The Responses And Mechanisms Of Ecosystem Carbon Cycle To Precipitation Variation In Songnen Leymus Chinensis Meadow

Global climate change intensifies the hydrological cycle and changes the precipitation pattern of terrestrial ecosystems,which are manifest in many ways,such as the increased variability of annual precipitation and altered seasonal distribution of rainfall events.In addition,climate models predict an increase in the frequency and magnitude of extreme precipitation.As one of the key driving factors of ecosystem processes,precipitation change affects plant physiological metabolism and soil physical,chemical and biological processes,with far-reaching consequences for the carbon cycle of terrestrial ecosystems and its feedback relationship with climate change.Grassland accounts for about a quarter of the total land area and plays a crucial role in the carbon cycle of terrestrial ecosystems.The structure,process and function of grassland are very sensitive to the altered precipitation.Compared with the single or combined effect of CO₂concentration and increasing temperature,precipitation change is predicted to have greater impact on the grassland ecosystem.Therefore,understanding the response of carbon exchange of the grassland ecosystem,allocation and turnover processes to precipitation changes will help improve our ability to predict the global carbon cycle in the context of climate change.Most studies on the effects of precipitation change on ecosystem processes more focus on inter-annual and inter-location precipitation changes.This method evaluates the responses of ecosystem to the altered precipitation regimes using rainwater,which could be confused by other factors（such as temperature）.Key environmental variables other than precipitation change with time and space,which may significantly change the response trend and degree of ecosystem carbon cycle to precipitation change.Recently,there has been an increasing trend of in-situ field experiments to study the response and adaptation strategies of ecosystem functions to altered precipitation through simulating increases or decreases in rainfall.However,there is a lack of systematic knowledge on the response of ecosystem carbon cycles to increases and decreases in precipitation.In order to fully comprehend the effect of precipitation change more accurately and systematically on the grassland ecosystem,and to solve the corresponding issues such as the effect of increasing precipitation and decreasing precipitation is not equivalent,it is urgent to cover the field experiment of increasing precipitation and decreasing precipitation.Moreover,since organic carbon is mainly stored in soil of grassland ecosystems,belowground carbon cycle may determine the response of grassland to precipitation change.Most of the studies on how precipitation changes regulate the carbon cycle in grassland ecosystems have focused on the response of aboveground carbon dynamics to precipitation changes,but the research on belowground carbon processes is scarce.In this study,a field experiment was conducted to simulate precipitation changes in Leymus chinensis meadow to investigate the response of key processes of carbon cycle（especially belowground carbon processes）to altered rainfall gradient,and its regulation mechanism in the Songnen grassland.In this study,a three-year field experiment with six levels of increasing（+30%,50%and 70%）and decreasing precipitation magnitude（-30%,50%and 70%）in the growing season（From May to September）was conducted.By measuring the main carbon cycle indicators of the studied grassland ecosystem,the response of carbon exchange to simulated rainfall gradient was analyzed in the semi-arid grassland ecosystem.Meanwhile,the stable carbon isotope labeling technique was used to characterize the strategy of photosynthate allocation in the plant-soil system in response to precipitation change,and the response mechanism of soil aggregate carbon accumulation to precipitation change was analyzed.This study can provide effective experimental data for predicting the response of ecosystem rainfall changes,and provide key parameter verification for modeling grassland carbon cycle.The main findings and conclusions of this study are as follows:（1）Simulated precipitation significantly changed carbon exchange in the Songnen grassland ecosystem,and the ecosystem carbon exchange had a nonlinear response to precipitation change in the Songnen grassland.The ecosystem carbon exchange showed obvious seasonal dynamics and was affected by the interannual effect of precipitation.After three years of precipitation treatment（2016-2018）,the variation range of ecosystem carbon exchange increased significantly.Total ecosystem production and ecosystem respiration were significantly positively correlated with increasing precipitation,while the opposite trend was observed for net ecosystem CO₂exchange.Due to the high sensitivity of ecosystem carbon exchange to drought（rain reduction treatment）,the carbon flux of ecosystem had a nonlinear response to precipitation change.Precipitation did not change plant diversity but significantly affected aboveground net primary productivity.The aboveground net primary productivity and soil water content were significantly correlated with total ecosystem production,ecosystem respiration and net ecosystem CO₂exchange.Further structural equation model analysis confirmed that soil water content was the key factor controlling the nonlinear response of ecosystem carbon flux to precipitation change.