Study On Carbon Budget Process Of Grape Farmland In Arid Oasis Region

The fragile ecological environment makes the process of ecosystem carbon budget extremely sensitive to the interference of natural factors and human activities in the arid area of Northwest China,especially the oasis farmland ecosystem is strongly disturbed by human production activities.Therefore,it is of great significance to deeply understand the dynamics of productivity and respiration process of the vineyard ecosystem in this region for accurately evaluating the process of farmland carbon budget in arid oasis region of China.Taking the vineyard in Nanhu Oasis of Dunhuang as the research object,the net ecosystem exchange and soil respiration of the vineyard were measured by combining the vorticity covariance system with soil carbon flux measurement system in 2019.Gross primary productivity（GPP）,net ecosystem productivity（NEP）and ecosystem respiration（R_e）were obtained by the net ecosystem exchange.Soil respiration（R_t）was divided into below-ground autotrophic respiration(R_ab)and heterotrophic respiration（R_h）by root exclusion method,above-ground autotrophic respiration(R_aa)and total autotrophic respiration（R_a）were calculated.Based on this,the dynamics of productivity and respiration process of the vineyard in arid oasis area were explored,and the comprehensive effects of various environmental factors on them were determined by using correlation analysis and structural equation model.Finally,the process of carbon budget in grape farmland was quantitatively evaluated.The main results are as follows:（1）The diurnal and seasonal variation of gross primary productivity（GPP）and net ecosystem productivity（NEP）of grape fields in the oasis in 2019 showed unimodal characteristics,but there were some fluctuations in different weather condition.The correlation analysis showed that the main influencing factors of the two were leaf area index（LAI）,air temperature（T_a）,photosynthetic effective radiation（PAR）,soil water content（SWC）,water vapor pressure deficit（VPD）,etc.The structural equation models between GPP,NEP and environmental factors were established to quantify the direct and indirect effects of various factors.The final model could explain 67%and60%of the changes of the two factors.The models showed that the most important factor was VPD,whose standardized total effect coefficients for GPP and NEP were-0.63 and-0.71,respectively.In addition,the enhancement of T_aand PAR could promote photosynthesis and improve farmland productivity to a certain extent.Although the total effects of SWC on GPP and NEP were lower,the coefficients of effect were only0.15 and 0.22,it would affect GPP and NEP through the negative effect on VPD.Finally,we focused on the response of NEP to the main environmental factors.PAR,as the direct driving force of photosynthesis,could explain 27～54%of NEP changes with the right-angle hyperbolic model,but high T_a,high VPD and low SWC will weaken the response of NEP to PAR.（2）For each respiratory process,the seasonal variation of R_e was consistent with that of LAI and T_a.The maximum value of R_e was 9.20 g C m^-2 day^-1 in July.R_tand R_a fluctuated strongly from June to September,but decreased gradually from October to December.In terms of R_t components.R_h was the dominant in R_t,while R_aawas the dominant in R_a.The primary influencing factors of R_e,R_t and R_awere T_a,T_sand PAR,respectively,which were determined by structural equation models.The total standardized effects of VPD to R_e,R_t and R_a were relatively small.The results showed that there were differences in the main controlling factors of each respiratory process and productivity.In addition,SWC had weak negative and positive effects on R_t and R_a,and the effect coefficients were-0.15 and 0.36,respectively.Therefore,sufficient irrigation in this area would have different effects on R_t and R_a.In conclusion,a precise understanding of the driving mechanism of carbon budget process could provide some reference for carbon cycle model calibration.In addition,the fit goodness of R_e,R_t and R_a were 62%,28%and 43%,respectivily,and the model had lower ability to explain the variation of soil respiration,which was related to the complexity of soil and underground processes.（3）The annual accumulation of GPP,NEP and R_e were 1520,429 and 1091 g C m^-2 in 2019,The annual carbon consumption intensity was about 72%,while the carbon consumption intensity in growing season was only 55%.Compared with other dryland ecosystems（including cotton fields,desert thickets and grasslands,etc）,grape field had strong photosynthesis and respiration,and better carbon sequestration capacity.During the observation period,the cumulant of R_t,R_h,R_ab,R_aa and R_a from June to December were 475,311,164,228 and 392 g C m^-2,respectively.And their contributions to R_ewere 68%,44%,24%,32%and 56%,respectively,which indicated R_t was the main component of R_e in vineyard.