Predicting biochar production and carbon sequestration for soil productivity in eastern Uganda

Nakubulwa, D. (2019). Predicting biochar production and carbon sequestration for soil productivity in eastern Uganda. Kampala, Uganda: Makerere University (67 p.).

Biochar, a product of biomass gasification is used to sequester carbon in soils for long periods of time and intensification of agricultural productivity. Crop residues are an important source of biomass for biochar production but availability in smallholder farming systems across Sub Saharan Africa (SSA) is unknown due to the cost and labor involved in obtaining related empirical data. The study presented here was carried out in Pallisa district, Uganda and (1) mapped crop residue yields and current usages, (2) developed allometric equations to quantify crop residues and (3) assessed the amounts of biochar carbon that can potentially be sequestered. The study focused on residues from major staple crops being; maize, millet, rice, sorghum and groundnut considering 12 mono-cropped farmer fields per crop, over two growing seasons. It was estimated that the availability of raw residues measured on average; 3.71, 4.99, 5.5, 5.87 and 12.96 tons ha-1 for groundnut, maize, sorghum, rice and finger millet respectively. Allometric models based on plant height and density without interaction were found to provide reliable estimates of the total yield of individual crop residues. Hence, these tools can help save time and cost to map the sources of crop residues at large scale. The amounts of carbon that would be sequestered through crop residue derived biochar were determined using a soil carbon balance model that accounts for the loss over time. Results show that residues from millet production had the highest potential for sequestering carbon, measuring on average 3.35 tons C ha-1 yr-1, whereas that of residues from maize, sorghum and rice amounted to 1.12 tons C ha-1 yr-1. This supports the viability for producing biochar from crop residues and sequestering carbon in these smallholder farmer systems.