Long-Term Grazing Intensity Impacts Belowground Carbon Allocation and Mycorrhizas Revealed by ¹³CO₂ Pulse Labeling

Despite the importance of grasslands for carbon storage and climate regulation, there is uncertainty about the effect of livestock grazing intensity on aboveground carbon assimilation and belowground carbon partitioning. Furthermore, the relationship between belowground carbon allocation and arbuscular mycorrhizal fungi, which serve as a conduit for carbon movement through the plant and soil, is unclear. To investigate this, we used an in situ ¹³C stable isotope pulse-chase labeling approach in plots under seven rates of sheep grazing intensity in a steppe grassland in northern China. We quantified the allocation of carbon to plants, soil, and soil-respired CO₂ along with measurements of mycorrhizal hyphal density in the soil. With increasing grazing intensity, carbon assimilation per unit shoot biomass increased significantly, whereas carbon allocation to roots marginally decreased. Soil-respired CO₂ appeared to be independent of grazing intensity. Mycorrhizal hyphal density decreased with increasing grazing intensity and was correlated significantly with new carbon input to roots 2 d after labeling and marginally related to that of soil 1 d after the ¹³C-CO₂ pulse. Our study suggests that grazing intensity alters the distribution of carbon among different carbon pools within the plant-soil system. The results also underscored the key role of mycorrhizas as a fast route for carbon transfer from plant to soil.