| In view of the environmental problem of whether dry soil absorbs greenhouse gas CH4,it is urgent to clarify the action mechanism of grassland soil on CH4 in arid and semi-arid regions.First of all,based on the measured CH4 flux,soil and vegetation parameters,the variation of CH4 flux and its correlation with environmental factors were analyzed.Secondly,this paper analyzes the Spatiotemporal characteristics of CH4 flux in the study area by using DNDC model simulation data.Finally,using scenario analysis method to select the most favorable prediction scheme for the realization of CH4 sink in the grassland in the next 10years,and analysing its warming potential and annual variation characteristics.The main research contents and results are as follows:(1)Characteristics of measured CH4 flux and its correlation with environmental factors.This study measured CH4 fluxes from 11 sites in July 2019 and July 2020 by using static chamber-gas chromatography,meanwhile measured the soil parameters(Soil bulk density,pH,clay ratio,SOC content)and plant parameters(the root/stem/leaf/grain biomass,biomass ratio and C/N ratio of annual and perennial plants)by experiments,and the soil temperature(0cm and 5cm)by field survey from 11 sites.The measured CH4 fluxes were all negative,indicating that the grassland soil in July was a CH4 absorption sink.The daily variation of CH4 absorption flux was high in day and low in night.The high value time of CH4 absorption flux was from 13:00 to 18:00,which could accumulate about 43 percent of the whole day's CH4 absorption flux.The CH4 flux at 7:00 is closest to the average daily CH4flux.Methane was absorbed by the types of woodland,grassland,bare land,sand land,cultivated land and transportation land.Moreover,the CH4 absorption flux of natural land use types(woodland,grassland,bare land and sandy land)was higher than that of cultivated land and transportation land.However,excessive soil moisture in wetlands caused the release of CH4 in an anaerobic environment.Soil SOC,clay proportion and soil bulk density had no significant effects on CH4 flux,while soil pH and soil temperature at 5cm depth had significant positive effects on CH4 absorption flux.Excessive aboveground plant biomass would inhibit the CH4 uptake rate by soil,while higher aboveground plant biomass would promote the CH4 uptake rate by soil.And the increase of plant C/N ratio was not conducive to soil absorption of CH4.(2)Sensitivity analysis and accuracy verification of DNDC model.Based on measured soil parameters,vegetation parameters,rainfall N concentration,the meteorological data of Chinese meteorological data website,CO2 concentration data from GOSAT official website and Grazing statistics data,this study analysed sensitive environmental factors of CH4 flux simulated by DNDC model.And the sensitive environmental factors of CH4 fluxes were screened out as temperature,precipitation,soil clay ratio,soil bulk density and soil SOC content.The increase of temperature and soil SOC content can promote the absorption of CH4by soil.The increase of precipitation,soil bulk density and soil clay ratio would slow down the absorption of CH4 by soil.Secondly,the measured CH4 fluxes in 2019 and 2020 were used to verify the accuracy of DNDC simulation values,and the correlation coefficient R between the two was 0.77(P<0.01),indicating that there is a very significant correlation between the simulated and measured values.Furthermore,the simulated and measured values of CH4 flux were interpolated in Arc GIS.The measured and simulated values of CH4 flux in the region were extracted by using 500 random points,respectively,and the linear fitting result of the two was R2=0.72.Therefore,the CH4 flux simulated by DNDC has high accuracy,and the CH4 flux in grassland can be simulated by DNDC model.(3)Temporal and spatial distribution of CH4 fluxa)Temporal variation of CH4 daily flux simulated by DNDC for two years.From March to November in 2019 and 2020,the soil showed CH4 absorption.It is basically the same that the change trend of 11 sample plots in each month.However,the CH4 flux of various plots varied in each month,which was related to the environmental factors such as soil and vegetation among the plots.The change curves of CH4 daily flux for the same month in 2019and 2020 are different,mainly due to the difference of precipitation time and quantity.However,the CH4 flux in the same month of the two years varied in a similar range,because the temperature varied in a similar range.In addition,influenced by the annual changes of temperature and precipitation in two years,the maximum CH4 absorption flux in 2019appears in July,and the seasonal absorption flux is shown as summer>autumn>spring>winter;the maximum CH4 absorption flux in 2020 appeared in May,and the seasonal absorption flux showed spring>summer>autumn>winter.The average value of cumulative CH4 absorption flux in 11 sample plots in 2019(0.6529kg-1·h-1)was higher than that in 2020(0.6391kg-1·h-1).b)Spatial distribution of CH4 daily flux simulated by DNDC for two years.In 2019,the monthly,seasonal and annual CH4 absorption fluxes showed a spatial distribution pattern of low in the northwest and high in the southeast in the study area;in 2020,the Monthly,seasonal and annual CH4 absorption fluxes in the study area showed a spatial distribution pattern of high in the north and low in the south.The difference of CH4 flux spatial pattern is closely related to the spatial pattern of temperature and precipitation.The regions with high temperature and annual precipitation of about 300mm have a higher CH4 absorption flux,indicating that these environmental conditions are conducive to the absorption of CH4 by soil.The total CH4 uptake in the grassland in 2019 and 2020 was 1.1714×106kg C and1.1135×106kg C,respectively.This indicates that the typical grassland is a large CH4 sink.(4)Scenario analysis of CH4 flux and warming potential(GWP)a)Based on the sensitivity test of DNDC model,the sensitive environmental factors of CH4 flux were screened out as meteorological factors(temperature and precipitation)and soil factors(soil SOC,soil bulk density and soil clay ratio),and the five environmental factors were taken as the variable factors of scenario analysis.From 2001 to 2020,Temperature increases volatility,precipitation decreases volatility.From 2000 to 2020,soil SOC showed a decreasing trend,while soil bulk density and clay ratio showed an increasing trend.Based on this,the scenario of the change of each factor in the next 10 years was set.Sixteen prediction schemes were obtained by free combination of scenario factors.and the CH4 flux of each prediction scenario in the next 10 years was simulated by DNDC.The interannual variation trend of CH4 flux was affected by both meteorological and soil factors,and the influence of meteorological factor group was greater than that of soil factor group.b)Under the prediction scheme?T1P1S1D1C1,the 10-year cumulative value of CH4absorption flux from 2021 to 2030 is relatively high,that is,to maintain the level of temperature and precipitation in the past 10 years and the average level of soil factors(soil SOC,bulk density,clay ratio)in the past 20 years,This scenario combination was not only conducive to the CH4 absorption sink of the grassland soil,but also met the needs of the stable and healthy development of the grassland ecosystem.Therefore,this scenario combination is preferred as the best forecast scheme.c)Under the scenario combination of the best prediction scheme,the average annual GWP of CH4 flux in the next 10 years shows a decreasing trend,with the average GWP of-18.273kg·hm-2,which is slightly larger than in 2019(GWP=-18.281kg·hm-2)and smaller than in 2020(GWP=-17.895kg·hm-2).It showed that the prediction scheme can effectively promote the absorption of CH4 in the grassland.Therefore,only by restoring the grassland environment to the average level of the 20 years before 2020 could the grassland maintain the optimal environment,which will be more conducive to the absorption of CH4 by soil of the typical grassland,so as to achieve the goal of slowing down the greenhouse effect. |
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