Effects Of Water And Nitrogen Addition On Plant Community Structure,Compositionand Function In Meadow Steppe

Global environmental change caused by intensive human activity has been more and more severe since the industrial revolution.How to maintain biodiversity and ecosystem functioning under future global change scenarios is an important issue of modern ecology.Ecologists are currently tasked with evaluating and predicting ecosystem structure and function responses and feedbacks to global change.As the most extensive terrestrial ecosystem,grassland is vulnerable to environmental change.Water and nitrogen(N)are two limiting factors for most grassland ecosystems.Change in precipitation pattern and increasing N deposition have been changing the inputs of water and N,resulting in pronounced consequences to grassland ecosystem structure and function.Investigating the mechanism of grassland in response to changes in precipitation and increasing N deposition could improve our understanding about responses of terrestrial ecocsystem to global change,and is beneficial to manage natural resources.In this study,we used two manipulative experiments conducted in meadow steppe located on eastern edge of Eurasian steppe to examine the response of plant community structure and function to precipitation and N addition.Specifically,a six-year(2010-2015)water and N addition factorial design experiment was conducted on a Stipa baicalensis meadow steppe in Tumugi National Natural Reserve(Zhalaite Qi,Xing’an League,Inner Mongolia,China)to examine community composition,productivity stability and carbon exchange in response to water and N resources alteration.In addition,a four-year(2008-2011)gradient N addition experiment was conducted on a degraded Leymus chinensis meadow steppe(Tongyu County,Jilin Province,China)to investigate the effect of N enrichment on restoration succession of saline-alkaline grassland.The objectives of this study are to address the following primaryquestions:(1)Under future global change scenarios with increasing precipitation and N deposition,how the plant community structure and composition of meadow steppe change over relatively long time?(2)Howproductivity stability of meadow steppe responds to water and N addition? What are the primary mechanisms for maintaining community stability in meadow steppe?(3)How dominant and subdominant species hierarchy mediates the response of carbon exchanges to water and N addition in meadow steppe?(4)Whetherrestoration succession of degraded meadow steppe in response to N enrichment is regulated by soil salinization-alkalinization backgroud? Based on these two field manipulative experiments,we obtained the main results and conclusions as follows:(1)InS.baicalensis meadow steppe,over the six years period,water addition had no significant effects on community structure(species richness,Shannon-Wiener diversity index,Pielou evenness index and Simpson dominance index).However,N addition significantly decreased species richness,Shannon-Wiener diversity,and Pielou evenness.Simpson dominance index was significantly increased by N addition.For changes in the magnitude of treatment effects on community structure over time,water addition did not exhibit obvious trend.N addition showed slight positive effect on species richness during the first two years(2010-2011),and showed negative effect since the third year.The magnitude of negative effects of N addition on Shannon-Wiener diversity and Pielou evenness increased significantly during the first four years(2010-2013).Response dynamicsof community composition to water and N addition exhibited continuous directional pattern,that is to say,the effects of treatment-induced changes in community composition enhanced over time.Regression analysis indicated that mechanism driven by functional groups differed between the responses of community composition to water and N addition.Specifically,enhanced effect of water addition on community composition was mainly attributed to enhanced positive effect on perennial bunchgrasses(PB)and negative effect on perennial rhizome grasses(PR).Enhanced effect of N addition on community composition was mainly attributed to enhanced positive effect on perennial rhizome grasses and negative effect on perennial forbs(PF).Moreover,water and N addition did not influence the interannual variability rate of community composition.Variability in community composition is mainly determined by regional species pool,so water and N treatments had no effect on variability rate of community composition in single site belong to a regional species pool.