Multi-year nitrogen accumulation weakens the stabilizing effect of species asynchrony on drought resistance in a Tibetan alpine meadow

Abstract

Although the impacts of exogenic nitrogen (N) enrichment and drought on grassland community productivity are well known, their interactive effects on community stability remain insufficiently understood. In this study, a seven-year N enrichment experiment was conducted in a Tibetan alpine meadow to examine the effects of N-altered species richness, species dominance, species asynchrony, and pre-drought biomass on community stability, resistance, and resilience during two natural drought stresses (the first severe drought stress in 2015, FDS, and the second light drought stress in 2019, SDS). Our results indicate that N enrichment did not alter species richness but significantly reduced species asynchrony and dominance. Community stability showed a significant correlation with species asynchrony and dominance while remaining independent of species richness. Drought resistance, which was positively associated with community stability, was primarily influenced by species asynchrony and pre-drought biomass, rather than species dominance. Remarkably, the slopes of resistance in relation to the aforementioned factors were steeper in SDS than in FDS, despite SDS being lighter than FDS, indicating a more sensitive response to drought resistance to long-term N accumulation. In contrast, resilience was unaffected by species richness but increased with the rate of N addition. During the FDS, drought resistance and community stability were primarily governed by species asynchrony, while during the SDS, pre-drought biomass overwhelmingly determined drought resistance, surpassing the role of species asynchrony. These findings demonstrate the opposite effects of multi-year N accumulation on species asynchrony and pre-drought biomass, both of which reduce drought resistance and community stability over time. Therefore, our results contribute to a deeper understanding of grassland community stability in the face of N enrichment and natural drought stresses, aiding in predictions of grassland responses to ongoing global environmental changes.