Responses Of Plant Community And Soil Organic Carbon To Climate Change And Grazing In Alpine Meadow

Climate change and human activities have been accelerating to alter structure and function of natural ecosystem.And alpine meadow in Qinghai-Tibetan Plateau,in term of its sensitivity to ecological disturbance,is considered to be a fundamental study region in terrestrial ecosystem.Previous studies have reported the effects of climate change and grazing on grassland community structure and ecosystem function.However,little is known about the interactions between climate change and grazing.Therefore,in the study,a field warming-grazing experiments,a regional investigation and a model(DNDC,denitrification-decomposition)were conducted to analyze the response of seedling regeneration,plant productivity and soil organic carbon(SOC)to warming and grazing and their interaction,respectively.Based on this study,we aim to explore the potential mechanism underlying climate change and grazing impact on alpine meadow ecosystem,and predict their tendency in future climate change scenarios,which further clarify the comprehensive effects of climate change and grazing intensity on alpine meadow ecosystem.Main results are as following:(1)In long-term enclosure(14 years)condition,community coverage increased by30.06%,SOC increased by 16.51%(0~10cm)and 30.67%(10~20cm),total nitrogen content(TN)increased by 13.26%(0~10cm)and 15.24%(10~20cm),and soil bulk density(BD)decreased by 30.18%(0~10cm)and 20.0%(10~20cm),and grass biomass(P<0.001)and root(P<0.01)was significantly increased,respectively.However,long-term enclosure reduced plant species richness and density by 17.31%and 20.57%,respectively.In addition,compared with moderate grazing,heavy grazing reduced the above-and belowground biomass,grass proportion,litter mass,SOC,and TN(P<0.01),but yet root:shoot ratio was increased by 19.82%.(2)Warming increased significantly species richness and density of plant seedlings(P<0.02),and increased the proportion of asexual recruitment to sexual recruitment.Moreover,grazing reduced the proportion of asexual recruitment to sexual recruitment(P<0.05),resulting in shift in the dominant plant functional groups from graminoids and legumes to forbs,and more importantly,grazing partly offsets increased seedling species richness caused by warming,but not seedling density.(3)Warming significantly increased aboveground biomass(AGB)(P<0.05),and grazing significantly reduced aboveground plant biomass(P<0.01).Compare with Control treatments,effect of warming on SOC was not significant(P>0.05),but warming reduced SOC by 2.79%in 2016 and 1.85%in 2017.Grazing increased SOC by 3.35%in 2016 and 1.89%in 2017.(4)DNDC model have powerful ability to predict response of AGB and SOC to climatic factors and grazing intensity(R~2=0.88,RMSE=17.11,MBE=-0.04,P<0.001;R~2=0.71,RMSE=1.43,MBE=0.06,P<0.001).Factor analysis showed that,compared with grazing intensity,climate change including temperature and rainfall,explained49.8%and 61.9%for AGB and SOC.By contrast,grazing intensity explained 26.4%and 1.6%for AGB and SOC.(5)By 2050,AGB under RCP 4.5 and RCP 8.5 future climate scenarios will increase by 15.00~169.01%and 7.74~135.27%,compared to 2017;SOC(0~20cm)will increase by 0.98~2.85%under RCP 4.5 future climate scenarios,and decrease by0.26~1.30%in RCP 8.5 future climate scenarios by 2050.Taking together,we concluded following insights:(1)Moderate grazing will be beneficial to plant diversity,biomass and soil environment,and is an effective land management for pasture in Qinghai-Tibetan Plateau.By contrast,enclosure is an effective management strategy for vegetation recovery and nutrient sequestration for deteriorated grassland,but not serves species diversity.(2)Warming has a positive impact on species richness and density of seedlings in alpine meadow,but yet grazing reduced the proportion of seedling regeneration,resulting shift among functional groups.Moreover,grazing offsets the positive effects of warming on seedling regeneration,suggesting suitable grazing may buffer the positive effects of warming on plant community structure and function.(3)Warming increased significantly aboveground plant biomass,but produced a negative impact on SOC.Grazing influenced the response of aboveground biomass and SOC.(4)Compare with grazing,climate factors such as temperature and rainfall,is main influence factor for altering aboveground biomass and SOC in alpine meadow.In term of base value in 2017,DNDC model predicted that grazing can promote grassland utilization under RCP 4.5 condition during 2017-2050,and grazing under RCP 8.5condition should reduce increasingly year by year,in order to sustain carbon sink function.In summary,the study suggested that moderate grazing ensured sustainable development of grassland,and understanding climate change impact on grassland ecosystem need focus on the interaction between climate change and grazing.To explore how grassland should be managed in future is an important issue under climate change background.