Chronic nitrogen enrichment decreases soil gross nitrogen mineralization by acidification in topsoil but by carbon limitation in subsoil
Lei Song, Jinsong Wang, Junxiao Pan, Yingjie Yan, Shuli Niu
Abstract
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 mineralization 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.