Canopy and understory nitrogen additions differentially regulate soil organic carbon fractions via litter–microbe–mineral interactions

The effects of nitrogen (N) deposition on forest soil organic carbon (SOC) are largely unclear, likely due to the divergent responses of particulate (POC) and mineral-associated carbon (MAOC). Conventional understory inorganic N (UIN) additions neglect canopy processes and the impacts of organic N, potentially misevaluating N deposition effects. This study was conducted in a long-term N addition experiment established in a Moso bamboo forest, which included six treatments combining canopy and understory N additions with organic (urea + glycine) and inorganic (NH₄NO₃) forms at a rate of 50 kg N·ha^-1·yr^-1. Litterbags were installed for a two-year decomposition experiment and collected at quarterly intervals, together with concurrent soil sampling under litterbags at 0–10 cm depth. We aimed to examine the effects of canopy vs. understory N addition and organic vs. inorganic N form on soil POC and MAOC concentrations. Our results showed that canopy N additions significantly reduced POC (-15.9%) but did not affect MAOC (P > 0.05). Conversely, understory N additions significantly increased POC (+30.9%) and decreased MAOC (-28.9%). Canopy N additions decreased POC by enhancing peroxidase activity and fungal diversity (FuD), while understory N additions promoted POC by inhibiting litter decomposition. Additionally, understory N addition-induced soil acidification decreased soil Ca²⁺ concentration, microbial carbon use efficiency, and bacterial necromass C, as well as the release of litter water-soluble compounds, thereby inhibiting MAOC. Moreover, nitrogen forms (organic vs. inorganic) had no effect on SOC fractions. Our findings underscore that canopy and understory N addition approaches differentially regulate SOC fractions by altering litter decomposition–microbial–mineral interactions, and the understory approach may overestimate soil POC gain and MAOC loss driven by atmospheric N deposition.