Effects of Grazing, Extreme Drought, Extreme Rainfall and Nitrogen Addition on Vegetation Characteristics and Productivity of Semiarid Grassland

Grassland use patterns, water and nutrients are the main determinants of ecosystem structure and function in semiarid grasslands. However, few studies have reported how the interactive effects of rainfall changes and nitrogen deposition influence the recovery of semiarid grasslands degraded by grazing. In this study, a simulated grazing, increasing and decreasing rainfall, nitrogen deposition test platform was constructed, and the regulation mechanism of vegetation characteristics and productivity were studied. We found that grazing decreased plant community height (CWM(height)) and litter and increased plant density. Increasing rainfall by 60% from May to August (+60%) increased CWM(height); decreasing rainfall by 60% from May to August (–60%) and by 100% from May to June (−60 d) decreased CWM(height) and coverage; −60 d, +60% and increasing rainfall by 100% from May to June (+60 d) increased plant density; −60% increased the Simpson dominance index (D index) but decreased the Shannon–Wiener diversity index (H index); −60 d decreased the aboveground biomass (ABG), and −60% increased the underground biomass (BGB) in the 10–60 cm layer. Nitrogen addition decreased species richness and the D index and increased the H index and AGB. Rainfall and soil nitrogen directly affect AGB; grazing and rainfall can also indirectly affect AGB by inducing changes in CWM(height); grazing indirectly affects BGB by affecting plant density and soil nitrogen. The results of this study showed that in the semiarid grassland of Inner Mongolia, grazing in the nongrowing season and grazing prohibition in the growing season can promote grassland recovery, continuous drought in the early growing season will have dramatic impacts on productivity, nitrogen addition has a certain impact on the species composition of vegetation, and the impact on productivity will not appear in the short term.