Soil Carbon and Agricultural Productivity: Perspectives from Sub-Saharan Africa - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 135

often observed to be related positively with
high crop yield (Carsky et al ., 1998).

(shoots and roots) to the soil will result in
an increase in soil carbon. These practices
include recycling of crop residues, transfer
of plant materials from non-cropped areas
to arable land, from biological N fixation
through leguminous crops, utilization of
animal manure and occasional application
of inorganic fertilizers (Bekunda et al ., 2002).
On the contrary, strategies to decrease carbon
losses from the system include reducing soil
erosion, practising minimum tillage, reducing
burning or removal of crop residues during
land clearing and harvesting, and controlled
grazing, among other practices.

Management of Soil Carbon and
Productivity

The amount of carbon in a soil is influenced
by the balance between inputs (plant res-
idues) and losses, mainly microbial decom-
position and associated mineralization (Batjes,
2001). This amount will vary with factors
such as soil clay content, soil depth and bulk
density and management. There are a wide
range of management options and farming
practices that can increase SOC levels by
either increasing inputs or decreasing losses
(Bruce et al ., 1999; Batjes, 2001), as summar-
ized in Table 10.2. Bruce et al . (1999) reviewed
the management practices for increasing car-
bon sequestration in the soil and grouped
them into five broad classes: (i) reduction in
tillage intensity; (ii) intensification of crop-
ping systems; (iii) adoption of yield-promoting
practices, including improved nutrient amend-
ments; (iv) soil and water conservation meas-
ures; and (v) re-establishment of permanent
perennial vegetation. Any practice that enhances
productivity and the return of plant residues

Organic Resource Concept and
Soil Organic Carbon Pools

The ultimate contribution of these practices
to SOC is controlled by the type (quality)
and amount (quantity) of plant residue
added to the soil (Palm et al ., 1997, 2001;
Vanlauwe, 2003). Studies have been con-
ducted to characterize the quality param-
eters (nutrient, polyphenol and lignin) of
organic resources and how these affect de-
composition rates. This led to the develop-
ment of the Organic Resource Database (ORD)

Table 10.2. Management practices that can increase soil organic carbon levels in
agricultural soils.

Management category Management practices to increase soil carbon

Crop management Soil fertility enhancement

Crop rotation and intercropping
Soil erosion control

Conservation tillage Crop residue retention

Reduced tillage
Crop rotation and intercropping

Pasture management Grazing management

Fertilizer management
Improved grass species
Introduction of legumes
Introduction of perennial pastures

Organic amendments Use of animal manure

Green manure application
Compost and recycling of organics

Agroforestry Improved fallows

Alley cropping
Biomass transfer
Parkland systems
Silvopastoral systems