Influence of simulated warming on soil nitrogen fractions in a Tibetan alpine meadow

Purpose Tibetan Plateau is ecologically fragile and sensitive region to global climate change. However, the impacts of climate change on nitrogen (N) cycling in soil remain unclear, which have restricted our knowledge of how N cycling in alpine ecosystems responds to climate change in the future. Methods In this study, a 5-year field simulated warming experiment using open top chambers was performed in a Tibetan alpine meadow to explore the impacts of winter warming (WW) and year-round warming (YW) on total N and N fractions in soil. The relationships between N fractions and environmental factors were evaluated. Results Relative to unwarmed control (CK), WW and YW did not significantly affect total N, NH4+-N, NO3--N, and total organic N contents. The alkali-hydrolyzable N and acid-hydrolyzable N (ammonia N, amino acid N, amino sugar N, and hydrolyzable unknown N) contents were significantly lower while acid-insoluble N content was significantly higher in WW and YW than in CK. Redundancy analysis showed that plant aboveground biomass was the most significant factor in shaping total N and N fractions. Structural equation modeling showed that fungi had direct negative effect but actinomycetes exhibited direct positive effect on hydrolyzable unknown N, which exhibited direct positive effect on plant aboveground biomass. Conclusion Our results suggested that climate warming promoted accumulation of highly stable organic N fraction, which may cause the decline of soil N supplying capacity. This study provides a novel understanding about the responses of alpine grassland to future climate change in terms of N cycling.