Changes in plant phosphorus demand and supply relationships in response to different grazing intensities affect the soil organic carbon stock of a temperate steppe.

Ongoing climate change and long-term overgrazing are the main causes of grassland degradation worldwide. Phosphorus (P) is typically a limiting nutrient in degraded grassland soils, and its dynamics may play a crucial role in the responses of carbon (C) feedback to grazing. Yet how multiple P processes respond to a multi-level of grazing and its impact on soil organic carbon (SOC), which is critical for sustainable grassland development in the face of climate change, remains inadequately understood. Here, we investigated P dynamics at the ecosystem level in a 7-year-long multi-level grazing field experiment and analyzed their relation to SOC stock. The results showed that, due to the greater P demand for compensatory plant growth, grazing by sheep increased the aboveground plants' P supply (by 70 % at most) while decreasing their relative P limitation. The increase in P in aboveground tissue was associated with changes in plant root-shoot P allocation and P resorption, and the mobilization of moderately labile organic P in soil. Affected by the altered P supply under grazing, corresponding changes to root C stock and soil total P were two major factors impacting SOC. Compensatory growth-induced P demand and P supply processes responded differently to grazing intensity, resulting in differential effects on SOC. Unlike the light and heavy grazing levels, which reduced the SOC stock, moderate grazing was capable of maintaining maximal vegetation biomass, total plant biomass P, and SOC stock, mainly by promoting biologically- and geochemically-driven plant-soil P turnover. Our findings have important implications for addressing future soil C losses and mitigating higher atmospheric CO threats, as well as maintaining high productivity in temperate grasslands.