Nitrogen addition regulates community biomass resistance and recovery to drought by altering species asynchrony rather than the diversity response

Extreme climatic events often co-occur with persistent environmental disturbances, such as nitrogen enrichment, which may influence the resistance and recovery of plant communities to extreme climatic conditions. However, most studies have focused on the resistance and recovery of community functions (e.g. biomass) to climatic events while neglecting the corresponding responses of diversity. Here, we performed a soil nitrogen addition experiment in an alpine meadow from 2011 to 2020, with 2015 characterized by extreme drought. We explored the effects of nitrogen addition on the resistance and recovery of plant community biomass and diversity in response to extreme drought using measures including community biomass, taxonomic diversity (TD), phylogenetic diversity (PD), and functional diversity (FD). We found that nitrogen addition decreased biomass resistance, mainly due to species asynchrony rather than the diversity resistance, even though PD and FD resistance also declined. Meanwhile, nitrogen addition enhanced the recovery of biomass to drought. This was mainly attributable to the direct, positive impact of nitrogen on biomass recovery, coupled with an indirect influence of species asynchrony, without any diversity (TD, PD, FD) recovery’s effects. Our results indicate that soil nitrogen enrichment mainly influences plant biomass responses to extreme drought, with a relatively small effect on plant diversity. Additionally, the mechanisms driving diversity and biomass responses may operate independently, as changes in diversity response did not scale up to changes in biomass. We anticipate that maintenance of plant community biomass during extreme drought would be more challenging in conditions of high nitrogen deposition.