52
SSA. Birch-Thomsen et al. (2007) examined the trends in the spatial distribution of
SOC stocks following the conversion of natural and semi-natural savannahs in the
semi-arid parts of Tanzania over a period of 50 years. Predictably, the results showed
that soils that had been cultivated for 50 years had on average less than 50% SOC
compared to the uncultivated ones. The documented rate of SOC losses from such
conversions is, on average, 2.77 and 0.82 Mg C ha
−1
year
−1
in the East Sudanian
savannah and southern Africa regions, respectively (Vågen et al. 2005). Detwiler
(1986) attributed the shrinkage of the SOC pool after conversions of natural forests
and savannahs to the changes in quantity and quality of organic material inputs to
the soil, soil erosion, leaching, and soil disturbances.
In contrast, Majaliwa et al. (2010) reported an increase in SOC after conversion
of forests to tea (Camellia sinensis) plantations around Kibale National Park in
Uganda. The observed differences in SOM were ascribed to the high level of organic
material input (i.e., residues from regular pruning and leaf fall) in tea plantations
than in natural forests coupled with better soil management practices, such as application of inorganic fertilizer, which stimulates decomposition and mineralization of
plant residues compared to natural conditions. In other words, the rate of SOC loss
after conversion depends on multiple factors, including the method of clearing, terrain, climate, soil types, and land use (type and duration) after clearing (Vågen et al.
2005; Girmay et al. 2008).
5 Sustainable Land Management Options for Soil C
Sequestration in SSA
From Sect. 4, it is evident that the conversion of native ecosystems to agroecosystems in SSA depletes SOC, which in turn influences the concentrations of C
in the atmosphere. However, the SSA agricultural landscapes can be sustainably
managed with a view to mitigating CO 2 emissions, enhancing C sinks by removing
CO 2 from the atmosphere for later storage as SOM, and creating resilient systems
among other socio-economic and environmental benefits. There are several SLM
technological solutions for conserving, replenishing, and enhancing C in the SSA
agricultural soils (Fig. 6).
To begin with, a fundamental SLM practice for soil C sequestration in SSA is
conservation agriculture (CA), which aims at accumulating SOC and creating a
healthy soil ecosystem by not tilling the soil prior to planting. That is, crops are
established on the residues left on the ground after harvesting. By minimizing soil
disturbance, retaining crop residues as surface mulch, managing nutrients, and
diversifying the cropping system through intercropping and incorporating cover
crops in the rotation cycle (Aune and Coulibaly 2015; Lal 2015; FAO 2013), CA
increases SOC, which in turn improves soil tilth, fertility, biological activity, and
infiltration capacity while curtailing soil erosion, soil compaction, and the release of
C to the atmosphere. CA also ameliorates soil moisture capacity, hence increasing
K. Were et al.
SSA. Birch-Thomsen et al. (2007) examined the trends in the spatial distribution of
SOC stocks following the conversion of natural and semi-natural savannahs in the
semi-arid parts of Tanzania over a period of 50 years. Predictably, the results showed
that soils that had been cultivated for 50 years had on average less than 50% SOC
compared to the uncultivated ones. The documented rate of SOC losses from such
conversions is, on average, 2.77 and 0.82 Mg C ha
−1
year
−1
in the East Sudanian
savannah and southern Africa regions, respectively (Vågen et al. 2005). Detwiler
(1986) attributed the shrinkage of the SOC pool after conversions of natural forests
and savannahs to the changes in quantity and quality of organic material inputs to
the soil, soil erosion, leaching, and soil disturbances.
In contrast, Majaliwa et al. (2010) reported an increase in SOC after conversion
of forests to tea (Camellia sinensis) plantations around Kibale National Park in
Uganda. The observed differences in SOM were ascribed to the high level of organic
material input (i.e., residues from regular pruning and leaf fall) in tea plantations
than in natural forests coupled with better soil management practices, such as application of inorganic fertilizer, which stimulates decomposition and mineralization of
plant residues compared to natural conditions. In other words, the rate of SOC loss
after conversion depends on multiple factors, including the method of clearing, terrain, climate, soil types, and land use (type and duration) after clearing (Vågen et al.
2005; Girmay et al. 2008).
5 Sustainable Land Management Options for Soil C
Sequestration in SSA
From Sect. 4, it is evident that the conversion of native ecosystems to agroecosystems in SSA depletes SOC, which in turn influences the concentrations of C
in the atmosphere. However, the SSA agricultural landscapes can be sustainably
managed with a view to mitigating CO 2 emissions, enhancing C sinks by removing
CO 2 from the atmosphere for later storage as SOM, and creating resilient systems
among other socio-economic and environmental benefits. There are several SLM
technological solutions for conserving, replenishing, and enhancing C in the SSA
agricultural soils (Fig. 6).
To begin with, a fundamental SLM practice for soil C sequestration in SSA is
conservation agriculture (CA), which aims at accumulating SOC and creating a
healthy soil ecosystem by not tilling the soil prior to planting. That is, crops are
established on the residues left on the ground after harvesting. By minimizing soil
disturbance, retaining crop residues as surface mulch, managing nutrients, and
diversifying the cropping system through intercropping and incorporating cover
crops in the rotation cycle (Aune and Coulibaly 2015; Lal 2015; FAO 2013), CA
increases SOC, which in turn improves soil tilth, fertility, biological activity, and
infiltration capacity while curtailing soil erosion, soil compaction, and the release of
C to the atmosphere. CA also ameliorates soil moisture capacity, hence increasing
K. Were et al.