Simultaneous Nitrogen Addition and Warming Cause More Plant Species Loss Than Single Factors Alone

Aims Warming and nitrogen enrichment are among the most important global environmental changes. While both warming and nitrogen enrichment may reduce plant species diversity in many herbaceous communities, their combined effects remain inadequately quantified. Methods In this study, we conducted a 9‐year warming and nitrogen addition experiment in a semiarid grassland, focusing on the potential mechanisms underlying plant species loss. Results Our findings revealed that while neither warming nor nitrogen addition alone altered plant species richness, their combination effect significantly reduced species richness and Shannon–Wiener index by 12% and 20%, respectively. Structural equation modeling identified soil water content as the primary mediator of change in plant diversity under nitrogen addition and warming, with a decline in species richness as soil water content decreased. We propose soil water limitation as a novel mechanism driving decline in plant diversity under nitrogen addition and warming. Conclusion Our study suggests that simultaneous nitrogen addition and warming cause more plant species loss than single factors alone and thus highlights the necessity of reducing nitrogen inputs under future warmer climate.