Soil Carbon and Agricultural Productivity: Perspectives from Sub-Saharan Africa - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 133

For centuries, many farming systems
have relied on the SOM to sustain production.
However, with increasing intensification, land
degradation and climate change, the quantity
of SOM has declined rapidly, thereby threat-
ening the capacity of the land to produce sus-
tainably. In the past 25 years, one-quarter of
the global land area has suffered a decline in
productivity and in the ability to provide eco-
system services because of soil carbon losses
(Bai et al ., 2008). The situation is made worse
in tropical soils, which are considered as more
risky because cropping is synonymous to nu-
trient removal in the already impoverished
soils with insufficient replenishment. There
is considerable concern that, if SOM concen-
trations in soils are allowed to decrease too
much, then the productive capacity of agri-
culture will be compromised. Soils exhibit
different behaviour, and as such we would ex-
pect to have different SOC levels. However,
there is a general consensus among scientists
that a 2% soil carbon (3.5% SOM) is a critical
level for temperate soils below which poten-
tially serious decline in soil quality will occur
(Loveland and Webb, 2003).

chemical and biological properties of soils
(Baldock and Skjemstad, 1999; Fig. 10.1) .
Physically, it promotes aggregate stability,
and therefore water infiltration, percolation
and retention. It impacts on soil chemistry
by increasing cation exchange capacity, soil
buffer capacity and nutrient supply. Bio-
logically, it stimulates the activity and di-
versity of organisms in soil (Allison, 1973).
While SOM is primarily carbon, it also
contains nutrients essential for plant growth,
such as nitrogen, phosphorus, sulfur and
micronutrients. Organisms in the soil food
web decompose SOM and make these nutri-
ents available (Brussaard et al ., 2007). The
rate of SOM decomposition and turnover
depends mainly on the interplay between
soil biota, temperature, moisture and a soil's
chemical and physical composition (Taylor
et al ., 2009). In addition to soil's clay con-
tent, there is evidence to suggest that an in-
crease in organic carbon content can increase
the amount of water present in the soil. Man-
aging soil water is critical in enhancing crop
productivity, especially with the adverse ef-
fects of climate change and rainfall variabil-
ity in most regions of the world. A marginal
increase in the amount of plant available
water of a soil can help maintain or enhance
potential productivity by allowing the soil
to retain more water, applied either as rain
or irrigation.

Functions of Soil Organic Carbon

Organic matter is of great importance in
soil, because it impacts on the physical,

Biological functions

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Provides energy to biological processes
Provides nutrients (N, P and S)
Contributes to resilience

Functions of
soil organic
matter

Physical functions

Chemical functions

• Improves the structural stability of the soil

Influences water retention properties
Alters soil thermal properties

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•

Contributes to cation exchange capacity
Enhances pH buffering
Complexes cations

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