Recovery through proper grazing exclusion promotes the carbon cycle and increases carbon sequestration in semiarid steppe

Grazing exclusion changes soil physical-chemical characteristics, rapidly affects microbial community composition and function, and alters biogeochemical processes, e.g., carbon (C) cycle, over time. However, the temporal dynamics of CO₂ emission and CH₄ uptake during grassland restoration chronosequences remain poorly understood. We investigated soil CO₂ emission and CH₄ uptake, the genes related to CO₂ and CH₄ production and reduction (cbbL, cbbM, chiA, and pmoA), and associated microbial communities under different durations of grazing exclusion (0, 7, 16, 25, and 38 years) to reveal the mechanisms and potential of soil CO₂ emission and CH₄ uptake in a semi-arid steppe. The results showed that a proper exclusion period could significantly improve soil physical-chemical conditions, vegetation community, and soil C-cycling. The abundance of C-cycling functional genes (cbbL, cbbM, chiA and pmoA), CH₄ uptake and CO₂ emission rates showed a single-peak pattern with increasing duration of grazing exclusion, peaking at 16 years and then decreasing in the period between 25 and 38 years, indicating that the effect of exclusion weakened when the exclusion period was too long. The changes in C-cycling functional genes and microbial communities are primarily influenced by aboveground net primary productivity (ANPP), and are associated with CO₂, CH₄, soil water content (SWC), and soil organic carbon (SOC). Structural equation modeling showed that increases in SOC content and pmoA abundance caused by an increase in ANPP accelerated CO₂ emission and CH₄ uptake rates, respectively. Our results provide valuable insights into the critical role of grazing exclusion in promoting grassland restoration and carbon sequestration, and have potential implications for sustainable land management practices.