Soil Carbon‐to‐Nitrogen Ratio Can Predict the Grassland Biodiversity‐Productivity Relationship: Evidence From Local, Regional, and Global Scales

Soil elemental stoichiometry serves as an inherent link between soil biogeochemistry and the structure and processes within plant communities, and thus is at the core of ecosystem functions. Yet, the regulatory role of soil stoichiometry, particularly the carbon-to-nitrogen (C:N) ratio, in shaping biodiversity-productivity relationships remains poorly understood. By integrating data from our regional field surveys (58 sites) and a local complementary N addition experiment in temperate grasslands, together with a global grassland dataset (74 sites), here we showed that plant productivity exhibited a unimodal response to increasing soil C:N ratios, with peaking values at the C:N ratio of approximately 15. At this critical value, the determinants driving grassland productivity undergo a fundamental shift: below the soil C:N of 15, plant diversity was positively related to productivity, while above this threshold, bacterial and fungal diversity showed a positive linkage with plant productivity. This divergence implies a stoichiometric "switch" in biodiversity-productivity relationships: high soil C:N ratios strengthen the reliance of productivity on soil bacterial and fungal diversity to mitigate N deficiency, while low C:N ratios shift the emphasis to plant diversity to exploit resource-rich environments. Our findings highlight that soil stoichiometry can predict biodiversity-productivity relationships, with important implications for grassland restoration and management.