The Effects And Regulation Mechanism Of Grazing On Grassland Carbon Sequestration

Grassland is one of the most important terrestrial ecosystems,which covers about 40% of the earth's surface,and its carbon dynamic is closely related to the global climate change.Grazing,as the most prevalent land use of the world's grassland,may substantially affect all of the carbon cycle processes in grassland,thereby grassland carbon sequestration.Therefore,a better understanding of the effect and regulation mechanism of livestock grazing on grassland carbon sequestration is of great importance for guiding the grassland grazing management,improving grassland carbon sequestration and mitigating climate change.The effects of livestock grazing on carbon source-sink relationship in the grasslands has significant uncertainty,which was modified by many factors including grassland types,grazing intensity,study duration and research methods.Recent studies showed that different herbivore assemblages had varied effects on plant community composition,aboveground biomass,soil available nitrogen and so on.Therefore,besides grazing intensity,herbivore livestock species and combination(herbivore assemblages)could be a crucial factor regulating grazer impacts on uncertainty of grassland carbon sequestration.In order to improve grassland carbon sequestration,how to select adapted herbivore assemblages in grasslands with different plant characteristics has not been explored.Consequently,in this study,our goal was to examine the effects of herbivore assemblages(sheep grazing,cattle grazing,mixed grazing by sheep and cattle)on grassland carbon fluxes and soil carbon storage in grassland communities with different plant diversity,which will benefit to obtain deep insight into the effects and regulation mechanism of grazing on grassland carbon sequestration.We obtained the main finding as follows.1)The effects of grazing on ecosystem carbon input over growing season strongly depended on herbivore assemblages and grassland community types.For whole growing season,mixed grazing by sheep and cattle increased ecosystem carbon input regardless of community types with low or high plant diversity,while single species grazing by sheep or cattle alone increased ecosystem carbon input in low diversity community but decreased it in high diversity community,and the effect strength slightly varied in each month.The effects of grazing on ecosystem carbon input were mainly attributable to changes in grazing-induced soil water availability and aboveground biomass.2)Herbivore assemblage significantly affected ecosystem carbon output over growing season,and these effects were different in both low and high diversity communities.The effects of herbivore assemblages on ecosystem carbon output only occurred at the mid-growing season.For whole growing season,sheep grazing only increased ecosystem carbon output in the low diversity community,while mixed grazing by sheep and cattle only increased ecosystem carbon output in the high diversity community.Mixed grazing among grazing treatments had the largest ecosystem carbon output in the high diversity community.Grazing altered ecosystem carbon output via a series of biological and edaphic processes.Gross ecosystem productivity,root biomass,plant diversity,soil water content and soil total N content had positive effects on ecosystem carbon output,while soil p H value and electrical conductivity had negative effects on it.Among these factors,the effect of gross ecosystem productivity on ecosystem carbon output is the greatest.3)Herbivore assemblages obviously affected net ecosystem CO2 exchange over growing season,and the effect strongly depended on grassland community types.In the low diversity community,all grazing treatments increased net ecosystem CO2 exchange.In the high diversity community,mixed grazing by sheep and cattle only increased net ecosystem CO2 exchange,while grazing by sheep or cattle alone decreased it.Herbivore assemblages altered net ecosystem CO2 exchange primarily via modifying ecosystem carbon input.The effects of grazing on net ecosystem CO2 exchange was largely driven by the changes in herbivory-induced aboveground biomass and soil water content.4)Herbivore assemblages generated varying effects on soil carbon(C)storage,and the effects were subject to community context of grazed grassland.In the low diversity community,none of three herbivore assemblages studied had obvious effects on soil C storage.In the high diversity community,however,sheep grazing significantly decreased soil C storage,and cattle grazing had no effects on soil C storage,while mixed grazing of sheep and cattle significantly increased soil C storage.Overall,soil C storage was highest in mixed-grazed high diversity grassland community.Different herbivore species and combination had varying effects on plant community composition and primary production due to different diet selection,which further affected soil C storage.Moreover,herbivore assemblages also had obvious effects on soil C storage by the changes in soil physical and chemical properties.Taken together,we concluded that herbivore assemblages significantly affected ecosystem carbon fluxes and soil carbon storage.The effects of herbivore assemblages on grassland carbon sequestration exhibited substantial variability,which depended on grassland community types.Moderate grazing utilization might compensate the reduced C input when plant diversity is declined,and high plant diversity in conjunction with high herbivore diversity is critical to maintain and improve the C-fixation functioning of grassland ecosystems.Difference in food preference intensity among herbivore assemblages is an important factor for controlling grassland carbon sequestration.Caution should be exercised when using grazer species with high food selectivity in grazing management,especially in the grasslands with abundant high-quality forbs and high plant diversity,as sheep grazing tended to reduce grassland carbon sequestration in our study.The study indicated that mixed grazing by sheep and cattle in moderate grazing intensity could be considered as an optimal grazing management strategy to grassland carbon sequestration in Songnen meadow steppe.