Biochar‐Induced Shifts in Soil Nitrogen Fractions and Microbial Nitrogen Demand After a Decade of Conservation Tillage

ABSTRACT The effects of field‐aged biochar on soil nitrogen (N) transformation differ from those of fresh biochar. The modification of predominant N turnover processes subsequently influences soil N accumulation and availability. However, the long‐term impacts of aged biochar on in situ native soil N fractionation (excluding biochar‐derived N) and the mechanisms by which microbial communities influence available N supply remain largely unexplored. An 11‐year experiment investigated the effects of different biochar application rates (0, 30, 60, and 90 Mg ha −1 ) on total native N, soil organic N (SON) fractions, total dissolved N (TDN), microbial carbon to nitrogen imbalance (MI C/N ), and gross N mineralisation rates (G N‐min ) in calcareous soil (0–15 and 15–30 cm) under a winter wheat‐maize rotation. The SON fractions included labile pools (ammonia‐SON and amino acid‐N), less labile pools (amino sugar‐N and hydrolysable unknown‐N), and a recalcitrant pool (acid insoluble‐N). Results demonstrated that biochar application significantly increased soil native total N (by 7%–8%) and native recalcitrant SON (by 52%–104%) in the 15–30 cm layer. Biochar also significantly increased labile SON pools (except at 30 Mg ha −1 ) and reduced less labile SON pools compared to unamended soil at both depths. During rapid crop growth stages, biochar‐amended soil showed an upward trend in TDN content and G N‐min value in the top 15 cm. Structural equation modelling (SEM) showed that increased available N was primarily driven by accelerated turnover of labile SON influenced by biochar content, rather than by higher labile SON content. In November, biochar application significantly decreased TDN contents and MI C/N values, indicating reduced microbial N‐mining in aged biochar soils. SEM further implied that MI C/N played a crucial role in limiting the degradation of less labile SON over winter. In conclusion, long‐term biochar application modified soil N fractions and influenced N turnover across different growth stages by regulating soil microbial N demand. These findings underscore the potential of biochar to sustain soil fertility and support the ecological sustainability of agricultural systems, particularly when combined with soil organic carbon management.