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

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 N-15 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 mu m). Microbial biomass N (MBN) increased by 21.5-130.9 % in soil aggregates, while the delta N-15 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 mu m) 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 N-min in bulk soil and the> 53 mu m size class, while the MBN is more important to net N-min in the < 53 mu m 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.

Automated summary

The addition of aged biochar decreased net N mineralization rate and hydrolytic enzyme activities, but increased microbial biomass N in soil aggregates. Moreover, biochar increased the labile C:N ratios in soil, indicating higher microbial N demand.