Research On CO₂ Exchange For Different Ecosystems And Its Influence Factors In The Badain Jaran Desert

Arid and semi-arid areas are important parts of terrestrial ecosystem and play a very important role in the carbon cycle of terrestrial ecosystem.The Badain Jaran Desert(BJD)is located in the arid area of northwest China,which has three typical underlying surfaces:lakes,sand hills and grassland.Due to harsh natural conditions and backward transportation and communication,the CO₂ budget balance in the BJD is not clear,and the mechanism of carbon cycle is also lacking.Based on this,lake,desert and grassland ecosystems in the Badain Jaran desert ecosystem were studied,and used the eddy covariance(EC)technique to conduct long-term monitoring of CO₂ flux of the three ecosystems.The variation characteristics of CO₂ flux at different time scales were analyzed.The carbon absorption capacity of the ecosystem was quantified and the nature of its source and sink was clarified.The responses of net CO₂ exchange(NEE)in desert and grassland ecosystem to precipitation events of different intensities were analyzed.The main environmental factors affecting CO₂ fluxes at different time scales were clarified.The main conclusions are as follows:(1)During the observation period,the carbon sequestration capacity of the lake ecosystem was the weakest,and the annual CO₂ flux was-97.07 g C m^-2a^-1.The average annual NEE of desert ecosystem was-127.54 g C m^-2a^-1.Grassland ecosystem had the highest carbon sequestration capacity,with an annual NEE of-138.21 g C m^-2a^-1.According to the carbon sink rate and area of the three ecosystems,it is estimated that the annual carbon uptake in the whole BJD is about 6.652×10~6 t.(2)The temporal variation characteristics of CO₂ flux in different ecosystems are different.CO₂ uptake in the lake ecosystem mainly occurred from 00:00 to 19:30.The diurnal and diurnal variations of NEE in desert and grassland ecosystems were u-shaped,with CO₂ absorbed during the day and released at night.The lake ecosystem has obvious CO₂ release phenomenon in summer,and the maximum CO₂ uptake in autumn.The amount of CO₂ absorbed by desert ecosystem was the highest in summer,and CO₂was released in winter.Under the influence of drought,grassland ecosystem will change from carbon sink to carbon source in the growing season.(3)Abiotic factors are the primary reason that the lake ecosystem acts as an atmospheric CO₂ sink.Inorganic processes are the main mechanisms controlling the CO₂ absorption and releases in the lake,which is dominated by p H and driven by temperature to regulate CO₂ exchanges at the interface between lake and air.(4)The threshold value of the NEE response to precipitation was?1.4 mm,indicating that desert ecosystem is extremely sensitive to precipitation events.The higher the precipitation intensity,the stronger the carbon fixation ability,the longer the time to reach the maximum CO₂ absorption and the duration of the residual effect.In the context of the warm and humid climate in Northwest China,total precipitation and the frequency of extreme precipitation are expected to increase;thus,the carbon sequestration capacity of the desert ecosystem will be enhanced.(5)Precipitation events have a strong control effect on the carbon flux in grassland ecosystem.Minor precipitation events did not trigger an NEE,gross primary productivity(GPP),or ecosystem respiration(R_eco)pulse,whereas large precipitation events or continuous minor precipitation events caused subsequent high assimilation phenomena.In the warm and humid climate of northwest China,if the frequency of minor precipitation events increases,the R_eco will be greatly affected.However,more frequent heavy precipitation events,especially extreme events,would increase the carbon sequestration capacity of the grassland ecosystem.(6)The main meteorological factors controlling CO₂ flux are different in different ecosystems.In the lake ecosystem,CO₂ flux was highly correlated with vapour pressure deficit(VPD)and relative humidity(RH)and air temperature(T_a)and water surface temperature(T_w)at the half-hour scale.On the daily and monthly scales,CO₂ flux was highly correlated with T_a,T_w,net radiation(R_n)and VPD.In desert ecosystems,the correlation between NEE and R_n was the strongest at the half-hour scale,followed by soil surface temperature(T_s).There was a strong correlation between R_n,soil volumetric water content at 30 cm deep(VWC₃₀),VPD and NEE on daily scale.On the monthly scale,the correlation with R_n and VWC₃₀ was higher.In grassland ecosystems,the correlation between NEE and photosynthetically active radiation(PAR)was highest on both half-hour and daily scales.In this paper,the temporal variation characteristics of CO₂ flux,carbon source/sink properties,precipitation response and environmental factors of the lake,desert and grassland ecosystems in the Badain Jaran desert ecosystem were studied.The results are helpful to reveal the mechanism of carbon cycle for the whole ecosystem,which is of great significance to enrich the carbon cycle of different ecosystems and search for missing sink,and can provide a certain basis for estimating the carbon budget of desert ecosystems.