Biochar Application to Soil for Increased Resilience of Agroecosystems to Climate Change in Eastern and Southern Africa

Abstract

With the current unreliable rainfall pattern, which is expected to worsen due to climate change, agricultural production might become more challenging especially among resource-poor farmers in Eastern and Southern Africa. This calls for adaptation of farming systems to overcome this emerging challenge. Biochar, a product of biomass pyrolysis, with long-term evidence from Amazonia, might contribute to a climate-resilient farming system. This is due to its positive effects on soil chemical and physical properties resulting in increased crop yields, which has been experimentally demonstrated largely within the last two decades. In acidic low cation exchange capacity (CEC) soils, biochar derived from corncob at 5% application rate, for example, increased pH by ≥1 unit and CEC by ≥2 cmol_c kg⁻¹ in addition to direct nutrient supply. Increased CEC may be linked to the observed increase in soil organic carbon content (biochar carbon/sequestered carbon) due to biochar addition. Sequestration of carbon due to biochar has been reported to be stronger in soils that have low pH and low carbon contents, with greater effects from biochars produced from woody materials or those produced at high temperature. Such soils with low pH and carbon contents are common in tropical areas. Another effect of biochar at field-relevant doses of ≤5% is the improvement of physical properties such as increased aggregate stability by up to 7%, increased aggregate mean weight diameter by 8–13%, increased soil water contents and reduced soil temperature fluctuations. Similar changes to soil properties have been found to increase yield by 10% based on global dataset, but up to fourfold increases have also been reported for acidic low CEC soils. One key challenge to implementation of biochar technology for increased yields is probably the unavailability of large quantity of biochar often in tens of Mg ha⁻¹ that is required for field application. Conservation agriculture (CA) with its three principles of minimum tillage, residue retention and crop diversification may partly address this key challenge of biochar implementation. This can be achieved by applying biochar only in the tilled part of land in a minimum tillage operation, which can be only 10% of the land surface. In this way, biochar may increase the positive attributes of CA such as increased rainwater harvesting and crop yields. Further solution to the quantity of biochar material for field application is the application of limited quantity of biochar in combination with traditional amendment such as manures either in mixtures or after co-composting. In this way, biochar is expected to deliver its benefits to manure resulting in improved soil properties and increased yields. Overall, incorporation of biochar in CA-based farming system alone or in combination with manures might contribute to building of climate-resilient agroecosystem.