Anthropogenic Nitrogen Deposition Increases Soil Carbon by Enhancing New Carbon of the Soil Aggregate Formation

Anthropogenic nitrogen (N) deposition can most likely increase temperate forest soil organic carbon (SOC) storage. Increased SOC is usually suggested to be associated with the suppression of SOC decomposition, which has been hypothesized to be due to the decrease in the activity of lignin‐degrading extracellular enzymes and/or the decrease in soil acidity under N addition. However, the potential mechanism of SOC protection derived from N addition is less understood. Here in a low‐deposition temperate montane forest, short‐term N addition could increase SOC storage in the aggregate fraction but not in the bulk soil. N‐induced SOC accumulation was partly associated with the suppressed SOC decomposition (indicated by lower soil respiration) that resulted from the reduced microbial biomass rather than from decreased lignin‐degrading enzyme activity or from reduced soil acidity. In addition, N addition promoted soil aggregate formation, which could partly suppress SOC decomposition by protecting new carbon that originated from plant litter residue to a greater degree, while dissolved organic carbon retention in the mineral soils played a limited role in the SOC sequestration derived from N addition, at least in the short term. A conceptual model was proposed and highlighted a new underlying mechanism of new carbon protection by enhanced aggregate formation, other than the role of microbial suppression, to explain the positive effect of anthropogenic N deposition on forest SOC.