Shifts in plant composition mediate grazing effects on carbon cycling in grasslands.
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 below-ground 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 study highlights the importance of the functional linkages between community characteristics and ecosystem processes, that is, shifts in plant community composition play a key role in regulating grassland carbon cycling. These findings provide a useful field-observed resource for model development and could improve the guidelines for livestock management and policies regarding climate mitigation.