Biochar-driven biological regulation dominates acid-hydrolyzable nitrogen accumulation in plantation soils under acid rain stress

Abstract Acid-hydrolysable nitrogen (AHN), a crucial fraction of bioavailable soil organic nitrogen (N), is highly sensitive to soil acidification. Alkaline biochar (BC) has been shown to effectively mitigate acid rain (AR)-induced soil acidification. However, its regulatory effects and underlying mechanisms on AHN fractions remain largely unexplored. In this study, a field-scale simulated AR experiment was conducted in a Quercus acutissima plantation, utilizing BC derived from Q. acutissima litter to evaluate its impacts on AHN fractions and associated soil chemical-biological drivers. The results showed that after 2 years of simulated AR spraying, BC application elevated soil pH by 0.19 units under AR stress and increased total AHN content by 64.8%. Specifically, acid-ammonia N, acid-amino sugar N, acid-amino acid N, and acid-hydrolyzable unidentified N increased by 45.0%, 61.3%, 80.6%, and 60.7%, respectively. BC-amended soils under AR exhibited the highest bacterial network complexity (0.8), whereas fungal network connectivity was reduced. Soil chemo-biological interactions explained 23.1−39.7% of the variations in AHN fractions. Random forest modeling identified microbial N use efficiency as the primary factor influencing acid-ammonia N, and microbial biomass N as the key factor governing the accumulation of acid-amino acid N and acid-amino sugar N. Furthermore, the regulatory effects of BC on AHN fractions (0.77–0.98) surpassed those of AR stress. This study elucidates the mechanistic pathways through which BC modulates acid-induced N dynamics, providing insights for sustainable N management in plantation ecosystems affected by AR. Graphical Abstract