The Effects Of Grazing And Drought On Vegetation And Soil Characteristics And Greenhouse Gas Flux In The Desert Steppe

Desert steppe is the aridest grassland ecosystem in the transition from steppe to the desert,which has a wide distribution and a large area,so it is highly sensitive to various disturbance activities.In recent years,this ecosystem faces serious threats due to the combined impact of global climate change and long-term unreasonable human utilization,especially the impacts of extreme drought and long-term overgrazing utilization.Under the background of climate change,the dynamic changes of this ecosystem will profoundly affect the livestock production,regional and global carbon balance and ecological service function.However,there is still a lack of comprehensive and systematic understanding of the response of different components of the desert grassland ecosystem to extreme drought and free-grazing disturbance and their regulatory mechanisms.Therefore,the grazing(free grazing:FG,control:CK)and the extreme drought test(reduce66%of precipitation from May to August:W1,reduce 100%of precipitation in June and July:W2,control:CK)platform was established in the Xilamuren desert steppe of Inner Mongolia in 2014 and 2016 respectively to study the effects of extreme drought and long-term free grazing on vegetation,soil physical and chemical properties and greenhouse gas flux deeply and the main driving factors of greenhouse flux comprehensively with the combined method of field sampling investigation and indoor measurement analysis.The main results are summarized as follows:(1)The soil moisture content,total carbon content,organic carbon content and soil C/N ratio(C/N)were significantly decreased by the free grazing.However,the soil?¹³C value,?¹⁵N value,bulk density and total nitrogen content were significantly increased.Soil organic carbon content was negatively correlated with its?¹³C value(P<0.05)significantly,while the total nitrogen content was positively correlated with its?¹⁵N value significantly(P<0.05).The soil surface moisture content was only decreased significantly by 66%precipitation reduction due to the influence of sampling time.In contrast,soil bulk density,organic carbon content,total nitrogen content,soil C/N ratio(C/N),?¹³C and?¹⁵N values were not sensitive in response to extreme drought.Moreover,the total nitrogen content was positively correlated with its?¹⁵N value significantly,while there was no significant linear correlation between soil?¹³C and organic carbon content.The aboveground stock of the plant community showed seasonal and annual dynamic changes from 2015 to 2018,and the aboveground stock of the plant community significantly reduced by free grazing.(2)Free grazing significantly reduced the coverage and height of the plant community,but had no significant effect on the density and species richness of the plant community during the peak period of the plant growing season from 2015 to 2018.In addition,free grazing did not significantly change the structure of plant community and functional group composition.The effects of grazing,year and their interaction on the Simpson dominance index,Shannon-Wienner diversity index and Margalef richness index were not significant,but the Pielou evenness index showed significant differences among the inter-annual effects(P<0.05).However,the height,coverage,density and plant richness of the plant community were significantly reduced by the two extreme drought treatments at the peak of the growing season in 2018.At the same time,the structure and composition of the plant community were changed by the short-term extreme drought measures,and the perennial rhizome grasses and annuals and biennials gradually become the dominant plants under the extreme drought.The Margalef richness index and Shannon-Wiener diversity index were significantly decreased by two extreme droughts,but it does not affect the Pielou evenness index.(3)Soil temperature and soil moisture content were not sensitive in response to grazing and year,but only showed significant inter-year differences in the growing season of 2017-2018.Moreover,compared with CK,the soil moisture content was significantly decreased by 27.65%and 40.01%by W1 and W2,respectively.At the same time,the extreme drought had no significant effect on soil temperature,and soil temperature only had significant differences between the two years.(4)During the growing season of 2017-2018,soil CO₂ flux was significantly decreased by 79.27%and 69.36%under free-grazing,respectively,compared with the control.However,grazing,year and their interactions had no significant effects on soil CH₄ flux(P>0.05).Simultaneously,extreme drought,year and their interaction had significant effects on soil CO₂ emission flux and CH₄ absorption flux,and the two extreme droughts significantly reduced CO₂ emission flux in the two years.However,the CH₄ flux was significantly reduced by the two extreme droughts in 2017,but the extreme drought had no significant effect on the CH₄flux in2018.(5)During the growing seasons of 2017 and 2018,there was a significant quadratic nonlinear regression between the soil CO₂ and CH₄ fluxes and the soil moisture content under grazing and extreme drought treatment(P<0.05).There was a significant quadratic nonlinear regression law between the CO₂fluxes and soil temperature under different grazing treatments in the two years,except for the CO₂ and CH₄fluxes under CK in 2017 and the CH₄fluxes under the free-grazing in 2018.Soil CO₂ flux and soil temperature showed a significant exponential relationship under different extreme drought treatments.However,there was no significant relationship between soil CH₄ flux and soil temperature under different extreme drought treatments.(6)Grazing disturbance inhibited the CO₂greenhouse gas flux indirectly by reducing soil moisture content and increasing soil total nitrogen content and inhibited the CH₄ absorption indirectly by reducing soil moisture content.However,the change of soil moisture content caused by extreme drought not only had a direct inhibition on soil CO₂flux but also had an indirect inhibition on soil CO₂flux by reducing species richness.Moreover,soil moisture content had an indirect inhibition on soil CH₄flux mainly by decreasing species richness.Therefore,soil moisture content plays a indirect influence factor in the effects of grazing and extreme drought on soil greenhouse gases.To sum up,this study clarifies the effects of free grazing and extreme drought on vegetation,soil characteristics and greenhouse gases fluxes comprehensively and systematically,and it also analyses the regulation mechanism of carbon-bearing greenhouse gases in the Xilamuren desert steppe of Inner Mongolia.It was found that extreme drought and free grazing could reduce the aboveground biomass,plant diversity and soil nutrients of the desert steppe.However,they slow down the carbon loss of the ecosystem by reducing the diffusion of carbon greenhouse gases at the atmosphere and soil level,thus reducing the risk of further degradation of the desert steppe.