Quantifying the Positive Effect of Ungulate Herbivory on Living Root-Derived Soil Organic Carbon Formation: Evidence From an Eight-Year Simulated Grazing Field Experiment With ¹³C Pulse Labeling

Ungulate grazing encompasses multiple components, including defoliation, trampling, and excreta return, all of which affect soil organic carbon (SOC) dynamics by influencing the balance between rhizodeposition and the subsequent C input and release. However, it remains unclear how ungulate grazing regulates SOC through living roots, especially as evidence from the field is lacking. A ¹³CO₂ pulse labeling experiment was conducted on an 8-year simulated grazing field experiment, involving separate or combined treatments of defoliation, excreta return, and trampling from grazing animals. We investigated the fate of newly assimilated C in different soil C pools and quantified CO₂ release under grazing treatments. Defoliation enhanced C assimilation in soil microorganisms and promoted fungal necromass formation, thereby increasing the microbial carbon pump (MCP) "capacity" (i.e., the net microbial necromass accumulation), contributing more C to SOC (+32%) and mineral-associated organic C (+34%) while reducing soil respiration (-19%). Excreta return stimulated C incorporation into bacterial necromass, enhanced MCP "efficacy" (i.e., the contribution of microbial necromass to SOC) and "capacity", and reduced heterotrophic respiration (-19%). Significant interactions existed between defoliation and excreta return on ¹³C recovery of SOC and CO₂: excreta return reduced the positive effect of defoliation on ¹³C recovery of SOC, while defoliation mitigated the inhibitory effect of excreta return on ¹³C recovery of CO₂. Trampling increased the contribution of plant-derived C to particulate organic C (POC, +26%) and significantly interacted with defoliation by weakening its positive effect on ¹³C recovery of POC. This study advances our understanding of root-derived C formation and stabilization in grazing grassland by disentangling the effects of defoliation, excreta return, and trampling from ungulates. Our work offers new insights for optimizing management practices to effectively utilize the soil MCP for C sequestration in grasslands in response to global climate change.