Long-term Effects Of Warming And Nitrogen Addition On The Stability Of Plant Community In A Desert Steppe

Global warming and increased nitrogen deposition due to human activities have great impacts on the biodiversity of terrestrial ecosystems and the composition,function and stability of plant communities.Grassland is one of the most important ecosystems in the world.Thus it is important to study the effects of climate warming and increased nitrogen deposition on grassland ecosystem function both now and in the future.The Stipa breviflora desert steppe is located in the transition zone between the temperate steppe and the desert in central Asia.It has unique composition,structure,function and dynamics under arid conditions.However,long-term effects of climate warming and increased nitrogen deposition on the composition,function and stability of plant community in the desert steppe are still unclear.This study is focused on the Stipa breviflora desert steppe in Inner Mongolia.A simulated warming and N addition experiment was conducted in the field from 2007 to 2018,using a split-plot design with warming(warming vs.no warming)as the main plot factor and N addition(N addition vs.no N addition)as the subplot factor.Thus,24 subplots were established with the following treatments:control(CK),N addition(N),warming(W)and warming with N addition(WN),with each treatment replicated six times.I analyzed the effect of long-term warming and N addition on changes in soil physical and chemical properties,plant community composition and function,species diversity,species asynchrony,and community stability,and explored the underlying mechanisms.The main results are summarized as follows:(1)During 2007-2018,warming significantly affected the temperature in the 0-10 cm soil,but N addition and their interaction had no effect on it.Compared with CK,soil temperature was significantly increased by 0.76? and 0.73? by W and WN,respectively.In addition,warming and N addition and their interaction had no significant effect on the volumetric water content in the 0-10 cm soil layer.(2)Aboveground net primary productivity of plant communities was significantly increased by N addition,but was not affected by warming and their interaction.N addition significantly increased the aboveground biomass of perennial forbs and annuals and biennials,but not that of perennial grasses and shrubs and semi-shrubs.However,warming and the interaction of warming and N addition did not affect the aboveground biomass of different functional groups.(3)Warming increased the aboveground biomass of C4 species,but had decreased that of C3 species during the experimental period.N addition and the interaction of warming and N addition had no significant effect on them.Warming did not change the aboveground biomass of C3 and C4 species when N was absent,but decreased aboveground biomass of C3 species by 17.3%while increased that of C4 species by 34.4%when N was added.In addition,warming decreased the aboveground biomass of Stipa breviflora and Convolvulus ammannii,but increased that of Kochia prostrate and Cleistogenes songorica.However,N addition and the interaction of warming and N addition did not affect the aboveground biomass of these four dominant species.(4)Warming significantly decreased species richness but increased Pielou's evenness index.However,N addition and the interaction of warming and N addition did not affect species richness,Shannon-Wiener's index,Margalef s index and Pielou's evenness index.In addition,species asynchrony was decreased by warming and N addition but was not affected by their interaction.(5)During 2007-2018,community stability was decreased by warming and N addition but was not affected by their interaction.The stability of perennial grasses,perennial forbs and shrubs and semi-shrubs was decreased by N addition but was not affected by warming and their interaction.On the other hand,warming decreased the stability of C3 species but did not affect that of C4 species.However,N addition decreased the stability of C4 species,but did not affect that of C3 species.(6)During 2007-2018,N addition significantly increased GEP and NEE,but did not affect ER.However,warming and the interaction of warming and N addition did not affect NEE,ER and GEP.Averaged over 12 years,N addition significantly increased NEE and GEP by 31.1%and 10.3%,respectively,under ambient temperature;N addition only significantly increased GEP by 13.0%under warming.N addition only had a positive effect on ER in specific years.(7)Warming decreased community stability by decreasing the stability of C3 species,but N addition decreased community stability by decreasing the stability of C4 species.In addition,both warming and N addition decreased community stability by decreasing species asynchrony.However,warming-induced decrease of species richness and N addition-induced increase of plant aboveground net primary productivity had no relationship with community stability.(8)The aboveground biomass of the community had positive relationships with ER and GEP.In addition,the aboveground biomass of perennial forbs had a positive relationship with ER,and annuals and biennials had a positive relationship with NEE.There were no relationships between ecosystem carbon fluxes and species richness,plant diversity indexes and species asynchrony.N addition increased GEP and ER by increasing the community aboveground biomass,and N addition increased NEE by altering species asynchrony and community stability.The warming-induced decrease in species richness had no significant effect on NEE,ER and GEP.Overall,plant aboveground productivity and ecosystem carbon exchange were not negatively affected by warming through adjusting the aboveground biomass of functional groups in the desert steppe,whereas nitrogen addition promoted plant productivity and carbon sequestration,and reduced the risk of further desertification of the desert steppe.On the other hand,both warming and nitrogen addition affected community stability by altering the stability of plant functional groups and species asynchrony.This research shows that climate warming and increased atmospheric N deposition will reduce the stability of plant communities in the studied desert steppe.