44
(1 Pg is equal to 10
15
g), which is equivalent to 20% of global land use CO 2 emissions to the atmosphere. Vågen et al. (2005) also noted from their review of soil C
sequestration literature that SOC stocks can diminish by 0.90 Mg C ha
−1
year
−1
(1 Mg is equal to 10
6
g) following the clearance and cultivation of forest areas in
SSA. These figures highlight the critical role of land use change and forestry in the
SSA and global C balance; however, this role is still loaded with large uncertainties
attributable to scarcity of studies and reliable long-term data needed for complete
accounting.
The objective of this chapter is to provide a synthesis of the impact of land use
changes (i.e., conversion of forests to croplands) on the SOC pool, as well as the
sustainable land management (SLM) strategies with potential for sequestering C in
SSA agricultural soils. The goal is to increase awareness and inform agricultural
development programs that aim at mitigating climate change, advancing food and
nutritional security, and sustaining life in SSA. We precede the synthesis with an
overview of SOC (i.e., its components, dynamics, and functions) for better understanding of the subject matter.
2 An Overview of SOC
2.1 Components of SOC
Walcott et al. (2009) defined soil as the thin mantle comprising organic materials,
inorganic materials, and living organisms that cover the Earth’s surface to a depth of
about two (2) meters. The constituent organic materials are derived from leaf litter,
branches, plant roots, soil organisms, and manure, which together form SOC. The
plants, animals, and microbes constitute the living organic matter, which upon
death and decomposition become the non-living organic matter. The latter is measured either as particulate organic matter (POM), dissolved organic matter (DOM),
humus, or inert (recalcitrant) organic matter.
SOC and the soil inorganic C (SIC) form the soil C (pedologic) pool, which is the
largest C reservoir in terrestrial ecosystems (FAO 2004; Lal 2002, 2004; Post and
Kwon 2000). The pedologic pool contains three (3) times more C (i.e., 1550 Pg of
SOC to 1 m depth and 950 Pg of SIC) than in the biotic C pool (i.e., 560 Pg C) and
twice as much C as in the atmospheric C pool (i.e., 760 Pg C) (Lal 2008). Unlike
SOC, SIC is made up of elemental C and carbonate minerals, such as calcite, dolomite, and gypsum, derived either from the weathering of parent material or the dissolution of atmospheric CO 2 into carbonic acid and its reaction with Ca
2+
and Mg
2+
brought into the soil system by calcareous dust, irrigation water, fertilizer, or
manure. Whereas SIC is an important constituent of the soils in arid and semi-arid
lands (ASALs), SOC is high in the soils of temperate regions and extremely high in
the organic (peat) soils. SOC also varies widely among eco-regions, being higher in
the cool and moist areas than in the warm and dry ones.
K. Were et al.
(1 Pg is equal to 10
15
g), which is equivalent to 20% of global land use CO 2 emissions to the atmosphere. Vågen et al. (2005) also noted from their review of soil C
sequestration literature that SOC stocks can diminish by 0.90 Mg C ha
−1
year
−1
(1 Mg is equal to 10
6
g) following the clearance and cultivation of forest areas in
SSA. These figures highlight the critical role of land use change and forestry in the
SSA and global C balance; however, this role is still loaded with large uncertainties
attributable to scarcity of studies and reliable long-term data needed for complete
accounting.
The objective of this chapter is to provide a synthesis of the impact of land use
changes (i.e., conversion of forests to croplands) on the SOC pool, as well as the
sustainable land management (SLM) strategies with potential for sequestering C in
SSA agricultural soils. The goal is to increase awareness and inform agricultural
development programs that aim at mitigating climate change, advancing food and
nutritional security, and sustaining life in SSA. We precede the synthesis with an
overview of SOC (i.e., its components, dynamics, and functions) for better understanding of the subject matter.
2 An Overview of SOC
2.1 Components of SOC
Walcott et al. (2009) defined soil as the thin mantle comprising organic materials,
inorganic materials, and living organisms that cover the Earth’s surface to a depth of
about two (2) meters. The constituent organic materials are derived from leaf litter,
branches, plant roots, soil organisms, and manure, which together form SOC. The
plants, animals, and microbes constitute the living organic matter, which upon
death and decomposition become the non-living organic matter. The latter is measured either as particulate organic matter (POM), dissolved organic matter (DOM),
humus, or inert (recalcitrant) organic matter.
SOC and the soil inorganic C (SIC) form the soil C (pedologic) pool, which is the
largest C reservoir in terrestrial ecosystems (FAO 2004; Lal 2002, 2004; Post and
Kwon 2000). The pedologic pool contains three (3) times more C (i.e., 1550 Pg of
SOC to 1 m depth and 950 Pg of SIC) than in the biotic C pool (i.e., 560 Pg C) and
twice as much C as in the atmospheric C pool (i.e., 760 Pg C) (Lal 2008). Unlike
SOC, SIC is made up of elemental C and carbonate minerals, such as calcite, dolomite, and gypsum, derived either from the weathering of parent material or the dissolution of atmospheric CO 2 into carbonic acid and its reaction with Ca
2+
and Mg
2+
brought into the soil system by calcareous dust, irrigation water, fertilizer, or
manure. Whereas SIC is an important constituent of the soils in arid and semi-arid
lands (ASALs), SOC is high in the soils of temperate regions and extremely high in
the organic (peat) soils. SOC also varies widely among eco-regions, being higher in
the cool and moist areas than in the warm and dry ones.
K. Were et al.