Chronic nitrogen enrichment decreases soil gross nitrogen mineralization by acidification in topsoil but by carbon limitation in subsoil

Continuously increasing nitrogen (N) deposition alters soil N mineralization, which is of vital importance in regulating soil N availability and satisfying plant N demands. Neither the response pattern of soil N minerali-zation to the chronic N enrichment nor the related mechanism has been well studied, particularly in deep soils. In this study, we measured the gross N mineralization rate (GNMR) as well as the corresponding soil abiotic and biotic properties along a chronic N addition gradient (7 years) at different soil depths in an alpine meadow. The results showed that GNMR was negatively correlated with the N addition rate and soil available NO3- content at each soil depth (P < 0.05). GNMR was positively correlated with soil pH, which was significantly decreased by N addition in the topsoil (0-10 cm, P < 0.05), but GNMR was not related to pH in the subsoil (10-20 or 20-40 cm). Instead, it was positively correlated with the soil carbon/nitrogen ratio (C/N), which was significantly decreased by N addition in subsoil (P < 0.05). Soil pH and C/N mainly explained the decreased GNMR with N enrichment in the topsoil and subsoil, respectively, and the contribution of the soil C/N to the decreased GNMR at the 20-40-cm soil depth was larger than that at the 10-20-cm soil depth. The results demonstrated that acidification, increased N availability, and C limitation all decreased soil N mineralization under chronic N deposition, but acidification dominated the decrease in the topsoil, while C limitation dominated the decrease in the subsoil. These findings extend our understanding of the response pattern and mechanism of soil N mineralization with chronic N enrichment at different soil depths. The diverse mechanisms underlying soil N mineralization in response to chronically increasing N deposition at different soil depths should be considered when predicting soil N mineralization in the context of global change.