The Characteristics Of GHG Emissions In Five Cultivated Grasslands During The Growing Season In Arid Land Of Northwest China

As the source and sink of greenhouse gases,terrestrial ecosystems play a crucial role in global climate change.Northwest Arid Region is the major production bases for cultivated grasslands in China.Five cultivated grassland,which included single sown grassland of annual ryegrass,perennial smooth bromegrass and alfalfa and mixed-grassland of alfalfa/smooth bromegrass and alfalfa/tall fescue,was the experimental group,and the natural grassland was the control group.In order to clarify the relationship among soil physicochemical properties,hydrothermal condition,aboveground biomass and greenhouse gases,and finally to determine the main factors affecting the greenhouse gas emissions in different grassland,we measured the emission fluxes of the three major greenhouse gases(CO2,CH4,and N2O)and analysized the characteristics of soil physicochemical properties during the growing season in 2016 and 2017.This study can clarify the rules and characteristics of CO2,CH4,and N2 O emissions in different type of grassland during the entire growing season,which would provide reasonable scientific data for emission reduction and plays a guiding role in the improvement of saline-alkali land in arid areas.The main findings are as follows:(1)Both CO2 and N2 O were emission source during the whole growing season in different types of grassland,and the five cultivated grasslands showed the absorption status of CH4.Natural grassland showed two states of CH4 absorption and emission,and there were significant differences between two years.The emission fluxes of the three greenhouse gases had the same unimodal trend during the growing season,which increased in May and June,reached the maximum in July and decreased in August and September.(2)During the whole growing season,the CO2 emission flux of perennial mixed-sowing grassland and natural grassland in 2016 was significantly lower than that in 2017(P<0.05).The CO2 emission flux has no significant difference inthe single sown grassland of annual ryegrass between years.The absorption flux of CH4 in 2016 was significantly lower than that of 2017(P<0.05).The flux of N2 O of single sown grassland of annual ryegrass and alfalfa in 2016 was significantly lower than that of 2017(P<0.05).We calculated three greenhouse gases emission flux on the basis of CO2 as an equivalent unit,except that single the sown grassland of two perennial grasses of Bromegrass and Alfafa were significantly lower in 2016 than in 2017(P<0.05),there was no significant difference in other types of grasslandbetween years.(3)Surface soil(5cm)temperature was significantly negatively correlated with CH4 emissions and positively correlated with CO2 and N2 O emissions.The water content in different soil layers(0-10 cm,10-20 cm,20-30cm)was positively correlated with the emission of three greenhouse gases.CO2 fluxes were more sensitive to hydrothermal conditions than CH4 and N2 O.When the soil moisture content was between 15 to 25% and the temperature was in the range of 12-20°C,the CO2 and N2 O emission fluxes increased with the increase of soil moisture and temperature.The response of different grassland soil respiration to temperature are different,the annual rye grassland was the most sensitive,followed by single sown perennial grassland.(4)There was no significant change in soil physical and chemical indexes between years,and the content of SOM and TN in perennial grassland soil was higher than that of in annual grassland.Soil p H and COND were common factors affecting greenhouse gas emissions in all grasslands,and TN was the common factor influencing greenhouse gas emissions from the single sown grassland and mixed-grassland of alfalfa.The emission fluxes of the three greenhouse gases were linearly positively correlated with above-ground biomass.(5)Grassland warming potential has The regularities of global warming potential in different types of grassland were found as follows :,annual ryegrass < alfalfa/tall fescue < alfalfa/smooth bromegrass <natural grassland < perennial smooth bromegrass < alfalfa.The single sown annual grassland mixed-grassland of perennial alfalfa could reduce global warming potential,while single sown perennial grassland increasedglobal warming potential of soil.Compared with natural grassland,the global warming potentials of three grassland of ryegrass,alfalfa/tall fescue and alfalfa/smooth bromegrass decreased by 39.62%,23.83% and 18.61% respectively in 2016.On the other hand,The global warming potentials increased 1.81% and 6.87% in alfalfa and smooth bromegrass single sowngrassland.In 2017,the global warming potentials of ryegrass,alfalfa/tall fescue and alfalfa/smooth bromegrass grasslanddecreased by 40.40%,6.48% and 4.37% respectively than that of natural grassland.And alfalfa and bromegrass rassland increased by 6.98% and 10.77% respectively.