Divergent effects of fertilization and grazing exclusion on species diversity in alpine grasslands

Fertilization and fencing are two widely used practical methods for grassland management in Qinghai-Tibet Plateau of China. While previous studies have focused on the direct effects of these practices on species diversity, research examining their indirect effects through multiple abiotic factors such as soil nutrient, light availability, soil temperature and moisture, as well as the underlying mechanisms, remains insufficient. In this study, we conducted a five-year field experiment with fertilization and grazing exclusion treatments, measuring soil temperature, moisture, carbon-to‑nitrogen ratio (C:N), and light (canopy light transmittance) in 2021 and 2023 to assess their roles in driving species richness and beta diversity. Our results showed that both fertilization and grazing exclusion significantly reduced species richness and increased beta diversity, with a significant interaction between the two treatments. Species richness increased with soil temperature, moisture, C:N, and light, whereas beta diversity declined with soil temperature and light. Structural equation modeling (SEM) revealed that light and soil C:N were primary factors of species richness loss. Under fertilization, soil C:N reduced species richness mainly through its negative association with soil temperature; whereas under grazing exclusion, this effect was mediated through reduced soil moisture. Increased beta diversity primarily associated with intensified light limitation. In summary, our study demonstrates that both aboveground light availability and belowground soil nutrient availability contribute to the reduction in species richness under fertilization or grazing exclusion, whereas changes in species composition were mainly driven by light availability rather than other abiotic factors. These findings highlight that the effects of abiotic factors on species diversity operate through distinct pathways and involve complex mechanistic linkages under different management practices.