Effect of biochar amendment on soil enzymatic activities, carboxylate secretions and upland rice performance in a sandy clay loam Alfisol of Southwest Nigeria

Field studies were conducted over two years to determine changes in soil biochemical properties and rain-fed upland rice yield as influenced by biochar amendment. Treatments consisted of incorporation of rice husk biochar at different rates (B1 = 0, B2 = 3 t ha−1, B3 = 6 t ha−1, and B4 = 12 t ha−1). Soil enzymes viz invertase, alkaline phosphatase, urease, and catalase, were significantly (P ˂ 0.05) higher under the highest application rate of biochar (12 t ha−1). Higher rates of enzyme activities were recorded within the top soil layer (0–0.1 m). Soil organic carbon, available nitrogen, Bray 1 phosphorus, total nitrogen and soil moisture content were significantly (P ˂ 0.05) increased upon the application of biochar at a rate of 12 t ha−1 in the top 0–0.1 m layer. Similarly, application of biochar at 12 t ha−1 significantly (P < 0.05) increased rice rhizospheric carboxylate secretions in the top 0–0.1 m layer. Carboxylate exudates were increased in the order of citrate ˃ malate ˃ acetate ˃ oxalate, while increasing soil layer reduced carboxylate secretions. In general, biochar application improved soil enzyme activities, rhizospheric secretions involved in C, N, P cycling and nutrient solubilization which influence soil nutrient retention, availability and crop performance. Biochar application at 3–6 t ha−1 significantly (P ˂ 0.05) increased rice grain yield by 46% and straw yield by 47%. Keywords: Alfisol, Biochar, Carboxylates, Oryza sativa, Soil enzymes

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