 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
dif

ﬁ

cult 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

ﬁ

rst year and reaching

40% of the total area after 20 years, as opposed to 60% for arable
land (starting with 5%;

Fig. 2

).

Table 1

SOC sequestration parameters (four-parameter sigmoid function; Eq.

(1)

) for scenario 1 and 2; for the cumulative C-sequestration calculations, a soil pro

ﬁ

le depth of 25 cm and

a soil bulk density equal to 1.3 g/cm

3

was assumed.

Land use

Scenario

a

(

e

)

b

(

e

)

t

0

(yr)

Cumulative C-sequestration after 87 years (Mg/ha)

Notation in

Fig. 1

Arable land and permanent crops

2

1.202

9.8

7

26.2

High

1

0.697

11.5

4

13.3

Medium

Permanent meadows

&

pastures

2

0.697

11.5

4

13.3

Medium

1

0.350

11.5

3

6.4

Low

R. Sommer, D. Bossio / Journal of Environmental Management 144 (2014) 83

e

87

84

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 con-
founding effects, for an easier interpretation of results. Projections
were made for the coming 87 years, i.e. 2014

e

2100.

3. Results and discussion

The average annual C sequestration rates over the

ﬁ

rst 50 years

of the low, medium and high

ﬁ

gures (

Fig. 1

) would be 0.125, 0.258

and 0.515 Mg/ha, respectively. However, it is obvious that, espe-
cially for the medium and high rates, providing averages fails to
describe the notable sequestration dynamics. Using averages also
increases the risk of laymen misinterpreting

ﬁ

gures 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

ﬁ

gures 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

ﬂ

aring (

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 anthro-

pogenic 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 na-
tional 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

1

1.4

1.8

0

0.4

0.8

0

20

40

60

C-

)r

y/

a

h/

g

M(

n

oit

ar

ts

e

u

q

es

SOC (%)

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);

y

0 (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

y

0 are

exemplarily only and do not affect SOC sequestration rates.

40%

60%

0%

25%

50%

75%

100%

2014

2034

2054

2074

2094

% of

to

ta

l ar

ea

Year

Permanent meadows

and pastures

Arable land and
permanent crops

Fig. 2.

Assumed increase in area included in a SOC sequestration campaign expressed

in percent of the total area.

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.

R. Sommer, D. Bossio / Journal of Environmental Management 144 (2014) 83

e

87

85

sequestration rates (scenario 1), under which the two countries
could still mop up 20

e

22% of their emissions. Optimistically, these

percentages would go up to 41

e

45%. Most of the large-area, low-

emitting countries (lower part of table in supplementary material)
would be able to become CO

2

neutral or even net C sinks. Never-

theless, 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 afore-
mentioned mitigation potential of future projected (SRES-A2)
emissions. It is subject of further debate which

ﬁ

gures are more

plausible.

Our 2009 emission related average

ﬁ

gures, 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 3

e

7 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 in

ﬂ

uence.

We assumed that SOC sequestration would start leveling off

signi

ﬁ

cantly after about 20

e

40 years (compare

Fig. 1

), which cor-

responds 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 CO

2

eq./yr) potentially achievable by 2030 according to

Smith et al.

(2008)

. The

ﬁ

gures 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 equilib-
rium is reached over time), this is not further considered. Thus, it is
not surprising that our annual average, scenario-2 SOC sequestra-
tion 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

,

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 seques-
tration rate of 0.49 Mg/ha. Similar

ﬁ

gures were given for grasslands

turned into a SOC sink by improved grazing, fertilizer and

ﬁ

re

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)

ﬁ

gures. Depending on the underlying SRES scenario, their

crop area ranged between 2213 and 2430 Mha

e

signi

ﬁ

cantly

higher than our value, and their grass area between 2213 and
2531 Mha, i.e. signi

ﬁ

cantly 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
signi

ﬁ

cantly to their CO

2

-sink, is responsible for the signi

ﬁ

cant

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 per-
centage 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 de

ﬁ

ned 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 calcu-
lations clearly show that SOC sequestration would have a peak and
then over the subsequent 3

e

6 decades would reduce signi

ﬁ

cantly

and fading out at some point in time.

Clearly, the importance of SOC in the global carbon cycle goes far

beyond

ﬁ

lling the SOC sink available in agricultural soils only.

Emissions of CO

2

from drain peat/wetlands (

Joosten, 2010

) and the

release of CO

2

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 impor-

tance of maintaining or increasing SOM (and thus SOC) for sus-
taining soil health, agro-ecosystem functioning and productivity;
an issue of global signi

ﬁ

cance 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 1.9

e

3.9% of the SRES-A2

projected 87-year anthropogenic emissions, or, if broken down
into equal annual pieces, 4.4

e

8.9% of the 2009 emissions. However,

our study shows that it is reasonable to assume that SOC seques-
tration is highly dynamic. According to our two scenarios SOC
sequestration would peak 2032

e

33, then reaching 4.3

e

8.9% of the

projected annual SRES-A2 emission. About 30 years later the
sequestration rate would have reduced by half. Thus, SOC

Table 2

Estimates of potential SOC sequestration due to improved land management glob-
ally and in selected countries as compared to total carbon emissions.

Region

Total C
emission

Potential SOC
sequestration

% Of total
emission

Reference

(Gt/yr)

(%)

Global

8.27

0.37

e

0.74

4.4

e

8.9

This study

Global

9.1

0.44

e

0.88

5

e

10

Paustian et al., 2004

Global

7.91

0.44

e

1.15

5

e

15

Smith et al., 2008

Europe

1.2

0.104

8.3

Smith et al., 2000

USA

2

0.288

14

Lal et al., 2003

Australia

0.154

0.013

8.4

Chan et al., 2008

Central Asia

0.107

0.017

16

Sommer and
De Pauw 2011

R. Sommer, D. Bossio / Journal of Environmental Management 144 (2014) 83

e

87

86

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

.

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