Variations And Influence Mechanism Of Carbon And Water Flux In Horqin Dune And Meadow Wetland Landscape

Horqin Sandy severely affected by the sensitivity of regional climate change and human activities in northern China under typical arid and semi arid desertification area,Over grazing caused serious land desertification and the underlying surface difference,which made the carbon and water process complexing.The region covers a variety of special geographical landscapes and plays an important role in maintaining the eco-hydrology balance in arid areas.This study selected two typical ecosystems which including dune and meadow wetland ecosystem as the main research object,by using the eddy covariance technique and the bowen-ratio meteorological and soil monitoring system,it analyzed the dynamic variations in carbon-water fluxes and the responses to environmental factors,and the variation of evapotranspiration and water use efficiency,to understand the interannual variation characteristics and control mechanism of carbon and water fluxes and quantized environmental factors and biological function changes contribute to the interannual variability of carbon and water fluxes,The results were as follows:1.The average diurnal variation of net ecosystem CO₂ exchange?NEE?exhibited a single peak U-shaped curve of dunes and meadow wetland ecosystem in 2016 and the NEE diurnal variation was negative during the daytime and positive during the nighttime,But there are obvious differences among different months.It was an absorption of CO₂from May to mid-August and a release of CO₂ from late August to September of meadow wetland ecosystem,showed that the accumulated?NEE?was-766.18 g CO₂·m^-22 during the growth season,and the accumulated gross primary productivity?GPP?and ecosystem respiration?R_e?were 3379.89 and 2613.71 g CO₂·m^-2,respectively.Daytime NEE fitted fairly well with the photosynthetically active radiation?PAR?by using a rectangular hyperbola function,the relationship was affected by the environmental factors,including vapor pressure deficit?VPD?,soil water content?SWC?and air temperature?T_a?.Regression analysis showed that the VPD and SWC for the maximum daytime NEE were2.75 kPa and 4.25%in the dune,the VPD and SWC were 1.75 kPa and 35.5%in the meadow wetland.Daytime NEE increased with the T_a,and T_a had no inhibitory effect on the daytime NEE when it reached the maximum.During the entire growth season,the temperature sensitivity of the ecosystem respiration(Q₁₀)was 1.6 and 2.4 for the dune and meadow wetland ecosystem,respectively,nighttime NEE was controlled by both T_sand SWC.2.The daily variation of balance components in different month shows single peak curve but ratio of energy partitioning in different components are not same in different time scale.The results showed that:sensible heat flux is the main part of the net radiation on dune all the year,and the maximum in May with a value of 142.32 W·m^-2,that latent heat flux was the main form of energy loss of meadow wetland during the growing seadon and follows by sensible heat flux in ungrowing season,the maximum occurred in July with value of 311.22 W·m^-2.The yearly net radiation of the dune ecosystem was1669.72 MJ·m^-2,and the sensible heat flux and latent heat flux were 784.29 and 652.36MJ·m^-2,and the sensible and latent heat fluxes were 46.9%and 39%of the net radiation,respectively,the smallest part less 1%of energy was stored in the soil heat flux.The annual net radiation?sensitive heat flux?latent heat flux and soil heat flux of the meadow wetland ecosystem were 3032.10?1587.14?114.38 and 16.40 MJ·m^-2,respectively,and the latent heat fluxes almost was 52.3%of the net radiation.There were significant correlations between sensitive and latent heat flux andnet radiation,air temperature?soil temperature?vapor pressure deficit?soil water content and wind speed over the dune in all year,latent heat fluxwas sensitive to net radiation,air temperature?soil temperature?vapor pressure deficit and soil water content over the meadow wetland,while remarkable correlations werefound between sensible heat flux and wind speed,different factor play an important role in different time scales.3.The evapotranspiration and water use efficiency have obvious seasonal variation and the difference of the dune and meadow wetland ecosystem,The total evapotranspiration was 278.03 mm and 683.27 mm in 2016,respectively,and the evapotranspiration mainly occurs in the growing season.The evapotranspiration and water use efficiency were affected by net radiation,air temperature,vapor pressure deficit and soil water content,and they all influenced evapotranspiration and water-use efficiency to some degree,and varied greatly between the ecosystems.4.The net ecosystem carbon exchange?latent heat flux and sensible heat flux exhibited a single peak in the dune,with the maximum and minimum values occurring in summer and winter,respectively,while the NEE and LE also had similar seasonal variation of the meadow wetland,but the H has no obvious trend.The results of HOS modle analysis separated four components:the function change?the direct interannual climate variability?the direct effects of seasonal climatic variation and random error,the contribution of seasonal variation of environmental factors to NEE,LE and H were47.10,64.10 and 84.70%over the dune during 2013-2016,respectively,the contribution of function changes were 3.80,8.80 and 0.30%,respectively.The seasonal variation of environmental factors for meadow wetland on the seasonal variation of NEE,LE and H were 32.30,65.10 and 67.10%,as well as 3.70,3.90 and 3.00%of function changes,respectively.The HOS modle be closer to the comprehensive observation value than the multiple regression simulation.It suggests that the carbon-water fluxes can not be predicted from the climatic variability alone in the future,the function change must be considered into mechanistic models.