(2)In the S.baicalensis meadow steppe,water and N addition had no significant effect on community-level stability.Additionally,dominant and subdominant species stability were not affected by water and N addition.At functional group level,water and N addition did not affect the stability of PR,PF and shrubs and semi-shrubs(SS).Water addition had no significant effect on PB stability.N addition significantly decreased PB stability,however,this negative effect was weakened with water addition.In this study,water and N addition had no significant effects on compensatory dynamics among species or functional groups within community.Dominant species asynchrony and subdominant species asynchrony were not influenced by water and N addition.N addition decreased PB asynchrony,and this negative effect was diminished with water addition.We analysed the mechanism for maintaining community stability.In this grassland ecosystem,dominant species stability and compensatory dynamics are the two dominanted factors driving community stability.Many diversity manipulative experiments found positive relationship between species diversity and community stability,however,we found no obvious linkage between diversity and stability in this grassland ecosystem.Nevertheless,this result dose not refute the potential role of diversity for stabilizing community.Instead,our study suggest that relatively small gradient of species diversity especially in single site of natural community would result in lack of significant positive diversity-stability relationship.Combined with the analyses of community structure and composition,we predict that community composition and species diversity would alter dramatically under water and N addition in long term scale,eventually causing significant changes in ecosystem function and its stability.(3)During the growing season in 2012 and 2013,we measured ecosystem carbon fluxes(gross ecosystem primary productivity,GEP;ecosystem respiration,ER;net ecosystem carbon exchange,NEE)inS.baicalensis meadow steppe.Our results showed that the impacts of water addition on carbon exchange varied significantly between the two years.In 2012,water addition decreased GEP and NEE,and did not significantly affect ER.In 2013,all the three compoents of carbon exchanges were not affected by water addition.N addition had no effects on carbon exchange in both years.Dominant and subdominant species hierarchy mediated the response of carbon exchanges to water and N addition.In 2012,the dominant species biomass was equal to subdominant species biomass across our experimental plots.Carbon exchanges were positively related to subdominant species biomass.Water addition suppressed subdominant species though not significantly,causing decreased carbon exchanges.N addition did not affect subdominant species resulting little effects on carbon exchanges.In 2013,dominant species biomass was significantly higher than subdominant species across all plots,and carbon exchanges were positively related to dominant species biomass.However,carbon exchanges were not affected by water and N addition because treatments had no effects on dominant species biomass.(4)In degraded L.chinensis meadow steppe,there were few changes in the species composition of the high saline-alkaline community,with annual species dominating all the plots over the four years.However,most of the low-stress communities developed from the annuals-dominated stage to the perennials-dominated stage.N addition significantly increased aboveground biomass of both communities,however,the N-induced increases in aboveground biomass were higher in low saline-alkaline community than in high saline-alkaline community.N addition did not affect species richness in high saline-alkaline community,and decreased species richness in low saline-alkaline community.These results suggest that although this grassland is a N-limiting ecosystem,but the degree of N-limitation is stronger in low saline-alkaline community than in high saline-alkaline ones because plant strategy is nutrient competition in low saline-alkaline community whereas salt-alkali tolerance in high saline-alkaline community.However,N enrichment did not obviously promote plant succession towards perennial community(especially Leymus chinensis community).Interestingly,the replicates within the same treatment under low-stress conditions did not develop towards similar community composition,indicating the importance of environment heterogeneity on the small spatial scale.From a restoration standpoint,we recommend natural succession induced by fencing in practice for restoration of degraded L.chinensis meadow steppe.Overall,our study provide insights for understanding the potential ecological consequences of precipitation change and N deposition on grassland ecosystem.The results in S.baicalensis meadow steppe supported the hierarchical-response framework,suggesting that directional change in herbaceous ecosystems may occur more rapidly due to relatively high turnover rates of herbaceous plant species.Our results showed that community stability and carbon exchange are closely related community composition.Hence,as water and N treatments continue,ecosystem structure and function would change dramatically in S.baicalensis meadow steppe.In addition,we found response of secondary succession dynamics of grassland community to N enrichment was regulated by soil characteristic(e.g.salinization-alkalization).This study will be helpful forclimate change adaption and mitigation,and provide scientific basis for rational utilization and adaptable management of natural resources.