An understanding of nitrogen (N) mineralization is essential for tracing the supply of inorganic N for plant uptake. However, how microorganisms regulate N mineralization for carbon (C) sequestration under field-aged biochar amendment remains unclear. To address this, we investigated the soil net N mineralization rate (net N_min), contents and hydrolytic enzyme activities of C and N, microbial biomass N, and native ¹⁵N values in bulk soil and aggregate size classes for six years after biochar application (20 and 40 t ha^-−1) in a typical rice-wheat rotation. The results showed that aged biochar decreased net N_min (normalized by total N content) by 10.5 %–69.9 %, and C and N hydrolytic enzyme activities per unit of microbial biomass C by 4.8–71.1 % and 24.0–77.8 %, respectively, compared with N fertilization in all soil aggregates except for ClayF size class (<2 μm). Microbial biomass N (MBN) increased by 21.5–130.9 % in soil aggregates, while the δ¹⁵N values decreased following biochar addition compared with those under N fertilization. The labile C:N ratios were higher in the bulk soil and MacroA size class (250−2000 μm) following biochar addition than under N fertilization, which would increase microbial N demand as evidenced by the lower enzymatic C:N ratios and higher MBN. Microorganisms obviously restrained net N_min but did not increase N hydrolytic enzyme activity to meet their stoichiometric N demands. Structural equation modeling revealed that enzymatic C:N stoichiometry is a dominant indicator of net N_min in bulk soil and the >53 μm size class, while the MBN is more important to net N_min in the <53 μm size class. We conclude that the addition of aged biochar could meet microbial stoichiometric requirements and regulate extracellular enzyme production, resulting in the decline of net N_min in soil aggregates, especially in MacroA size class.

了解氮（N）的矿化对于追踪植物吸收无机氮的供应至关重要。然而，在田间生物炭的田间老化过程中，微生物如何调节氮的矿化以实现碳（C）的固存尚不清楚。为了解决这个问题，我们研究了生物炭施用后六年土壤中的净氮矿化率（净N _min），碳和氮的含量和水解酶活性，微生物生物量氮以及散装土壤和集料尺寸类别中的天然¹⁵ N值在典型的稻麦轮作中为（20和40 t ha ^--1）。结果表明，老化的生物炭降低了净N _min（以总氮含量标准化）为10.5％–69.9％，每单位微生物量碳的碳和氮水解酶活性分别为4.8–71.1％和24.0–77.8％，与所有土壤团聚体中的氮肥相比，除ClayF尺寸等级（<2μm）。微生物生物量N（MBN）增加了在土壤中聚集体21.5-130.9％，而δ ¹⁵的N值与下氮肥相比以下生物炭除了减少。生物炭添加后，散装土壤和MacroA大小等级（250-2000μm）中不稳定的C：N比要高于氮肥，这将增加微生物对氮的需求，这由较低的酶促C：N比和较高的MBN所证明。微生物明显抑制了净N _min但并未增加N水解酶的活性以满足其化学计量的N需求。结构方程模型显示，酶促C：N化学计量是散装土壤和> 53μm尺寸类别中净N _min的主要指标，而MBN对于<53μm尺寸类别中的净N _min更重要。我们得出的结论是，添加老化的生物炭可以满足微生物化学计量要求并调节细胞外酶的产生，从而导致土壤团聚体中净N _min的下降，尤其是在MacroA大小等级中。