Soil organic carbon in agricultural systems of six countries in East Africa – a literature review of status and carbon sequestration potential

S Namirembe,K Piikki,R Sommer,M Söderström,B Tessema,Ss Nyawira

doi:10.1080/02571862.2019.1640296

Abstract

Cropland soils are considered to have the potential to sequester atmospheric CO2 through agronomic best management practices (BMPs). To estimate this potential in East Africa, the authors reviewed 69 published studies from Ethiopia, Kenya, Rwanda, Tanzania, Uganda, and Burundi assessing the effect of land use conversion from native vegetation to cropland on soil organic carbon (SOC) and the extent to which carbon sequestration is feasible through BMPs. Reported losses of SOC in the top 30 cm of the soil profile in short (<10 years), medium (10–25 years), and long (>25 years) term were 6.7 ± 6.0, 13.0 ± 9.2, and 2.8 ± 1.0 t C ha–1 year–1, respectively, for forest-to-cropland; and 16.0, 2.1 ± 2.2 and 0.3 ± 0.8 t C ha–1 year–1 respectively, for woodland-to-cropland conversion. Duration to steady-state SOC was 21–38 years for forest-to-cropland conversion. Short-term SOC sequestration (t C ha–1 year–1) in the 0–30 cm layer as a result of BMPs was 19.7 ± 3.9 from crop residues, 14.8 ± 8.7 from farmyard manure, 3.5 ± 4.5 from inorganic fertilizers, 2.7 from agroforestry, and 2.5 from improved fallow. However, the studies reviewed were mostly short-term and concentrated to a few locations. Future research should address these gaps.

Highlights

Soil carbon sequestration is considered to be a promising way of mitigating climate change by taking up atmospheric carbon dioxide (CO2) through plants and storing it as soil organic carbon (SOC) in decomposable plant residues, living biomass, and recalcitrant organic matter (Johnson et al 2007; Shelukindo et al 2014; Paustian et al 2016)
Soil organic carbon stocks in cropland and other vegetation types In the 0–30 cm depth interval, SOC stock (t C ha–1) in cropland ranged from 36.3 ± 27.7 to 116.3 ± 37.1 in semi-arid and sub-humid conditions, respectively, and was significantly lower than that in forests
This review sought to use available evidence to establish whether SOC in cropland can be built up through best management practices (BMPs)

Summary

Introduction

Soil carbon sequestration is considered to be a promising way of mitigating climate change by taking up atmospheric carbon dioxide (CO2) through plants and storing it as soil organic carbon (SOC) in decomposable plant residues, living biomass, and recalcitrant organic matter (Johnson et al 2007; Shelukindo et al 2014; Paustian et al 2016). Following conversion of native vegetation to cropland, soils are reported to lose SOC, which reaches a lower equilibrium of about 25–75% less carbon than in undisturbed native vegetation (Lal 2004). Soil organic carbon loss in cropland is mainly due to disturbance of soil aggregates through cultivation, which accelerates soil microbial activity, organic matter oxidation and mineralization (Six et al 1999; Loke et al 2012; Swanepoel et al 2016). Runoff, and leaching lead to further loss of SOC from cropland (Roose and Barthes 2001). The degree of SOC loss due to these processes is influenced by many factors, including climate conditions (Bispo et al 2017), altitude (Lemenih and Itanna 2004; Pabst et al 2013), soil texture (Tiessen et al 1984; Bruun et al 2015), and soil structure (Nsabimana et al 2009; Canarini et al 2017)

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