Canopy mitigates the effects of nitrogen deposition on soil carbon-related processes in a subtropical forest

Highlights: • Forest soil carbon responds to canopy and understory nitrogen addition is different. • Canopy nitrogen addition does not enhance soil organic carbon content. • Understory nitrogen addition significantly increased soil organic carbon content. The rapid increases in atmospheric nitrogen (N) deposition have greatly affected the carbon (C) cycles of terrestrial ecosystems. Most studies concerning on the effects of N deposition have simulated N deposition by directly applying N to the understory and have therefore not accounted for the possibility of N absorption, retention, and transformation by the canopy. In this study, we compared the effects of understory addition of N (UN), canopy addition of N (CN) at 25 and 50 kg N ha⁻¹ yr⁻¹, and ambient addition of N (CK) on soil carbon-related processes in a subtropical forest. After seven years of addition, the contribution of new C from litter (F_new) was more than 2× greater with UN treatments than with CN treatments. UN treatments significantly increased the activity of β-1,4-glucosidase (BG) but reduced the activities of β-1,4-N-acetylglucosaminidase (NAG), polyphenol oxidase (PPO), and peroxidase (PER). CN treatments, in contrast, did not alter the activities of extracellular enzyme. Compared to CN, UN treatments significantly enhanced soil organic carbon (SOC) and mean weight diameter (MWD, represents soil aggregate stability). Differences in the responses of SOC and MWD to CN and UN treatments were positively correlated with F_new but negatively correlated with the activities of PPO and PER. The results imply that forest canopy mitigates the effects of atmospheric N inputs on SOC, and that conventional understory N addition might overestimate the positive effects of N deposition on forest soil C-related processes. We suggest that CN rather than UN should be used to simulate the effects of atmospheric N deposition on soil C dynamics in subtropical forests.