Radiocarbon evidence of organic carbon turnover response to grassland grazing: A soil aggregate fraction perspective

Grassland grazing, driven human activities, represents a prevalent form of land use conversion. While numerous studies have examined the impact of such conversions on soil carbon cycling, they primarily focus on the content and storage of soil organic carbon (SOC). However, research on the turnover dynamics of SOC and the underlying mechanisms triggered by land use conversions remains relatively scarce. In this study, radiocarbon (¹⁴C) tracing technology was applied to investigate the effects of grassland grazing on SOC turnover in the Saihanba area of Hebei province, China. The results revealed that the turnover time of SOC in pasture grassland was shortened by approximately 250 years compared to meadow grassland, suggesting that grazing diminishes the ability of topsoil to stabilize SOC. Furthermore, our findings indicate that grazing leads to a decrease in soil CO₂ flux by 0.50 g C m⁻² y⁻¹ under aggregates larger than 250 μm and those between 63 and 250 μm. Conversely, the CO₂ flux under aggregates, specifically those between 2 and 63 μm and less than 2 μm, increased by 0.96 g C m⁻² y⁻¹. This shift suggests a significant increase in the contribution of older SOC pools to the overall soil CO₂ flux. Our study provides novel insights into SOC cycling in the context of grassland grazing, highlighting the importance of understanding SOC turnover dynamics for effective land management.