Six years of nitrogen addition reduced ecosystem carbon sequestration capacity in a subtropical forest

Nitrogen (N) deposition profoundly influences carbon (C) cycling in terrestrial ecosystems. However, integrated studies on dynamics of net ecosystem C stock (NEC) under N deposition in subtropical forests remain limited, creating uncertainty in assessing their C sequestration potential. We conducted a 6-year field experiment using a randomized block design to investigate the effects of N addition at three levels (0, 30, and 60 kg N·ha⁻¹·year⁻¹) on NEC and its components—aboveground C stock (AGC), belowground C stock (BGC), forest litter C stock (FLC), fine root C stock (FRC), and heterotrophic respiration C efflux (RhC). N addition significantly reduced AGC, BGC, FRC, and RhC, but increased FLC. As a result, NEC declined with N addition, with AGC contributing most to this reduction and FLC the least. The N-addition-induced reduction in soil water content appeared to be the primary driver of decreases in AGC and BGC and indirectly reduced FRC via suppressed fine root biomass. RhC dynamics were more strongly governed by fine root biomass than by microbial traits, thereby partially mitigating the NEC loss. While N addition rates had limited effects on NEC and most C stock components, RhC was significantly affected. These findings suggest that medium- to long-term N deposition may reduce the C sequestration capacity of subtropical forests. This study provides new insights for accurately assessing C sequestration potential under increasing N deposition.