The Effect Of Grazing And Climate Change On Characteristics Of Vegetation And Cycle Of Soil Carbon And Nitrogen In Grassland Ecosystem

Grassland is an important ecological security barrier in China,and it plays an irreplaceable role in water conservation,wind protection and sand fixation,soil conservation,climate regulation,maintenance and carbon sink.In recent years,the climate change and the increasing intensity of human activities have dramatically altered the structure and function of grassland ecosystems,which have been changed its ecological function.Accurately assessing the impact of climate change and human activities on grassland ecosystem dynamics can promote grassland ecosystem health and maintain grassland ecosystem services.The response of grassland ecological process to the driving factors of global change is highly complex,but most of the previous studies on grassland ecological process to the driving factors of global change focused on a single grassland type,a single environmental factor and other aspects.The ecological process of different types of grassland various responses to the same environmental factor,and the response of the same grassland ecosystem process to different environmental factors is frequently different.Therefore,this study used the same research method to explore the impact of global change on grassland ecosystem from two dimensions:regional scale（grazing）and growth season time scale（simulated climate change）.In order to explore the impact of grazing on different types of grassland ecosystems,the experimental treatments were divided into enclosed non grazing areas and over grazing areas.At the regional scale of the test sample site,six different types of grassland were selected from the northeast to the southwest of China,including plain hilly meadow steppe subgroup（located in Hulun Buir）,plain hilly temperate steppe（Xilinhot）,sandy grassland subclass（Wushen Banner）,desert steppe subtype of sandy land（Etuoqianqi）,subtype of mountain desert steppe（Xinjiang）,alpine meadow subclass（Hongyuan,Sichuan）.Also,an experiment was conducted to investigate the impact of temperature,precipitation and their interaction on the desert steppe ecosystem in the time scale of the growing season.The OTC was used to simulate climate change（short-term four years）,on Stipa breviflora desert,in Inner Mongolia,China.Temperature enhancement was achieved by open-top growth chamber（OTC）,and precipitation enhancement was achieved by self-made rain interception device.The experimental treatment of increasing temperature and precipitation was set,and the experimental treatment of temperature increase（T0:ambient temperature,T1:T0+2℃,T2:T0+4℃）,precipitation increase（W0:ambient precipitation,W1:W0+25%,W2:W0+50%）and their interaction（T*W）.The main results and conclusions as follows:（1）The effects of grazing on vegetation characteristics of different types of grassland ecosystems are consistent.Grazing significantly reduced the aboveground biomass（average decrease of 72%）and species richness index（average decrease of 27%）of six different types of grasslands,but had no significant impact on the evenness of the community,indicating that the aboveground biomass of vegetation in the grassland ecosystem responded more strongly to grazing,and the reduction of biomass caused by overgrazing was the most direct factor causing grassland degradation.（2）The effect of grazing on soil carbon transformation in grassland ecosystem varies with grassland type.Grazing decreased the content of soil organic carbon on the grassland types of plain hilly meadow grassland subclass,plain hilly temperate steppe and alpine meadow subclass,while grazing increased the content of soil organic carbon on the grassland types of sandy grassland subclass,sandy desert grassland subclass and mountain desert steppe subclass.The effect of grazing on the soil respiration rate of various grasslands is not significant,but grazing significantly increased the soil respiration rate on the mountain desert grassland in Xinjiang and alpine meadow in Sichuan,while the soil respiration rate of the alpine meadow subclass is the highest.Both the hilly meadow steppe subclass and the hilly temperate steppe show a carbon sink state.（3）Grazing had the same effect on soil nitrogen transformation in different types of grassland.Grazing significantly increased the soil content of NH₄⁺-N and NO₃^--N in various grassland ecosystems,and increased significantly by 43.6%and 27.3%respectively in 0-15 cm soil layer.Grazing significantly increased the denitrification rate of grassland soil,especially in plain hilly temperate steppe,sandy grassland subclass,sandy desert grassland subclass and mountain desert grassland subclass,which showed that grazing promoted the denitrification process of grassland soil,indicating that grazing increased greenhouse gas N₂O emissions.（4）Vegetation characteristics of Stipa breviflora desert steppe ecosystem are insensitive to short-term warming and increased precipitation and their interactions treatment.In the fourth year of treatments,warming and increased precipitation had no significant impact on the index of Species,Simpson,Shannon-Wiener,and Pielou of the vegetation community of Stipa breviflora desert grassland ecosystem.The vegetation species richness at warming by 4℃was significantly higher than that at warming by 2℃under increased precipitation by 25%in May,indicating that warming increased the species richness at the rejuvenation of plants while a temperature sensitive period.The warming significantly increased the vegetation aboveground biomass of Stipa breviflora desert steppe,and with the increasing of temperature,the aboveground biomass showed a trend of first rising and then declining.The aboveground biomass at warming by 2℃was the higher than that at warming by 4℃.The aboveground biomass was more sensitive to temperature,and continuous warming would lead to the degradation of desert steppe.（5）The soil transformation of carbon and nitrogen had different responses to warming and increased precipitation in Stipa breviflora desert steppe ecosystem.Warming significantly increased the soil content of carbon and nitrogen,while increased precipitation had no significant effect the change soil content of carbon and nitrogen.Warming and increased precipitation and their interactions treatment significantly affected soil nitrification-denitrification rate and respiration rate.The denitrification rate and total nitrification rate increased first and then decreased with the increasing of temperature,and decreased with increased precipitation.The soil respiration rate gradually increased with the warming,and showed a trend of rising first and then falling with increased precipitation,the maximum emission of soil respiration at the increased precipitation by 25%.These indicated that climate change will significantly affect the soil transformation process of carbon and nitrogen in the desert steppe.