Effects Of Grazing On Plant Community Assembly And Ecosystem Functions In Grasslands

Grazing by domestic livestock has been the largest land-use activities for utilization of semi-arid grasslands around the world over thousands of years.Occupying⁴0%of the Earth's land surface,the rangelands provide a critical service,for example,contributing⁴0%of global agricultural production and supporting³0%freshwater for more than 1.0 billion people.However,overgrazing intensification has become the most critical driving forces shifting community assembly and ecosystem functions,and thereby may consequentially threat the functioning and services of semi-arid grasslands.We conducted a long-term?2012-2018?sheep grazing experiment at the Typical Steppe grasslands of Inner Mongolia,including four grazing intensity,such as heavy-,moderate-,light-,and no-grazing with intensities at 8,4,2,and 0 sheep/day?ha.,respectively.We collected plant community assembly?e.g.,biodiversity and plant functional groups?and ecosystem functions?e.g.,shoot and root biomass,soil nutrients,carbon/water cycling?variables during the growing season from 2014 to2018,which quantitatively examined how changes of grazing intensities shift plant community assembly and regulate ecosystem functions.We also investigated what processes?i.e.,biotic vs.abiotic?determine ecosystem multifunctionality during the grazing-induced succession of grasslands.Our main results and conclusions are as follows:1.An increase of grazing intensity apparently shaped above-ground community assembly but not in below-ground productivity of grasslands.Above-ground biomass,canopy height,and standing density of grasslands had apparent variabilities both during the growing season and at interannual timescales,as well as below-ground root biomass at shallow and mediate soil depths?0–50 cm?.Above-ground biomass,canopy height,standing density,and litterfall biomass has been considerable decreased when increasing grazing intensity,except for standing density has been increased in the light-grazing plots.In addition to during the last five grazing years,these consequences have been exacerbated.However,the below-ground root biomass at the mediate and deep soil depths?30–100 cm?was increased only by the light-grazing intensity in comparisons to no-grazing plots.2.Effects of grazing intensity increase on plant taxonomic,functional,and phylogenetic diversity of grasslands were diversity identity-dependent during the growing season and over years.Plant taxonomic,functional,and phylogenetic diversity apparently changed both in intra-?i.e.,growing season?and inter-annual timescales.During the last five grazing years,effects of an increase of grazing intensity were constant on?,?,and?diversity?e.g.,light-grazing increase but heavy-grazing decrease?whereas were exacerbated on Shannon-Wiener,Simpson,and Pielou indexes?e.g.,light-grazing decrease but heavy-grazing increase?;effects of an increase of grazing intensity were shifted from a decrease-then-increase trend to a straight increase trend on functional dispersion and Rao's Q,as well as from decreasing to increasing on functional evenness,but has been mitigated on functional divergence.Despite grazing intensity had coupled effects with year and month on phylogenetic diversity,grazing aggregated the plant phylogenetic cluster.3.Grazing effects on plant functional groups in grasslands were group-dependent in their relative biomass and richness,which has been exacerbated for the last five grazing years.The composition of the dominant grasses?both bunch-and rhizome grasses?were fewer fluctuations than subordinate or transient species during the growing season.However,the increasing grazing intensity increased perennial bunchgrasses relative shoot biomass whereas decreased rhizome grasses'.Despite the relative species richness of perennial forbs was declining during the growing season,grazing significantly reduced the relative species richness and shoot biomass of perennial forbs.Conversely,grazing promoted an increase of relative species richness of annuals and biennials.The increase in grazing intensity reduced the relative species richness of C3 and the relative biomass of C4,but increased the relative species richness of C4 and the relative biomass of C3.4.Vertically differential distribution was more important than an increase of grazing intensity influencing on soil carbon and nitrogen in grasslands.Soil carbon at mediate depths?30–50 cm?was vulnerable to grazing,but soil carbon and nitrogen at shallow depths had large variabilities over years.Grazing reduced soil gravimetric water content,but year and month timescales had large effects on soil volumetric water content and soil temperature.5.Effects of grazing on carbon/water cycling in grasslands were process-dependent,which regardless of both year and month timescales had larger influence than an increase of grazing intensity,that had a reduction trend during the growing season and over years.Generally,the light-grazing accelerated but the moderate-and heavy-grazing decelerated CO₂ and H₂O exchange rates.Plant photosynthesis and transpiration had large variability over years,but soil respiration and evaporation significantly changed during the growing season.What's more,carbon/water use efficiencies apparently varied both the intra-and inter-annual timescales whereas no differentials among four grazing intensities.6.Ecosystem multifunctionality of grasslands changed over the years but was not significantly different among grazing intensities.Structural equation modeling indicated that soil temperature directly enhanced ecosystem multifunctionality rather than plant taxonomic,functional,and phylogenetic diversity.Our results synthetically suggest that the effects of an increase of grazing intensity on plant community assembly and ecosystem functions?e.g.,primary productivity,plant diversity,carbon/water cycling,and soil nutrients?were identity-dependent in Inner Mongolia grasslands both during the growing season and at interannual timescales.This study would be helpful to complement the fundamental theories of grazing ecology,ecosystem ecology,and other fields,and would be a useful application for managing multiple ecosystem services and functions in rangelands.