Extreme Drought and Herbivory Redirect Recently Assimilated Carbon From Storage to Rapid Turnover Pathways in Grasslands.

Environmental stresses can alter ecosystem carbon cycling by changing both carbon uptake and the subsequent allocation of assimilated carbon among plant, soil and consumer pools. However, how multiple stressors influence the fate of recently assimilated carbon remains poorly understood. We combined 13C pulse labelling with manipulations of extreme drought, grasshopper herbivory and plant functional composition in a semi-arid grassland of Inner Mongolia, China, to examine how climatic and trophic stresses regulate carbon allocation and turnover. Extreme drought shifted recently assimilated carbon from aboveground tissues to roots and reduced its recovery in soil pools. Drought also weakened the coupling between aboveground biomass and ecosystem CO2 exchange. Herbivory alone had limited effects on carbon allocation but increased the transfer of recently assimilated carbon to herbivores and respiratory pathways under drought. Plant functional composition further modified these responses, with communities containing Caragana microphylla maintaining greater recovery of recently assimilated carbon in soil pools under combined stress. These results show that drought and herbivory regulate grassland carbon cycling by altering post-assimilation carbon pathways, highlighting the importance of plant functional composition and trophic interactions in ecosystem responses to compound disturbances.