Effects Of Grazing On Meadow Grassland Communityunder Nitrogen Deposition

Grazing by domestic and wild ungulates is the the most globally widespread land use. Large herbivore grazing plays an important role in regulating plant community composition, diversity and productivity. Meanwhile, nitrogen deposition is a key issue for global change. Nitrogen deposition can strongly influence the nutrient dynamics and plant species relationships, the results, therefore, have the potential to exert effects on plant communities and ecosystem processes. It has been widely accepted that nitrogen deposition and grazing can affect plant communities respectively, while the combined effects of large herbivores grazing and nitrogen deposition has rarely been examined. We still understand poorly how different grazing systems(sheep grazing, cattle grazing, mixed grazing) affect grassland plant community, especially under the high nitrogen regimes.In this dissertation, we mainly focused on the research that under the condition of different nitrogen addition levels, the single grazing and mixed grazing of different large herbivores(Ujumqin sheep and Mongolian cattle) had effects on plant community characteristics, plant functional groups and plant diversity in Hulun Buir Grasslands. The results are as follows:Both nitrogen addition and grazing changed plant height and aboveground biomass, and the effects of grazing could be affected by nitrogen addition. Under no grazing condition, nitrogen addition significantly increased plant aboveground biomass(P < 0.05), and tended to increase plant cover and height, but reduced plant density. Under non-nitrogen fertilizer condition, grazing significantly reduced plant height, while sheep grazing and cattle grazing significantly reduced plant aboveground biomass, and the effects was reinforced by nitrogen addition(P < 0.05).2) Nitrogen addition and grazing both had different effects on all kinds of plant functional groups. Moreover, there was difference among the effects on plant functional group of different grazing systems. Under no grazing condition, nitrogen addition tended to increase the cover, height and aboveground biomass of all plant functional groups, but significantly increased the aboveground biomass of grasses and forbs(P < 0.05). Under non-nitrogen fertilizer condition, grazing tended to reduce the cover of Allium senescens,grasses and legumes, but increased the cover of shrubs and forbs. Under nitrogen fert addition, sheep grazing, cattle grazing and mixed grazing significantly reduced the cover of Allium senescens, legumes and grasses respectively(P < 0.05). Grazing significantly reduced the height of Allium senescens, legumes and forbs(P < 0.05), and the effects were reinforced by nitrogen addition. Grazing significantly reduced the aboveground biomass of Allium senescens and legumes(P < 0.05), and the effects were reinforced by nitrogen addition. Moreover, grazing significantly reduced the aboveground biomass of grasses and forbs under nitrogen fertilizer condition(P < 0.05).3) Different grazing systems exerted different influence on plant diversity, and the effects was affected by nitrogen addition. Under non-nitrogen fertilizer condition, grazing had no effects on plant species richness and Shannon-Wiener index, but Shannon-Wiener index was significantly higher in sheep grazing treatment than cattle grazing treatment(P < 0.05). Under nitrogen fertilizer condition, sheep grazing and mixed grazing significantly reduced plant species richness, and mixed grazing significantly reduced plant Shannon-Wiener index(P < 0.05).In conclusion, nitrogen addition exerted effect on the relationships between large herbivores and plant community.Moreover, the single grazing and mixed-grazing of cattle and sheep had different effects on plant community. Our dissertation confirmed that the interaction between grazing and nitrogen deposition had an effect on the plant community in grassland. Meanwhile, factors such as climate change and grazing system merit further exploration and have implications for the management of grasslands using large herbivore grazing.