Isotopic Constraints on Seasonal Nitrogen Transformation Dynamics along a Deposition Gradient in Subtropical Forests

The progression of subtropical forest ecosystems toward nitrogen (N) saturation under chronic atmospheric deposition remains poorly quantified, challenging the universality of temperate-derived N saturation frameworks. Here, we integrate multiisotopes with a novel isotope mass-balance model to resolve responses of N transformation pathways across seven subtropical forests spanning a natural N deposition gradient (15.6-77.9 kg N ha^-1 yr^-1). Results revealed significant organic-mineral N exchange via simultaneous heterotrophic nitrification and microbial assimilation in subtropical forests. Crucially, N-saturated sites exhibit intense monsoon-season denitrification (29.0-111 kg N ha^-1 yr^-1), removing >96% of deposited N and outstripping fluxes in N-limited sites by 44-fold. This seasonal pulse correlates with high nitrate (NO₃^-) availability from heterotrophic nitrification, transient soil anoxia during concentrated rainfall, and elevated temperatures. These findings provide novel mechanistic insights, revealing coupled organic-mineral N exchange and unexpectedly large seasonal denitrification pulses and refining the N saturation framework for humid subtropical ecosystems. They highlight the critical, previously underestimated role of seasonally activated anaerobic pathways and underscore the necessity of incorporating these climate-sensitive gaseous loss processes into regional and global N budgets and models.