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 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.

Methods
Details of the methods can be found in the manuscripts.