Influence Of Precipitation Events On Ecosystem Carbon Flux Dynamics In Eurasian Grassland

Climate change affects ecosystems through changes in precipitation patterns and fluctuations in the magnitude,frequency and intensity of extreme precipitation events.Precipitation changes alter plant physiological processes and thus affect CO₂ source-sink dynamics.Therefore,there is an urgent need to assess the response of ecosystem carbon fluxes to extreme precipitation events.In this study,natural precipitation events were analyzed using eddy covariance（EC）data,and carbon flux change patterns were analyzed using box method data for artificially simulated extreme rainfall events,respectively.The first approach in this research was that the carbon fluxes dynamics documented data from three open-path EC flux observation systems during consecutive three years（2014-2016）in Mongolia–typical steppe（TPL）,meadow steppe（MDW）and shrubland（SHB）.The IRGA was calibrated before field setup and early of the growing season each year over grasslands.Data gaps due to instrument malfunction,power failure and calibration schedule were also filled using linear interpolation.An individual precipitation event was defined as a series of one or more consecutive days of precipitation followed by at least 1 day without precipitation.The amount of precipitation classified into four levels:0.1-2,2-5,5-10,and 10-25 mm d^-1.The second approach was that the experiment was arranged in a randomized block design with four replications,with an area of 5 m×6 m for each plot in Hulunbuir,Inner Mongolia.We used experimental precipitation gradients that specifically took into account extreme values from climatological perspectives that focused on percentiles of the long-term climate record.Thus,the 5^th percentile of precipitation（drier than 95%of the recorded years）and below represents an extreme drought,while the 95^th percentile of precipitation（wetter than 95%of the recorded years）and above represents an extremely wet year in grassland ecosystems.Each precipitation level corresponded to a specific percentile of precipitation records,ranging from the 5^th（driest）to the 95^th（wettest）percentiles of the spring and summer seasons for 49-years record（1967-2015）.The median of long-term historical data（～mean value）was used as control treatment.Five precipitation treatments（control,dry spring,wet spring,dry summer and wet summer）were implemented in mowed and unmowed grasslands to create the extreme precipitation gradients.The manipulated precipitation events were calculated by dividing the total amount of precipitation events by the number of days of precipitation in each month.The water was added via an electrically powered water pump connected to nearby water that was supplied by a local water delivery service and stored in on-site tanks.Finally,the main results were identified as follows:First,the gross primary productivity reached highest on days with 2 mm d^-1 and 5 mm d^-1 of precipitation in 2015 and 2014,respectively in TPL,whereas the highest gross primary productivity observed at 10-25 mm d^-1precipitation size in both MDW and SHB.The ecosystem respiration changed positively with different sizes of precipitation events,except for the 5 mm d^-1and 10 mm d^-1 precipitation size in the study sites.There existed positive relationship between net ecosystem CO₂ exchange（NEE）and small size precipitation event in both TPL and MDW.The slight association was detected between NEE and small size precipitation（<2 mm）events in SHB during the study years.By contrast,the strong relationship was observed between large precipitation sizes and NEE（p<0.01）in SHB.Second,across treatments（i.e.control,dry spring,wet spring,dry summer and wet summer）,the mean Rs was increased by 24.9%and 24.1%in the wet spring and wet summer precipitation treatments,respectively,and decreased by-22.1%and-3.5%in dry spring and dry summer precipitation treatments in mowed grassland.The relative contribution of Rh and Ra to Rs showed a significant（p<0.05）change among simulated precipitation treatments with the highest value（76.18%）in wet summer and 26.41%in dry summer,respectively under mowed grassland.Rs was significantly（p<0.05）affected by the interactive effect of extreme precipitation and mowing treatments in 2020 and 2021.The effects of precipitation change via these biotic and abiotic factors explained by 52%and 81%in Ra and Rh,respectively in mowed grassland.The changes in microbial biomass carbon（MBC）and nitrogen（MBN）had significant（p<0.05）direct effects on Rh in both mowed and unmowed grasslands.Third,the dry spring treatment significantly decreased ER and GPP by-16.8%and-11.1%,respectively,across the three study years in mowed grassland（p<0.05）.In contrast,the wet summer treatment significantly increased ER and GPP by 93.5%and 92.2%,respectively,across the three study years in mowed grassland（p<0.05）.The ER relative response showed the highest values,76.2%and 73.5%under wet spring and wet summer treatments,respectively,and the lowest values were recorded in dry spring（-16.7%）and dry summer（14.2%）in mowed grassland.The relative response of WUE showed comparable changes in dry precipitation treatments in mowed grassland.The nighttime NEE showed a positive exponential increase with soil temperature（Ts）among different extreme precipitation treatments,with Q₁₀ values ranging from 1.26 to 2.44 in mowed grassland.Fourth,the least value in NEE was found at 10-25 mm d^-1precipitation size in Bayan site.Among four precipitation sizes,the highest ER observed at 10-25 mm d^-1 size of precipitation events.The highest daily averaged GPP was at the large precipitation size events during the study period in Bayan.Additionally,the ER and GPP increased with size of precipitation events.As precipitation size increased,the relative response of ET increased in Bayan.Similarly,the relative response values of gross primary productivity（GPP）also increased with wet precipitation gradients,and decreased with dry precipitation gradients in Hulunbuir grassland.The relative response of water use efficiency（WUE）began to increase more in wet treatments than in dry treatments.In summary,the main findings of this study emphasized that the carbon fluxes of ecosystem is mostly sensitivity to precipitation events,causing them to alter between net carbon sink and sources depending on their extremity and precipitation size events which implies that the assessment of carbon fluxes is the first step for carbon trading;hence,it is crucial for estimating the interaction of extreme precipitation and size events with ecosystem carbon fluxes.Overall,this study provided a new perspective on the importance of both extreme precipitation and size events in controlling ecosystem carbon exchange,which are crucial to further understand carbon cycles in meadow steppe and help to predict how grassland structure and characteristic will respond to climatic change.