Abstract: Plant microbiome plays a vital role in plant fitness and ecosystem functioning, yet its response to global environmental change remains poorly understood. Using an 18-year field experiment, we investigated the effects of climate warming and nitrogen deposition on the diversity of leaf epiphytic and endophytic bacterial communities in two dominant plant species (Stipa breviflora and Cleistogenes songorica) of a temperate desert steppe. We found that warming and nitrogen addition increased both epiphytic and endophytic bacterial diversity, but via different mechanisms. Specifically, epiphytic diversity increased with leaf temperature and transpiration rate, whereas greater endophyte diversity was linked to higher leaf carbon and nitrogen concentrations. Structural equation modeling revealed that both epiphytic and endophytic diversity were negatively associated with plant diversity. Our results demonstrate different mechanisms driving similar responses of leaf epiphytic and endophytic bacterial diversity to global change, and point to a negative feedback loop between phyllosphere bacterial and plant diversity.