Enhancing SOC sequestration:
myth or reality in Africa?
Rolf Sommer
r.sommer@cgiar.org

From the FAO SOC Symposium webpage:
…large losses of soil organic matter …have occurred in
soils from various global agroecosystems … where losses
ranged between 25–75 % of their original SOC pool.
These losses provide an opportunity: the recoverable
carbon reserve capacity of the world’s agricultural and
degraded soils is estimated to be between 21 to 51 Gt of
carbon.
Soil organic carbon sequestration aspirations
How much is recoverable, where, and
how long would it take and then last?
Total emissions
worldwide (2012)
51,840 Mt CO2 EQ
=
14.14
Gt C
Source: EcoFys 2016

Losses of SOC in response to Land use (change)
Source:
Don et al. 2011
Global Change
Biology 17,
1658–1670

Losses of SOC in response to Land use (change)
Source: Kinjangi, 2008
(PhD thesis, redrawn)
Forest to cropland (Western Kenya)
0
2
4
6
8
0 30 60 90 120
kgC/m²/10to12cm
Years since native forest conservation
Heavy-textured
North Nandi
Heavy-textured
South Nandi
Medium-textured
Kakamega

Losses of SOC in response to Land use (change)
Forest to permanent pastures (French Guiana Amazonia)
Pasture age (yrs)
Primary forest
Source:
Stahl et al. 2016,
Global Change Biology
doi:10.1111/gcb.13573

Observed SOC changes over time under "improved" land use
In SSA increases were between 0.28
and 0.96 Mg C ha-1 yr-1, but with much
greater variation and a significant
number of cases with no measurable
increase. …
CA should be promoted on the basis of
these factors and any climate change
mitigation regarded as an additional
benefit, not a major policy driver for
its adoption.
… data from 125 on-farm validation trials
across 23 sites in Malawi, Mozambique,
Zambia and Zimbabwe. … No consistent
differences in bulk density and soil C
concentrations were found. … These
results … indicate that there is a limited
potential for conservation agriculture to
significantly increase soil C stocks after
up to 7 years of conservation agriculture

Observed SOC changes over time under "improved" land use
Source: Sommer et al. submitted for publicationCIAT long-term trials, Western Kenya:
ISFM
17
19
21
23
25
1/Jan/05 1/Jan/08 1/Jan/11 1/Jan/14
SOC(g/kg)
FYM+ R+
FYM+ R-
FYM- R+
FYM- R-
LSD
Conservation Agriculture
17
19
21
23
25
1/Jan/05 1/Jan/08 1/Jan/11 1/Jan/14
0T R+
CT R+
CT R-
0T R-
LSDs, same
level of tillage
Instead of C-sequestration, we detect losses in both systems!

Observed SOC changes over time under "improved" land use
Source:
Don et al. 2011
Global Change
Biology 17,
1658–1670

Food for thought
• True C-sequestration or only "avoided losses"?
• How transferrable are results to other tropical agro-ecosystems?
• Importance of land use history!?
• Importance of climate, "base" SOC levels / landscapes / clay
content and other soil properties…
SOCcontent
Time
Improved management
Common practice
?

6/28/2017 10
A fundamental shift in global agriculture is required where
sustainability constitutes the core strategy for agricultural
development.
Rockström et al. Ambio (2017) 46: 4. doi:10.1007/s13280-016-0793-6

Create the right enabling environment and
incentives for change: policy and institutional
New systems and approaches: to motivate behavior
change and enable the right responses
Innovation: to tackle new coincidences of problems
Multi-scale delivery that manages trade-offs and build
synergies
Collaboration across research and
development communities is key!
Moving from Opportunity to Reality: What’s needed?
Photo © G. Smith

• Revisit the individual "best bet" cases
(above all grasslands)
• In sub-Saharan Africa food security and
sustaining soil fertility comes first, SOC
sequestration is a co-benefit only!
• Very different type of cropping systems
are required for "carbon" farmers
• Avoid deforestation!
• Avoid draining wetlands and peatlands!
Holistic view

1
Thank you!
Photo © A. Notenbaert