Responses Of Plant And Soil Microbial Communities To Nitrogen And Water Addition On The Stipa Baicalensis Steppe

Nitrogen?N?deposition and precipitation change are important phenomena in global climate change and they can strongly influence grassland ecology.There have been many studies on the response of plant and soil microbial communities to N addition.However,the interactive effects of N addition and irrigation on soil microbial communities are still largely unknown.Studying the effects of nitrogen?N?deposition and precipitation change on soil microbial community structure is of great significance for predicting the interactive effects of multiple climate factors on grassland ecosystems in the future.Here,we simulated N deposition and precipitation change by N addition(0,15,30,50,100,150,200,300 kg N hm^-2a^-1)and irrigation?no irrigation and irrigation equivalent to 100 mm extra summer rainfall?at a test area on the Stipa baicalensis steppe.A split-plot design was adopted.Soil samples were collected with a soil probe after four years of experimental treatments.Using high-throughput sequencing technology,we evaluated the interactive effects of N addition and irrigation on the plant diversity,plant primary productivity,soil properties,soil bacterial community and fungal community.The main results are as follows:1.Plant diversity and grassland productivity showed negative correlation after N and water addition.N and water addition changed the composition of plant community.Under single N addition,following the N enrichment,the plant diversity increased at N15-N30 but decreased at N50-N300.Under N and water addition,the plant diversity decreased following the N enrichment and significantly decreased at N30–N300.The grassland productivity increased with N enrichment under the two conditions and significantly at N50-N300.N and water interactively affected the plant diversity and grassland productivity.Water addition enhanced the negative effects of N addition on plant diversity and the positive effects on productivity.2.Under single N addition,soil organic carbon and total N significantly increased at N100-N150.NO₃^-?N concentration significantly increased at N100-N300.pH significantly decreased at N200-N300.When water was added,soil organic carbon and total N significantly increased at N50-N300.NO₃^-?N and NH₄⁺–N concentration significantly increased at N150-N300.pH significantly decreased at N100-N300.Soil C:N significantly shifted at N300.3.The dominant bacterial phyla were Verrucomicrobia?30.61%–48.51%?,Proteobacteria?21.37%–29.97%?,Acidobacteria?9.54%–20.67%?,and Bacteroidetes?4.96%–9.74%?.the relative abundance of 19 bacterial phyla significantly shifted under different N and water addition rates.Phyla Verrucomicrobia and Acidobacteria could be used as indicator species for changes in soil bacterial community structure.N and water addition significantly changed the structure of the soil bacterial community as well as interactively affected the soil bacterial diversity.Under single N addition,N100-N300 significantly increased the soil bacterial diversity.However,when water was added,bacterial diversity increased at N15-N50 but significantly decreased at N100-N300.4.The dominant fungal phyla were Zygomycota?22.0%—48.9%?,Basidiomycota?7.8%—18.5%?,Ascomycota?9.4%—20.1%?,Glomeromycota?0.7%—3.1%?and Chytridiomycota?0.1%—1.3%?.The relative abundance of all 5 fungal phyla and 11 fungal Classes shifted significantly in different N and water addition rates.Genus Mortierella,family Entolomataceae and genus Geastrum could be used as indicator species to indicate the changes in soil fungal community structure.The results of PCoA also showed that N and water addition significantly modified the structure of soil fungal community.N100-N300 significantly increased fungal diversity under normal precipitation.In contrast,when water was added,fungal diversity showed no significant change following N enrichment.5.The soil NO₃^-?N concentration and pH was the main factor that influence soil bacterial community.Changes in them either directly affected bacterial diversity or indirectly affected bacterial diversity through plant community.Plant community composition and plant species richness was the main factor of shifts in soil fungal community.Soil inorganic N and pH indirectly affected the structure of soil fungal community through plant community.6.Integrated analysis of the changes in plant and edaphic factors,different organisms responed differently to N and water addition.In present study,the grassland productivity in single 100 kg N hm^-2a^-1 treatment was 2.46 times that of control.Meanwhile,the concentration of soil organic carbon and total N,bacterial diversity and fungal diversity significantly increased.The plant diversity showed no significant decrease at N100.These indicated that the fertilizer threshold on the Stipa baicalensis Steppe should be 100 kg N hm^-2 a^-1 under the present precipitation pattern.Under N and water addition,N addition level exceeded 30 kg hm^-2 a^-1 increased soil organic carbon and grassland productivity but decreased plant diversity.The bacterial diversity also significantly decreased under N addition level exceeded N50.These indicated that the loss risk of grassland biodiversity possibly exist under elevated N deposition and precipitation.Hence,utilize individual biodiversity or productivity to evaluate the effects of N and water addition on grassland ecosystem is incomplete.Integrated above-and belowground biodiversity with grassland productivity as well as conduct long-term studies are needed in future.