Carbon and nutrient cycling responses to repeated application of biochar and NPK fertilizers depend on microenvironmental differences among hierarchical aggregate fractions

The conversion of bambara groundnut seed residues into biochar facilitated the bioavailability and retention of more nutrients (C, N, P, K and Mg) after repeated application of biochar. However, the mechanisms of microbially mediated biochar-C degradation and nutrient cycling responses to repeated biochar application, particularly in different hierarchical soil aggregates, are largely unknown. A 20-day incubation experiment was conducted on Ultisols from a 4-year cucumber field trial in Nsukka, Nigeria. The effects of repeated application of bambara seed residue biochar with or without NPK fertilizer on soil aggregate associated electrical conductivity (EC), basal respiration, microbial biomass, and soil enzyme activities were determined. The results showed that the concentration of organic matter in the bulk soils increased with biochar and NPK+biochar treatments. These treatments also increased the soil EC, cumulative CO₂ respiration, microbial biomass C and N concentrations, and the activities of tyrosine-aminopeptidase and sulfatase enzymes, compared to the biochar treatment and the control soil. The NPK+biochar treatment contributed 35 % more to β-cellobiosidase activity, but the biochar treatment resulted in 85 % reduction in N-Acetyl-β-glucosaminidase activity, indicating microbial N mining. The NPK and NPK+biochar treatments accounted for a higher percentage of N-, C and N-, and S- cycle enzyme activities, although their composition was relatively higher with the latter treatment. The overall soil biochemical responses were significantly higher in the micro-aggregates (< 0.25 mm) than in the macro-aggregates (≤ 4.75–0.25 mm); least of all in the small macro-aggregates (0.25–1.00 mm). Therefore, repeated application of biochar to N-deficient soils generally does not result in positive soil biochemical responses. However, repeated application of biochar together with NPK fertilizer modulates N limitation and optimizes microbial nutrient cycling processes, especially in micro-aggregates.