Herbivore Assemblage as an Important Factor Modulating Grazing Effects on Ecosystem Carbon Fluxes in a Meadow Steppe in Northeast China

A better understanding of how grazing by large herbivores, as the major land use worldwide, affects grassland carbon fluxes is critical for predicting future uptake of CO(2)in terrestrial ecosystem. Previous studies have focused on individual herbivore species; it remains poorly understood as to if and how herbivore assemblage (single vs. mixed species) would alter the impact of grazers on grassland carbon fluxes. Here we examined the effects of moderate grazing by different herbivore assemblages on net ecosystem CO(2)exchange (NEE), gross ecosystem productivity (GEP), and ecosystem respiration (ER) over the growing season in two types of grassland communities in a meadow steppe. We found that herbivore assemblages significantly affected NEE, and the effects varied in the two types of grassland communities. Compared to ungrazed grassland, mixed grazing of sheep and cattle increased NEE (i.e., increased CO(2)uptake) over the growing season regardless of community type, while sheep- or cattle-only grazing increased NEE in the low-diversity community and decreased it in the high-diversity community. Herbivore assemblages altered the NEE primarily via changing GEP. The different effects of herbivore assemblages on GEP may be attributed mainly to grazing-induced changes in soil water availability and canopy light availability. Our study indicates that mixed grazing of sheep and cattle might be an important grazing management practice to improve plant aboveground productivity and help mitigate CO(2)emissions during the growing season. It should be particularly used in diverse plant communities, where it might increase grassland carbon sequestration.