The Impact Of Climate Change And Grazing On Grassland Biomass And Soil Organic Carbon In The Sanjiangyuan

The Qinghai-Tibet Plateau?QPT?with the largest alpine meadow grassland ecosystem, which is the most unique type in the world.The average elevation of the Qinghai-Tibet Plateau is over 4000 meters and is one of the most important ecological functional areas in China.The QTP is remarked as one of the most vulnerable and sensitive regions to both of human disturbance and climate change.Over the past few decades,grassland degradation in the region has been exacerbated by the effects of climate change and human activities.However,the driving forces that cause changes in grassland ecosystems are still inconclusive.In particular,climate change and grazing patterns have negative impacts on grassland biomass and soil organic carbon?SOC?.Sanjiangyuan Region located in the center of QTP.The grassland is dominated landcover type of Sanjiangyuan,where also the water source of the Yellow River,the Yangtze River and the Lancang River,and plays an extremely important role in water conservation and ecological protection.Therefore,in this study,we use this region as the study area to study the impact of climate change and grazing on grassland.We applied the DNDC?Denitrification-Decomposition?model to simulate and analyze the dynamic changes of grassland biomass and soil organic carbon?SOC?under realistic climatic conditions and the future climate scenarios?representative concentration pathways:RCP4.5 and RCP 8.5?with the different grazing intensities.A multiple linear regression analysis was adopted to analyze the relationship between the annual changed biomass and SOC with corresponding temperature,precipitation and grazing intensity.Through this study,we concluded that:1)Model validation and sensitivity analysis based on the observed vegetation biomass and SOC sampling data from 2005 to 2016 in the Sanjiangyuan grassland ecological monitoring project.The validation results showed that the simulated total biomass and SOC are in good agreement with the measured values.There was a significant linear correlation between the simulated and measured values of total biomass(R²=0.71,RMSE=93.11 g C m^-2;P<0.001),and the simulated values of SOC were significantly correlated with the measured values(R²=0.73,RMSE=21.51 g C kg^-1;P<0.001).Therefore,The DNDC model could simulate the effects of climate and grazing conditions on grassland biomass and soil SOC with reasonable accuracy.2)The simulation results show that climate change may be the main factor leading to grassland biomass and SOC change.Temperature and precipitation explain 26.8%of grassland biomass and 48%of SOC variation,respectively,while grazing intensity explained 6.4%of the change in grassland biomass and 2.3%of the change in SOC.3)Under both RCP4.5 and RCP8.5 scenarios,total grassland biomass and SOC tended to decrease significantly in the study area.Compared with 1987,the simulated values of grassland biomass and SOC decreased by 16.91%-18.51?23.82%-26.44%and 3.94%-4.10%?4.03%-4.19%respectively under the climate scenarios of RCP 4.5 and RCP 8.5 in 2016.4)Grassland biomass had a negative relationship with grazing intensity.The grass biomass under the grazing scenarios G₀,G_-50 and G₊₅₀ increased by 4.75%,2.37%and-2.16%respectively.The grazing intensity and SOC were positively correlated.Compared with Baseline,SOC increased-0.43%?G₀?,-0.23%(G_-50)and+0.22%(G₊₅₀)respectively under different grazing treatments.