Besides carbon sequestration, biochar amendment in soils can regulate the loss of applied fertilizer-nitrogen (N) into the environment. Soil aggregates are essential in controlling soil N stocks' stabilization and supply. However, unraveling the rate-dependent impact of biochar on stable soil aggregates and their associated N in fertilized soils over an extended period is a prerequisite to understanding its implications on soil-N dynamics. We unraveled how high and low biochar application rates combined with inorganic fertilizer (20- and 40-tons ha−1 [FB1 and FB2]) affected soil N fractions, stable soil aggregates, aggregates associated-N, and microbial responses to regulate N supply for Nageia nagi after one year. Results revealed that biochar amendment, especially at higher rate, increased the concentration of inorganic N and some amino acids compared to sole fertilizer (F). Available N increased by 16.5% (p = 0.057), 23.8% (p = 0.033), and 34.8% (p = 0.028) in F, FB1, and FB2, respectively, compared to the control (C). Also, 28.1% and 32.8% significant increases in the availability of NH4+-N were recorded in FB1 and FB2, respectively, compared to F. NO3−-N availability was significantly increased by 15.2%, 21.8%, and 20.8% in FB1, FB2, and C, compared to F. Biochar amendment, irrespective of rate, increased stable microaggregates (< 0.25 mm). However, FB2 significantly increased macro- and intermediate-aggregate-N, and urease activity, and hence higher N supply capacity to meet the N need of N. nagi even after one year. Hence, the N content of N. nagi was 41.3%, 28.8%, and 12.2% higher in FB2, FB1, and F, respectively, compared to the control. Biochar amendment decreased bacterial species diversity but increased the proportion of NH4+-oxidizers (especially the Betaproteobacteria) to maintain the mineralization and slow release of N. Although a low biochar rate was more beneficial than sole fertilization, higher biochar application rate could sustain higher N supply by stabilizing soil microaggregates and increasing macro- and intermediate-aggregates N, its mineralization, and slow-release over longer periods.
