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4 Impact of Land Use Changes on SOC Stocks in SSA
The IPCC (2007) attributed the dramatic increases in global surface temperatures
and other observed climatic changes to anthropogenic emissions of GHGs through
land use changes, fossil fuel combustion, and cement production. POM is, especially, sensitive to land use changes (FAO 2004), which greatly influence its amount,
quality, and turnover (Post and Kwon 2000; Birch-Thomsen et al. 2007). The attendant climatic changes are also strongly linked to soil degradation (i.e., loss of SOM
and soil quality) because of the feedback between SOM and climatic elements.
Climate change aggravates depletion of SOM by altering the patterns of temperature, rainfall, solar radiation, and winds.
A few studies have been conducted in SSA with a view to understanding the
anthropogenic effects, particularly land use changes on the storage and fluxes of C
in the agro-ecosystems. The focus has mostly been on the impact of converting
natural forests to cultivated lands. For example, Demessie et al. (2013) evaluated the
changes in SOC stocks and concentrations under the chronosequences of 12, 20, 30,
40, and 50 years after conversion of natural forests to agro-forestry and agricultural
lands in southern Ethiopia. The results showed that depletion of SOC stock after
conversions from natural forests varied from 28.2 to 98.9 Mg C ha
−1
under the chronosequences of 12–50 years of agro-forestry and agricultural lands. The rate of
SOC loss after 12 years of agro-forestry was 6.2 Mg C ha
−1
year
−1
, which declined
to 0.9 Mg C ha
−1
year
−1
after 50 years. The corresponding losses for agricultural
lands were slightly higher (i.e., 6.6 and 1.3 Mg C ha
−1
year
−1
). A similar trend was
observed by Awiti et al. (2008) along a forest-cropland chronosequence in Kakamega
Forest and its environs in Kenya, where the topsoil SOC content declined from
7.27 kg C m
−2
in the forests to 2.67 kg C m
−2
in the croplands over a period of
60 years.
In addition, Were et al. (2015) analyzed the variations of SOC stocks under natural forests (NF), plantation forests (PF), bamboo forests (BF), and croplands that
had been converted from such forests (i.e., NF2C, PF2C, and BF2C) in a Kenyan
Afromontane ecosystem. The results indicated significant differences in SOC stocks
between NF and NF2C (p < 0.0001) and between PF and PF2C (p < 0.0001).
Specifically, the surface soils (0–15 cm) of NF had the highest SOC stocks
(71.6 Mg ha
−1
), while NF2C had the lowest (35.4 Mg ha
−1
) signifying a decline in
SOC stocks by about 51% after NF conversion. These results compare with others
from cognate studies; for example, Amanuel et al. (2018), Lemma et al. (2006), and
Kimigo et al. (2008) also examined SOC variations in relation to land use changes
in the upper Blue Nile River basin in Ethiopia, southwestern highlands of Ethiopia,
and Sasumua catchment in Kenya and found that SOC stocks reduced by 36%,
43%, and 8%, respectively, after NF conversion. All these results confirm the observation by Vågen et al. (2005) that SOC contents decrease by 063% following deforestation in SSA.
Besides natural forest conversions, a few other studies have also attempted to
shed light on the impact of converting natural savannahs to cultivated lands in
Land Use Changes and Sustainable Land Management Practices for Soil Carbon…
4 Impact of Land Use Changes on SOC Stocks in SSA
The IPCC (2007) attributed the dramatic increases in global surface temperatures
and other observed climatic changes to anthropogenic emissions of GHGs through
land use changes, fossil fuel combustion, and cement production. POM is, especially, sensitive to land use changes (FAO 2004), which greatly influence its amount,
quality, and turnover (Post and Kwon 2000; Birch-Thomsen et al. 2007). The attendant climatic changes are also strongly linked to soil degradation (i.e., loss of SOM
and soil quality) because of the feedback between SOM and climatic elements.
Climate change aggravates depletion of SOM by altering the patterns of temperature, rainfall, solar radiation, and winds.
A few studies have been conducted in SSA with a view to understanding the
anthropogenic effects, particularly land use changes on the storage and fluxes of C
in the agro-ecosystems. The focus has mostly been on the impact of converting
natural forests to cultivated lands. For example, Demessie et al. (2013) evaluated the
changes in SOC stocks and concentrations under the chronosequences of 12, 20, 30,
40, and 50 years after conversion of natural forests to agro-forestry and agricultural
lands in southern Ethiopia. The results showed that depletion of SOC stock after
conversions from natural forests varied from 28.2 to 98.9 Mg C ha
−1
under the chronosequences of 12–50 years of agro-forestry and agricultural lands. The rate of
SOC loss after 12 years of agro-forestry was 6.2 Mg C ha
−1
year
−1
, which declined
to 0.9 Mg C ha
−1
year
−1
after 50 years. The corresponding losses for agricultural
lands were slightly higher (i.e., 6.6 and 1.3 Mg C ha
−1
year
−1
). A similar trend was
observed by Awiti et al. (2008) along a forest-cropland chronosequence in Kakamega
Forest and its environs in Kenya, where the topsoil SOC content declined from
7.27 kg C m
−2
in the forests to 2.67 kg C m
−2
in the croplands over a period of
60 years.
In addition, Were et al. (2015) analyzed the variations of SOC stocks under natural forests (NF), plantation forests (PF), bamboo forests (BF), and croplands that
had been converted from such forests (i.e., NF2C, PF2C, and BF2C) in a Kenyan
Afromontane ecosystem. The results indicated significant differences in SOC stocks
between NF and NF2C (p < 0.0001) and between PF and PF2C (p < 0.0001).
Specifically, the surface soils (0–15 cm) of NF had the highest SOC stocks
(71.6 Mg ha
−1
), while NF2C had the lowest (35.4 Mg ha
−1
) signifying a decline in
SOC stocks by about 51% after NF conversion. These results compare with others
from cognate studies; for example, Amanuel et al. (2018), Lemma et al. (2006), and
Kimigo et al. (2008) also examined SOC variations in relation to land use changes
in the upper Blue Nile River basin in Ethiopia, southwestern highlands of Ethiopia,
and Sasumua catchment in Kenya and found that SOC stocks reduced by 36%,
43%, and 8%, respectively, after NF conversion. All these results confirm the observation by Vågen et al. (2005) that SOC contents decrease by 063% following deforestation in SSA.
Besides natural forest conversions, a few other studies have also attempted to
shed light on the impact of converting natural savannahs to cultivated lands in
Land Use Changes and Sustainable Land Management Practices for Soil Carbon…