Divergent responses of particulate and mineral-associated organic carbon stock to grazing in a Eurasian temperate meadow steppe

Grazing plays a pivotal role in shaping the carbon dynamics within grassland ecosystems. Although the impact of grazing on soil carbon dynamics has recently become a major focus, the mechanistic drivers of grazing effects on functionally distinct carbon fractions remain unclear. Here, we employed a combined density and particle-size fractionation approach to divide the soil carbon pool into three distinct functional fractions (fPOC: free particulate organic carbon, oPOC: occluded particulate organic carbon, and MAOC: mineral-associated organic carbon), and investigated their responses to four different grazing intensities (Non-grazing: G0.00 (0 AU ha⁻¹), light grazing: G0.23 (0.23 AU ha⁻¹), moderate grazing: G0.46 (0.46 AU ha⁻¹) and heavy grazing: G0.92 (0.92 AU ha⁻¹), where 1 AU = 500 kg of adult cattle) and further explored the potential mechanisms involved in the Inner Mongolia meadow grassland. Our results show that POC stock has greater sensitivity to grazing disturbance than MAOC stock. Light to moderate grazing promoted the increase in both fPOC and oPOC stocks (fPOCs, oPOCs) compared to non-grazing, while heavy grazing (G0.92) significantly decreased relative to moderate grazing. In contrast, all intensity levels decreased in mineral-associated organic carbon stock (MAOCs). The structural equation model (SEM) indicates that grazing increases POCs by increasing belowground biomass input and suppressing microbial processes (microbial and enzyme activities). In addition, grazing-induced soil environment deterioration and nutrient depletion inhibit the input of microbial biomass and necromass, ultimately reducing MAOC formation. Furthermore, the distribution of POC increased significantly with grazing intensity, indicating enhanced SOC activity. Overall, our results highlight that grazing-induced shifts in plant above-belowground biomass allocation strategy and microbial enzymatic activity collectively drive soil carbon dynamics.