Effects Of Water And Nitrogen Patterns On Plant Community Characteristics And Soil Microbial Communities In Desert Steppe

Global change is profoundly affecting the sustainable development of human beings and terrestrial ecosystems.Frequent extreme precipitation events and increased nitrogen deposition lead to serious problems,such as loss of biodiversity and reduced ecosystem stability,which further affect the food products provided by grassland ecosystems for humans,the function of ecological barriers,and the feedback regulation of ecosystems to climate change.Desert steppe is a unique zonal vegetation type in central Asia,which is located in the transition zone from steppe to desert and is also a sensitive area affected by global change.However,there is still a lack of in-depth understanding of the relationship with environmental factors,feedback mechanism and systematic assessment of plant,soil and microbial elements of desert steppe in the context of global change.In this study,an in-situ experiment was conducted to simulate precipitation pattern change and nitrogen deposition in Stipa klemenzii desert steppe in Inner Mongolia.A randomized block design was used with 4 replicates of 6 treatments: control(CK),decrease precipitation(DP),increase precipitation(IP),nitrogen addition(CN),precipitation decrease + nitrogen addition(DN),and precipitation increase + nitrogen addition(IN).From 2016 to 2018,the characteristics of plant community,aboveground net primary productivity,plant functional groups,soil physical and chemical properties,enzyme activities,soil bacteria and soil fungi were measured.In this study,the effects of changes in water and nitrogen patterns on plant community characteristics and soil microorganisms in desert steppe,as well as the response and feedback mechanisms of soil microbial communities to changes in plant communities and soil characteristics were explored.The main findings were as follows:1.During 2016-2018,precipitation pattern change,nitrogen addition and their interactions had significant effects on aboveground net primary productivity(ANPP)and species richness(SR).Moisture was the main factor affecting the changes of ANPP and SR.Precipitation pattern change and nitrogen addition significantly affected C4 plant biomass,but not C3 plant biomass.Among C4 plants,changes in precipitation pattern and nitrogen addition significantly affected Cleistogenes songorica and Salsola collina.2.The response of inorganic nitrogen to precipitation pattern change and nitrogen addition was asymmetric.Both precipitation pattern change and nitrogen addition significantly affected soil microbial biomass carbon and nitrogen,while water-nitrogen interaction had a significant effect on soil microbial biomass nitrogen but not on soil microbial biomass carbon.The increase of precipitation and N addition significantly increased N potential ratio.N addition significantly increased nitrate reductase activity,while precipitation reduction significantly decreased nitrate reductase activity.3.Actinobacteria was the dominant phylum of soil bacterial community under the conditions of precipitation pattern change and nitrogen addition.The decrease of precipitation significantly affected the relative abundance of Actinobacteria,Chloroflexi,Acidobacteria,Planctomycetes,Nitrospirae,Armatimonadetes and Bacteroidetes.Nitrogen addition significantly increased the relative abundance of Actinobacteria,but decreased the relative abundance of Acidobacteria and Armatimonadetes.4.Structural equation modeling(SEM)analysis indicated that nitrogen addition indirectly affected the composition of soil bacterial communities by changing plant species richness,soil p H,soil water content(SW)and soil total nitrogen(TN)content.Decreased precipitation directly affected soil bacterial community structure,while increased precipitation indirectly affected and structure bacterial community by changing soil TN.SW,soil p H and ANPP co-regulated the changes in bacterial community composition,and SW and ANPP were the main driving factors determining the changes in bacterial community composition.5.Moisture was the main factor driving fungal communities change.Changes in precipitation patterns and nitrogen additions affected soil fungal diversity and community composition by altering plant and soil properties.Ascomycota is the dominant group of fungi in desert steppe.From the perspective of plant functional groups,C4 plant biomass was the main factor affecting fungal diversity and community composition.6.Precipitation patterns played a dominant role in affecting fungal diversity and community composition.The fungal Shannon-Wiener index was mainly affected by C4 plant biomass.The fungal Shannon-Wiener index was less affected by the plant Shannon-Wiener index under the conditions of precipitation pattern change and nitrogen addition.7.Under the background of changes in precipitation patterns and nitrogen deposition,soil microbial communities effectively alleviated environmental changes through changes in dominant phyla(such as Actinobacteria)and significantly improved the stability of desert-steppe ecosystems.