Carbon cycling in grasslands can be impacted by livestock grazing, partially as an indirect result of herbivory-induced compositional shifts in the plant community. However, the underlying mechanisms of how these shifts impact carbon cycling are not well documented.

We conducted a long-term grazing experiment with four sheep stocking rates in the semi-arid grasslands of Inner Mongolia, China, to examine grazing effects on the ratio of C₃ to C₄ species (C₃:C₄), shoot biomass, root biomass, root:shoot, soil respiration, soil C, soil N, and soil C:N between 2014 and 2018. We explored the responses of these carbon metrics to C₃:C₄ under different grazing treatments and the mechanisms driving grazing-induced carbon loss using structural equation models.

Livestock grazing directly shifted plant community composition (i.e., increasing C₃:C₄) and reduced vegetation carbon (i.e., shoot biomass), whereas grazing effects on belowground carbon were mediated by the interactions of the soil profile (i.e., depth-dependence) and year-to-year variation (e.g., rainfall regulation). Grazing-induced increases in C₃:C₄ suppressed soil carbon loss by inhibiting the rate of soil respiration. Furthermore, grazing intensity indirectly altered these relationships. Specifically, C₃:C₄ was positively related to shoot biomass and negatively associated with root:shoot, soil C, and soil N, whereas these relationships were only significant in no-grazed plots. Meanwhile, soil respiration was negatively associated with C₃:C₄, soil C, soil N, and soil C:N, but a positive relationship with shoot biomass; these relationships were significant only in grazed-plots.

Synthesis and applications. Our findings emphasize the functional linkages between community characteristics and ecosystem processes, i.e. shifts in plant community composition play a key role in regulating grassland carbon cycling. These results provide a useful field-observed resource for model development and could improve the guidelines for livestock management and policies regarding climate mitigation.

Details of the methods can be found in the manuscripts.