Ecosystem Carbon Exchange In A Desert Steppe In Ningxia Under Precipitation Change And Nitrogen Addition

As the consequence of global change,the change of precipitation pattern and increase in atmospheric nitrogen deposition directly affect soil moisture and nutrient contents,and thus affect plant community structure and the migration and transformation of soil-microbial elements.Both plant community structure and soil-microbial element balance could directly or indirectly affect the capacity of carbon fixation through plant photosynthesis,resulting in the changes in ecosystem carbon exchange.Desert steppe is characterized by low precipitation,high evaporation,low critical load of nitrogen deposition and the total amount of acceptable nitrogen deposition,so they are sensitive to the change in precipitation and the increase of nitrogen deposition.However,there still lacks a better understanding on how precipitation,nitrogen deposition and their interactions affect ecosystem carbon exchange in desert steppe,especially under extreme precipitation conditions.To deeply understand the response mechanisms of the ecosystem carbon exchanges in desert steppes under changing precipitation and increasing nitrogen deposition,a field experiment involving five precipitation treatments(50%reduction,30%reduction,natural,30%increase,and 50%increase)and two nitrogen addition treatments(0 and 5 g·m-2·yr-1)was conducted in a desert steppe of Ningxia in 2017.The temporal dynamics of net ecosystem carbon exchange(NEE),ecosystem respiration(ER),and gross ecosystem productivity(GEP)were monitored from May to October of 2019.Together with the changes in plant biomass,plant diversity,soil and microbial C:N:P ecological stoichiometry,and other key soil properties,the main influencing factors of ecosystem carbon exchange were analyzed too.The main findings are as follows:1)Changing precipitation did not clearly change the temporal dynamics of ecosystem carbon exchange.Under 0 and 5 g·m-2·yr-1 of nitrogen addition,monthly dynamics of NEE,ER,and GEP increased first and then decreased,with the peak values appearing in late August or early September.Ecosystem carbon exchange changed from carbon source to carbon sink during the growing season.The daily dynamics of NEE,ER and GEP also showed the same trends,with the highest values of NEE and GEP showing at 9:30 am and that of ER showing at 12:00 am;2)The decreasing precipitation reduced ecosystem carbon exchange,while the increasing precipitation promoted ecosystem carbon exchange,especially under 5 g·m-2·yr-1 of nitrogen addition.Under 0 and 5 g·m-2·yr-1 of nitrogen addition,compared with natural precipitation,the decreasing precipitation reduced NEE,ER,and GEP to a certain extent,whereas increasing precipitation showed different effects,especially under 30%increase of precipitation.Under the same precipitation condition,nitrogen addition enhanced NEE,ER and GEP to different degrees,and its effect was more obvious with natural and increasing precipitation;3)The decreasing precipitation inhibited plant growth,while the increasing precipitation and adding nitrogen changed plant biomass,and their effeces were species specific.The Shannon-Wiener diversity index and Pielou evenness index increased firstly and then decreased with the increase of precipitation.The reducing precipitation increased soil total nitrogen and phosphorus,while the increasing precipitation by 30%decreased soil total nitrogen and phosphorus,and increased soil organic carbon,C:N,and C:P.Both changing precipitation and adding nitrogen could affect microbial biomass C:N:P ecological stoichiometry,especially under extremely increasing precipitation(50%);4)ecosystem carbon exchange closely related to plant community structure.Specifically,-NEE,ER and GEP were positively correlated with the community biomass and the population plant biomass of Lespedeza potaninii,Astragalus melilotoides,Pennisetum centrasiaticum,Stipa capillata,Cleistogenes squarrosa(P<0.05).-NEE was positively correlated with Patrick richness index and Shannon-Wiener diversity index(P<0.05),had significant negative correlation with Pielou evenness index(P<0.05).Both ER and GEP had significant positive correlations with Patrick richness index(P<0.05)and significant negative correlation with Pielou evenness index had(P<0.05);5)ecosystem carbon exchange also related to microbial biomass C:N:P ecological stoichiometry.Specifically,-NEE was significantly negatively correlated with microbial biomass C:N(P<0.05),ER was significantly positively correlated with microbial biomass nitrogen(P<0.05),GEP was significantly negatively correlate with microbial biomass phosphorus(P<0.01);6)Ecosystem carbon exchange were more greatly affected by soil C:N,C:P,phosphatase activity,water content,organic carbon,electrical conductivity,total phosphorus,total nitrogen and NH4+-N(P<0.05).Specifically,-NEE and GEP were positively correlated with soil C:N,C:P,phosphatase activity,water content,organic carbon,electrical conductivity and total phosphorus,and negatively correlated with soil total nitrogen and NH4+-N,whereas ER showed the opposite correlations with the soil properties above.The results above indicate that,decreasing precipitation reduces soil water,changes soil nutrient availabilities,inhibits plant growth,and thus limiting ecosystem carbon exchange.An appropriate increase in precipitation can promote plant growth and species diversity through increasing soil water content,stimulating soil enzyme activities,regulating soil C:N:P stoichiometric balance,etc.,consequently improving the ecosystem carbon sink function;nitrogen addition also promots ecosystem carbon exchange,but its interaction with precipitation is time-lag to a certain extent.Therefore,a long-term field observation is necessary.