Root, not aboveground litter, controls soil carbon storage under grazing exclusion across grasslands worldwide

Grazing exclusion is a common management strategy for improving grasslands degraded due to overgrazing. Although previous meta-analyses proved that grazing exclusion improved soil carbon (C) storage of grasslands in China, we know little about its effect on grassland ecosystems elsewhere and whether litter accumulation induced by grazing exclusion regulates soil C storage. Using meta-analysis, we integrated 91 publications to examine the effects of grazing exclusion on community litter and soil C storage, accompanied by analysis of the regulating roles of body size of herbivore, climatic conditions, and grazing exclusion duration. We found that grazing exclusion enhanced litter biomass and decomposition rate but did not change litter chemical quality. Generally, grazing exclusion enhanced plant production and soil C storage across grassland worldwide. Exclusion of medium and small herbivores had stronger effects on aboveground litter production and soil C storage than exclusion of large herbivores, and grazing exclusion's enhancement on litter production and soil C storage tended to increase as grazing exclusion proceeded. Notably, grazing exclusion-induced increase in aboveground litter biomass was attributed to the increase in biomass of graminoids, while root biomass, not aboveground litter biomass, primarily regulated the increase in soil C storage. The stimulating effect of grazing exclusion on soil C storage was enhanced possibly due to enhanced root biomass response along a precipitation gradient. Generally, our study provided new insights that management practice under grazing exclusion should take plant community dynamics, grazing exclusion duration, preceding grazer type, and climate conditions into account simultaneously.