Contrasting responses of soil organic carbon dynamics to long-term canopy and understory nitrogen addition in a subtropical forest

Elucidating the impacts of chronic atmospheric nitrogen (N) deposition on soil organic carbon (SOC) is crucial for predicting the dynamics of terrestrial C sinks, particularly in N-rich subtropical forests. Experiments using understory N addition (UN) have provided valuable insights into these impacts, but unavoidably neglect processes such as interception and absorption of N within forest canopy. We assessed the effects of long-term (11-yr) fertilization via both canopy N addition (CN) and UN on SOC in a subtropical forest. Our results showed significantly different responses of SOC between the approaches, with UN displaying greater effects on SOC than CN. Specifically, both low and high rates of UN substantially increased the concentrations of particulate organic C (POC), whereas the high rate of CN significantly increased those of mineral-associated organic C (MAOC) rather than POC. Long-term CN and UN treatments had distinct effects on plant- and microbial-derived C processes. UN treatments significantly increased soil available N and improved the litter quality, enhancing the formation of POC, and suppressing microbial decomposition of POC due to the significant decreases in soil pH. However, CN treatments significantly improved litter quality and mitigated soil acidification, thus stimulating microbial C utilization and accelerating the microbial transformation of POC to MAOC. Our findings imply that the underlying mechanisms of natural N deposition influencing forest SOC may differ from those obtained from UN, and conventional fertilization experiments may overestimate the benefits of elevated N deposition to forest SOC.