（2）Due to the high sensitivity of ecosystem carbon exchange to rainfall reduction,the stable carbon isotope tracer technique was used to investigate the effects of drought（P-30%）on plant-soil photosynthate distribution in the Songnen grassland.The results showed that drought significantly reduced ¹³C abundance in plants（leaves,stems and roots）.The decreasing precipitation reduced the¹³C abundance of silt and clay components of soil,but had limited effect on the ¹³C abundance of soil microaggregates.The aboveground and belowground distribution of photosynthates had the high regulation ability.Under drought conditions,plants responded to the negative effects of soil water deficit by decreasing aboveground carbon distribution and increasing belowground carbon distribution.In addition,the decreasing precipitation increased the correlation between root and soil carbon distribution.Moreover,the carbon distribution between roots and silt and clay components of soil was more closely related than that of microaggregates.（3）Simulated precipitation significantly changed plant above-and belowground biomass,soil characteristics（soil water content,aggregate distribution,enzyme activity and microbial biomass）,and ultimately affected the carbon sequestration capacity of the ecosystem.The response of soil fertility and function depended on the variation of aggregate size and precipitation.With the increasing precipitation,the proportion of soil microaggregates decreased gradually.However,precipitation variation did not significantly change soil microaggregates carbon content and enzyme activities.With the increasing precipitation gradient,the proportion of soil silt and clay gradually increased,and the carbon content and enzyme activity also increased correspondingly.In addition,the effects of precipitation change depended on aggregate size on soil aggregate carbon accumulation:plant biomass was the main controlling factor of soil microaggregate carbon accumulation;soil aggregate distribution and enzyme activity were the main factors affecting carbon accumulation in silt and clay of soil.Changes in precipitation led to redistribution of soil aggregate components,which ultimately affected the carbon sequestration capacity of the ecosystem by changing soil aggregate carbon accumulation.In summary,precipitation variation significantly changed the carbon cycle of the Songnen grassland ecosystem.The carbon cycle is not only dependent on the variation of rainfall on the grassland ecosystem,but also affected by the interannual variation of natural precipitation and the duration of precipitation treatment.Precipitation changes affect carbon exchange,plant-soil carbon distribution and soil carbon accumulation by changing plant（aboveground and belowground biomass）and soil characteristics（soil aggregate distribution,enzyme activity）,and further consequence on the carbon cycle of grassland ecosystem.In this study,the nonlinear relationship between key processes of carbon cycle and rainfall gradient was confirmed by the responses of several processes of carbon cycle to rainfall change.Moreover,due to the high sensitivity to precipitation change in the Songnen grassland,changes in rainfall will affect the ecosystem carbon sequestration function.This will affect the carbon sequestration potential of the Songnen grassland by disproportionately changing the carbon input and output of the ecosystem.Therefore,the study of multi-gradient rainfall experiment is conducive to our understanding of the carbon cycle of the ecosystem,especially the dynamic process of belowground carbon,and helps to improve our ability to predict the impact of precipitation change on soil carbon sequestration.In this study,we focused on the response mechanism of the belowground carbon distribution process to rainfall gradient in semi-arid grassland ecosystems,and the results found that precipitation change promoted the redistribution of soil aggregate components by adjusting the plant-soil carbon distribution process,and ultimately affected soil aggregate carbon accumulation.The results provide reliable experimental evidence and theoretical basis for predicting and simulating the response of grassland ecosystem to rainfall change.