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

The rapid increases in atmospheric nitrogen (N) deposition have greatly affected the carbon (C) cycles of terres-trial 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 understoly 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 2x greater with UN treatments than with CN treatments. UN treatments significantly increased the activity of beta-1,4-glucosidase (BG) but reduced the activities of beta-1,4-N-acetylglucosaminidase (NAG), polyphenol oxidase (PPO), and peroxidase (PER). CN treatments, in contrast, did not alter the activities of extracellular enzyme. Com-pared 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. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

The study compared the effects of different nitrogen addition methods on soil carbon processes in a subtropical forest, finding that understory nitrogen addition was more beneficial for increasing soil organic carbon and had a positive impact on soil hardness compared to canopy nitrogen addition.