Differential response of C3 and C4 plants in temperate grasslands to different grazing intensities and nitrogen addition

Plant functional traits serve as key indicators of species' adaptive strategies to environmental conditions, which are employed to elucidate biomass responses to diverse ecological drivers. Grazing is the primary method of managing and utilizing grasslands, while nitrogen deposition is a growing aspect of global environmental change. While previous research has primarily focused on the impacts of grazing or nitrogen addition on C₃ or C₄ plants, relatively few studies have systematically compared the differences in their functional trait adaptation strategies under combined gradients of grazing intensity and nitrogen enrichment. This study was conducted for 4 years on temperate semi-arid grasslands. Three grazing treatments—NG (no grazing), LG (light grazing), and HG (heavy grazing)—were applied alongside two nitrogen addition gradients. Leymus chinensis (C₃) and Cleistogenes squarrosa (C₄). The results demonstrated that grazing intensity significantly reduced individual biomass by 70.5 % in L. chinensis and 78.2 % in C. squarrosa. Key functional traits such as plant height, leaf length, stem length, leaf area, stem dry weight, and leaf dry weight also declined significantly. The L. chinensis adapted to grazing stress achieved by reducing leaf weight and maintaining growth in a small and dense form. In contrast, C. squarrosa adapted by sacrificing leaf number and maintaining leaf width. Nitrogen addition mitigated the negative effects of grazing on both species. Under grazing conditions, C. squarrosa showed higher specific leaf area (increased by 16.3 %) and a more sensitive response to nitrogen, reflecting enhanced resource acquisition and stress tolerance. These findings suggest that moderate nitrogen addition can support the restoration and long-term sustainable utilization of the grazing ecosystem, and highlight the critical role of C₄ plants in grassland ecosystems in the context of future global change.