Unveiling the role of foliar fungi in mediating leaf photosynthesis under global change

Abstract Global nitrogen (N) deposition and changes in precipitation patterns profoundly influence leaf photosynthesis and carbon sequestration in terrestrial ecosystems. Foliar fungi, a critical microbial community on leaf surfaces, could simultaneously impact several leaf functions. However, the regulated effects of foliar fungi on leaf photosynthesis in response to nitrogen and water additions remain unclear. Here, using a long‐term field and a complementary pot fungicide experiment in an alpine meadow, we quantified the effects of nitrogen and water addition on leaf photosynthesis and foliar fungal communities in two dominant species. We found that, under the combined effects of nitrogen and water addition, the light‐saturated CO 2 assimilation capacity ( A sat ) of two dominant species ( Carex alatauensis and Leymus secalinus ) increased significantly. Moreover, nitrogen and water additions significantly increased leaf N content, which in turn reduced foliar fungal diversity, enhanced stomatal conductance, and ultimately improved A sat . Importantly, the fungicide experiment demonstrated that foliar fungi removal could improve A sat , highlighting the negative effects of higher foliar fungal Shannon diversity on leaf photosynthesis. Synthesis . Our findings found that foliar fungi mediate photosynthetic responses to resource enrichment. These findings extend the mechanistic understanding of how foliar fungi mediate leaf photosynthesis and plant growth under global change, offering insights into microbial regulation of photosynthesis in response to varied resource availability.