| As the consequence of global change,the change of precipitation pattern and the increase in atmospheric nitrogen(N)deposition directly regulate soil moisture and nutrient availabilities,and thus affect plant community characteristics and soil properties,which jointly drive the input and output processes of carbon(C)in plant-soil system and in turn lead to changes in ecosystem C sink function.Desert steppe is characterized by low precipitation,high evaporation,and low critical load of N deposition.Therefore,this ecosystem is sensitive to the change in precipitation and the increase of N deposition.However,there still lacks a better understanding on how the interaction of precipitation and N deposition affects C source/sink in desert steppes,especially under extreme precipitation condition.To deeply understand the response mechanism of C pool and C emission in desert steppes under changing precipitation regime and increasing N deposition,a field experiment involving five precipitation treatments(50%reduction,30%reduction,natural,30%increase,and 50%increase)and two N addition treatments(0 and 5 g·m2·yr-1)was conducted in a desert steppe of Ningxia in 2017,the characteristics of C pool in plants and soils in August of 2021 and the temporal dynamics of soil respiration rate from May to October in 2021 were studied.Together with the changes in plant biomass,plant diversity,and key soil properties,the main influencing factors of plant-soil C pool and soil respiration rate were analyzed too.The main findings are as follows:(1)Precipitation and its interaction with N addition had a significant effect on total C in plant communities and above-ground plant C storage.Under 0 g·m-2·yr-1 of N addition,the 50%reduction in precipitation significantly reduced total C,the reduced precipitation significantly reduced C storage.Under 5 g·m-2·yr-1 of N addition,both the 30%reduction the increase in precipitation significantly increased total C and C storage.Under the same precipitation condition,N addition significantly increased C storage when precipitation was reduced 30%.(2)Except soil light fraction organic C and microbial biomass C,precipitation,N addition and their interaction had no significant effects on organic C and its fractions.Under 0 g·m-2·yr-1 of N addition,the reduced precipitation decreased organic C and its fractions to different degrees and increased microbial biomass C.Under 5 g·m-2·yr-1 of N addition,the reduced precipitation significantly increased dissolved organic C in the soil shallow layer,while the increased precipitation decreased organic C fractions to different degrees.Under the same precipitation condition,the effects of N addition on soil organic C and its fractions were modulated by precipitation.(3)Precipitation had a significant effect on soil respiration rate,while N addition and its interaction with precipitation had no significant effects on soil respiration rate.In the way of daily dynamic,soil respiration rate increased first and then decreased.Whereas,it showed a fluctuating monthly dynamic.Under 0 and 5 g·m-2·yr-1 ofN additions,the increasing precipitation significantly increased soil respiration rate.Under the same precipitation condition,N addition significantly reduced soil respiration rate when precipitation was reduced by 30%.(4)Precipitation and N addition had a direct negative effect on total C in plant community,but neither had a direct effect on C storage.Additionally,precipitation also had an indirect effect on total C and C storage through its positive effect on plant community characteristics.(5)Precipitation and N addition have a direct positive effect on soil organic C storage,organic C and its components,or indirect effects on organic C and its components through their effects on environmental factors.Precipitation and N addition have indirect effects on organic C and its components through their negative effects on soil chemistry and biological properties.(6)Precipitation and N addition had no direct effects on soil respiration rates.However,they could indirectly affect soil respiration rates through their effects on environmental factors.Precipitation indirectly affected soil respiration rates through its negative effects on plant community characteristics,while N addition indirectly affected soil respiration rates through its positive effects on soil biological properties.The results above indicate that,the reduced precipitation(especially extreme reduction)may reduce plant-soil C fixation by reducing soil resource availability and thus inhibiting plant growth.The increased precipitation may increase plant-soil C fixation and soil respiration through increasing soil moisture and enzyme activity and thus stimulating plant growth;there are lack of consistent effects of N addition on plant-soil C fixation and soil respiration.There is a time-lag in the interaction ofN addition with precipitation.Therefore,a long-term field observation is necessary. |
PDF Full Text Request