Soil Carbon and Agricultural Productivity: Perspectives from Sub-Saharan Africa - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 134

Soil Carbon Changes/ Variation
Across Ecosystems

and peatlands are all being converted to ar-
able land at an increasing rate. While temper-
ate humid grasslands lose about 30% of their
SOC after 60 years of cultivation (Tiessen
et al ., 1983), soil carbon stocks in semi-arid
environments can decrease by 30% in less
than 5 years when native vegetation or pas-
tures are converted to cropland (Zach et al .,
2006). Cultivation of tropical forest soils
causes losses of more than 60% of original
SOC stocks in just a few years (Brown and
Lugo, 1990).
Soil erosion is the most widespread
form of soil degradation and has a strong im-
pact on the global C cycle (Lal, 2003). Being a
selective process, erosion preferentially re-
moves the light organic fraction of a low
density of <1.8 Mg m - 3 . A combination of
mineralization and C export by erosion causes
a severe depletion of the SOC pool on eroded
compared with uneroded or slightly eroded
soils. In addition, the SOC redistributed over
the landscape or deposited in depressional
sites may be prone to mineralization because
of breakdown of aggregates, leading to expos-
ure of hitherto encapsulated C to microbial
processes among other reasons. Adoption of
conservation-effective measures may reduce
the risks of C loss and promote sequestra-
tion of C in soil and biota.
Recent studies in Africa have shown
the existence of steep gradients in soil or-
ganic C status between fields located close
to homesteads and those located further
away (Prudencio, 1993; Table 10.1) . These
gradients reflect the site-specific manage-
ment practices the land users apply to the
respective areas under production. Fields
located closer to homesteads benefit more
from organic and inorganic nutrient appli-
cation compared to those located further
away. High SOM status in the home fields is

The worldwide distribution of SOC reflects
rainfall distribution, with greater accumula-
tions of carbon in more humid areas. Most
SOC is found in the northern hemisphere,
which contains more land mass in humid
climates than the southern hemisphere.
Temperature plays a secondary role in global
SOC distribution. Within climatic zones, the
amount of SOC is determined by soil mois-
ture, which in turn is influenced by relief,
soil texture and clay type. The soils of savan-
nahs are relatively low in SOC, but the car-
bon stocks of savannah soils are significant
globally due to the large land area covered by
this biome. In contrast, peatlands cover only
3% of the global land area but contain almost
one-third of global soil carbon, making them
the most space-effective carbon store among
all terrestrial ecosystems.
The current rate of change in SOC is at-
tributable mainly to worldwide land-use in-
tensification and the conversion of new land
for food and fibre production. The clearing of
forests or woodlands and their conversion
into farmland in the tropics reduces the soil
carbon content, mainly through reduced pro-
duction of detritus, increased erosion rates
and decomposition of SOM by oxidation.
Modern industrialized crop production re-
lies on monocultures of highly efficient cash
crops, which generally create a negative car-
bon budget. Alternative uses of crop residues
for fodder, fuel or industrial applications
exacerbate this trend of decreasing carbon
return to the soil. Intensive land uses are also
expanding into areas where SOC stocks are
less resilient or soil conditions are marginal
for agriculture. For example, semi-arid sa-
vannahs and grasslands, tropical rainforests

Table 10.1. Soil fertility of various fields within a farm in Burkina Faso. (From Prudencio, 1993.)

Organic C
(g kg -1 )

Total N
(g kg -1 )

Available P
(mg kg -1 )

Exchangeable K
(mmol kg -1 ) Field

Home garden 11-22 0.9-1.8 20-220 4.0- 24

Village field 5-10 0.5-0.9 13-16 4.1- 11

Bush field 2-5 0.2-0.5 5-16 0.6- 1

Home gardens are near the homestead, bush fields are the furthest away from the homestead and village fields at
intermediate distances. C, carbon; K, potassium; N, nitrogen; P, phosphorus.