Nitrogen Addition Alleviates Microbial Nitrogen Limitations and Promotes Soil Respiration in a Subalpine Coniferous Forest

Soil microbes are an important component of soil ecosystems that influence material circulation and are involved in the energy flow of ecosystems. The increase in atmospheric nitrogen (N) deposition affects all types of terrestrial ecosystems, including subalpine forests. In general, alpine and high-latitude ecosystems are N limited. Increased N deposition could therefore affect microbial activity and soil respiration. In this study, four levels of N addition, including CK (no N added), N1 (2 g m(-2) a(-1)), N2 (5 g m(-2) a(-1)), and N3 (10 g m(-2) a(-1)), were carried out in a Sichuan redwood forest at the eastern edge of the Tibetan Plateau. The dynamics of soil respiration, major microbial groups, ecoenzymatic stoichiometry, and microbial biomass carbon and nitrogen (MBC and MBN, respectively) were investigated over a year. The results showed that N application significantly increased soil respiration (11%-15%), MBC (5%-9%), MBN (23%-34%), N-acetylglucosidase (56.40%-204.78%), and peroxidase (42.28%-54.87%) activities. The promotion of soil respiration, N-acetylglucosidase, and peroxidase was highest under the N2 treatment. The carbon, nitrogen, and phosphorus metabolism of soil microbes in subalpine forests significantly responded to N application. In the latter stages of N application, microbial metabolism changed from being N restricted to phosphorus restricted, especially under the N2 treatment. Soil bacteria (B) and gram-positive (G(+)) bacteria were the dominant microbial groups affecting soil respiration. Structural equation modelling indicated that N application significantly promoted soil respiration and microbial biomass, whereas the main microbial groups did not significantly respond to N application. Therefore, we conclude that short-term N addition alleviates microbial nitrogen limitation and promotes soil respiration in the subalpine forest ecosystem that accelerates soil carbon (C) and N cycling. Continuous monitoring is needed to elucidate the underlying mechanisms under long-term N deposition, which may help in forecasting C, N, and P cycling in the alpine region under global climate change.