The Effects Of Nitrogen And Phosphorus Addition On The Species Diversity And Stability Of Plant Communities In Hulunbuir Grassland

As a result of the combined effects of industrial development and global climate change,atmospheric nitrogen deposition has gradually increased,leading to a gradual increase in terrestrial ecosystem availability of nitrogen（N）.Grasslands,as an important part of terrestrial ecosystems,are significantly affected by nitrogen deposition,and some N-limited species have“N saturation phenomenon”,In turn restricted by phosphorus（P）or other elements.The experiment was conducted in the platform of nitrogen and phosphorus addition in a meadow steppe ecosystem,near the Erguna Forest-Steppe Ecotone Research Station to study the P species on grassland plant community species diversity and stability under N-limited and non-limited Sexual influence.In this study,the use of phosphorus addition levels 0,2,4,6,8 and 10 g m^-26 levels in the platform interacted with nitrogen addition levels 0 and 10 g m^-2,a total of 12 treatments.A plant community survey was carried out during the maximum biomass period from 2017 to 2019 to analyze the effects of nitrogen and phosphorus addition on soil nutrient content,species diversity and productivity,and the related mechanisms of diversity-stability maintenance were also studied.The main conclusionsareasfollows:（1）Nutrient addition alters soil nutrient content.Nitrogen addition significantly increased soil organic carbon content,nitrogen and phosphorus addition had no significant effect on soil total nitrogen content,nitrogen and phosphorus addition significantly increased soil inorganic nitrogen content,phosphorus addition significantly increased soil total phosphorus content and available phosphorus content.The additionofnitrogenandphosphorushadnosignificanteffectonsoil pHandwatercontent.（2）Nitrogen addition significantly affected plant productivity.Nitrogen addition significantly increased community productivity,PR productivity,significantly reduced PF productivity,phosphorus addition had no significant impact on community and functional group productivity.The community productivity in N10 treatment was 1.6 times higher than that in N0 treatment.Nitrogen addition significantly increased the relative biomass of PR,significantly reduced the relative biomass of LE,PB and PR.The application of phosphatefertilizerhadnosignificanteffectontherelativebiomassoffunctionalgroupsinthisstudy.（3）Nitrogen addition significantly reduced plant species diversity.The addition of nitrogen significantly reduced the community species diversity index,while the addition of phosphorus only significantly affected the community species richness.The species richness of LE,PB and PF were significantly reduced by nitrogen addition.Nitrogen addition significantly reduced the Shannon-Wiener index of PB and the Pielou index of PF,and significantly increased the Shannon-Wiener and Simpson indexofPF.（4）Nitrogen and phosphorus addition significantly affected plant stability.Nitrogen and phosphorus addition,as well as N and Pr interactions,significantly affected the stability of three ecological classes:community,functional group and population.Nitrogen addition significantly affected the functional group stability except for legume plants,and phosphorus addition significantly affected the functional group stability except tufted grasses.There was no significant correlation between community,functional group and population stability and community species diversity.Nitrogen and phosphorus addition significantly affected the stability of each functional group,and there was no significant correlation between the stability ofeachfunctionalgroupandthespeciesdiversityofitsfunctionalgroup.（5）The mechanism of different nutrient additions to maintain community stability is also different.When phosphorus was added alone,the community had species asynchronous effect and portfolio effect,and portfolio effect was the main mechanism to maintain community stability.When nitrogen and phosphorus are added together,there are selection effect,portfolio effect and species asynchronous effect in thecommunity,andtheselectioneffectisthemain mechanismtomaintainthestability ofthecommunity.In summary,the addition of nitrogen and phosphorus significantly affected the soli available nutrients,species diversity and stability of community,and phosphorus addition had no significant effect on plant productivity.The mechanisms of maintaining community stability are also different with the addition of nutrients.Phosphorus addition did not alter the effect of nitrogen deposition on species diversity and productivity in grassland ecosystems.Phosphorus addition significantly changed community stability in the contextofnitrogen deposition.