Transformation of litter carbon to stable soil organic matter is facilitated by ungulate trampling

Abstract Plant litter is an important source of soil organic carbon (SOC) in terrestrial ecosystem. The formation of SOC from plant litter and SOC mineralization to atmosphere is a critical determinant of long-term net ecosystem C balance. While it is generally understood that grazing plays a role in SOC cycling, what mechanisms are involved in driving SOC dynamic are not clear. Trampling enhances microbial processes by incorporating litter into the soil, likely influencing the fate of litter C and SOC mineralization. We conducted a controlled microcosm experiment to assess the role of trampling on the fate of litter C, litter-derived SOC mineralization (priming effect), and net C balance through incubation of 13C isotopically labelled Stipa krilovii litter placed on the soil surface, or incorporated into soil via simulated trampling, for 6 months. Litter C transferred into the SOC pool was further fractionated into mineral-associated soil organic carbon pool (MASOC, 53 μm). We analyzed soil enzyme activities and litter-derived microbial biomass carbon (MBC) to determine microbial activities. We found trampling increased overall losses in litter mass (+16%) and litter C (+14%), with proportionally more decomposed litter C transferred to SOC pools (MASOC + 47% and POC + 157%), compared to absence of trampling. The litter-derived MBC was positively correlated with SOC formation, which was increased (+40.78%) by trampling, indicting a stronger microbial contribution to SOC formation. The disturbance of trampling did not induce significant positive priming effects, consistent with invariant soil total MBC and soil enzyme activities following trampling. As a result, trampling induced an increase in SOC formation and invariant priming effect, which contributed to positive net soil C balance (-230.74 ± 89.44 vs 100.33 ± 32.65 mg C/kg soil). Our results show trampling incorporates litter into soil and promotes microbial utilization of litter C and physiochemical stabilization of decomposed litter C, suggesting trampling is an important mechanism of SOC storage with litter C efficiently transferred into SOC pool. We demonstrate the importance of trampling in SOC formation and stabilization. Our findings indicate SOC formation efficiency from C input should be included in SOC predictive models in managed ecosystems.