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.
Ding et al. (Wed,) studied this question.