Heavy grazing intensifies antagonistic effects of nitrogen and phosphorus addition on the water use efficiency of grasslands in Inner Mongolia

Abstract Background Plant water use efficiency (WUE) is a key indicator of coupling between the carbon and water cycles in grassland ecosystems and is influenced by both grazing and nutrient availability. However, the regulatory effects of grazing intensity on plant WUE under nitrogen (N) and phosphorus (P) addition remain unclear. Methods This study employed stable carbon isotope techniques in a typical steppe ecosystem in Inner Mongolia, China, to investigate the effects of grazing, N and P addition, and their interactions on the foliar δ 13 C values of four dominant plant species. This research aims to assess the dynamics and underlying mechanisms of grassland WUE under grazing management in the context of global change. Results Grazing intensity and nutrient addition significantly affected the δ 13 C leaf values. Heavy grazing increased the δ 13 C leaf values of Cleistogenes squarrosa and Carex korshinskyi but decreased those of Leymus chinensis and Stipa grandis . N addition increased δ 13 C leaf across all species, whereas P addition significantly increased δ 13 C leaf in Cleistogenes squarrosa but decreased it in the other species. A significant antagonistic interaction between N and P addition was observed, where P inhibited the positive effects of N. This antagonistic effect was amplified under heavy grazing, leading to reduced plant WUE. Structural equation modeling analysis further identified soil temperature and leaf total N as the dominant factors governing plant WUE dynamics, with both factors contributing to an increase in plant WUE. Conclusions This study highlights that grazing can modulate the effects of N and P addition on WUE in grasslands. Specifically, heavy grazing exacerbated the antagonistic effects of N and P addition on plant WUE, suggesting that overgrazed grasslands under N and P enrichment could face increased water limitation. In light of concurrent environmental changes, future grassland management should consider the interactive effects between grazing intensity and nutrient dynamics on plant water relations, growth and plant community composition.