What is the potential for biogas digesters to improve soil carbon sequestration in Sub-Saharan Africa? Comparison with other uses of organic residues

Using bioslurry from anaerobic digestion as an organic fertilizer has great potential to increase carbon sequestration by supplying organic matter to the soil. This paper examines this potential in Sub-Saharan Africa compared to other uses of organic residues, including burning on pyrolysis cook-stoves and composting. Measurements of loss of carbon on treatment of organic residues indicate that the proportion of carbon lost from organic residue during treatment is greater for anaerobic digestion than for aerobic composting or pyrolysis. The stability of organic residue is increased by treatment, and is similar for composted and anaerobically digested material, but is higher for material treated by pyrolysis. Simulations using the RothC model, driven by parameters based on incubations of the organic residues with soil, suggest that on the basis of decomposability alone, treated organic residues sequester significantly more carbon than untreated organic residues, and despite the differences observed in stability, unless biochar contains a high proportion of inert organic material that does not decompose at all, the potential carbon sequestration by incorporating biochar is similar to that for compost or bioslurry. However, if losses of carbon during treatment are also taken into account, incorporating bioslurry sequesters only approximately the same amount of carbon as if the organic residue had been left untreated. By contrast, incorporating compost and biochar sequesters significantly more carbon than incorporating the untreated organic residue. Therefore using bioslurry as an organic fertilizer sequesters less of the carbon in the soil from organic residue than burning on pyrolysis cook-stoves or composting.