In situ effects of biochar field-aged for six years on net N mineralization in paddy soil

Abstract

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 Nmin), 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 Nmin (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 Nmin 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 Nmin in bulk soil and the >53 μm size class, while the MBN is more important to net Nmin 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 Nmin in soil aggregates, especially in MacroA size class.