The combined application of nitrogen and biochar reduced microbial carbon limitation in irrigated soils of West African urban horticulture

Background: Intensive wastewater irrigated urban horticulture in sub-Saharan West Africa receives high nutrient inputs, which lead to large gaseous and leaching losses. The addition of biochar to the usually sandy soils may reduce these losses and improve the habitat conditions for soil microorganisms. Two similar experiments focused on crop yields and nutrient balances have been carried out over a 2-year period in semi-arid Ouagadougou, Burkina Faso, and in sub-humid Tamale, Ghana, representing to some extent different but typical locations in West Africa. // Methods Biochar and N fertilization effects were measured on soil microbial biomass carbon (MBC), fungal ergosterol, and functional diversity, estimated by multi-substrate-induced respiration. It was additionally possible to study the effects of clean water irrigation on the respective microbial properties in Tamale soil. // Results Sole biochar addition did not affect any soil chemical or soil biological properties analyzed. In contrast, biochar application with N fertilization increased the mean respiratory response of the 11 substrates added by 23% in the Ouagadougou soil and by 13% in the Tamale soil. N fertilization decreased soil pH in both cities by 1.1 units. However, a pH-H₂O of 4.7 led to reduced MBC and ergosterol contents at Tamale. Also, the Shannon index of the respiratory response was positively correlated with the soil pH. Clean water irrigation decreased the ergosterol content and increased the respiratory response to organic acids. Conclusions Biochar addition with N fertilization improved habitat conditions for soil microorganisms. An N fertilizer-induced decline in soil pH < 5 should be avoided, as it decreased MBC and microbial functional diversity.