Distribution And Influencing Factors Of Soil Diazotroph In Alpine Grassland

Biological nitrogen fixation means that diazotroph convert atmospheric nitrogen into ammonia to replenish soil nitrogen pools through the catalysis of nitrogenase in the body.It is an important nitrogen input method for ecosystems and plays an important role in alleviating soil degradation and reducing the use of nitrogen fertilizers.The Qinghai-Tibet Plateau has a high altitude and a fragile habitat.Coupled with the impact of climate change and human activities,the alpine grassland has been seriously degraded and biodiversity has been lost sharply,which has brought great difficulties to people’s production and life.The nitrogen fixation of microbial can increase the nitrogen content of grassland,improve vegetation productivity and alleviate grassland degradation.Therefore,it is of great significance to study the distribution of soil diazotroph community in alpine grassland and its influencing factors for the formulation of alpine grassland management measures.In this study,three types of grasslands in Qinghai-Tibet Plateau(alpine meadow,alpine steepe and alpine desert steepe)were taken as the research object.Through field vegetation,soil investigation and indoor experimental analysis,q PCR and high-throughput sequencing technology were used to systematically analyze the diazotroph community.The main results on the distribution of alpine grassland and its influencing factors are as follows:(1)During the transformation from alpine meadow to alpine steppe to alpine desert steppe,the dominant species in the plant community gradually changed from Kobresia pygmaea to Stipa purpurea and to Stipa glareosa.The diversity,coverage and biomass of the plant community decreased,and the dominant functional group changed from sedges to forbs;the soil water content decreased,the bulk density and p H increased,and the water-soluble organic carbon,organic carbon,total nitrogen,water-soluble organic nitrogen,ammonium nitrogen,nitrate nitrogen and available phosphorus all decreased,while the content of available potassium increased.(2)The overall diazotroph community,rare species and abundant species in the alpine grassland were mainly composed of Proteobacteria(relative abundance was 76.6%,83.7% and 71.2%,respectively),Cyanobacteria(relative abundance was 3.5%,5.6% and1.2%,respectively)and Verrucomicrobia(relative abundance was 2.8%,2.7% and 2.2%,respectively).The relative abundance of Proteobacteria in the overall diazotroph community in alpine meadow is smaller than that of alpine steepe and alpine desert steepe,and its Cyanobacteria and Verrucomicrobial phyla are greater than those of alpine steepe and alpine desert steepe.The relative abundance of Cyanobacteria phyla of rare species was alpine desert steppe > alpine steppe > alpine meadow.The overall diazotroph abundance,α and βdiversity of the diazotroph community were alpine meadow > alpine steppe > alpine desert steppe,and the α and β diversity of rare species were greater than that of rich species.(3)Stochastic processes(drift and dispersal limitation)dominated the community assembly of diazotroph in alpine grassland soil.From alpine meadows to alpine steepe to alpine desert steepe,the relative importance of dispersal limitation gradually decreases,and the relative importance of drift gradually increases.The number of edges,the number of nodes and the average degree of the diazotroph co-occurrence network are alpine meadow >alpine steppe > alpine desert steppe,while the network density,modularity coefficient,close centrality and average path length of alpine meadow and alpine desert steppe is longer than alpine steepe.(4)The abundance of diazotroph had the strongest correlation with soil organic carbon,aridity and total nitrogen.The α-diversity of rare species of diazotroph was more correlated with environmental factors than that of abundant species and the total.Soil organic carbon,soil moisture content,and plant biomass were most strongly correlated withα and β diversity of total and rare species of diazotroph,while abundant species were most strongly correlated with organic carbon,soil moisture,and total nitrogen.Climatic factors had the largest direct effect on the α-diversity of diazotroph and the abundance of nitrogen-fixing genes,and the direct effect of species interaction on the β-diversity was the largest.Together,alpine meadows had higher vegetation productivity,diversity and soil nutrients than alpine steepes and alpine desert steppes,resulting in higher abundance of diazotroph abundance and α-and β-diversity of diazotroph in alpine meadows.In addition,rare species were most susceptible to environmental influences,and their response to environmental factors was consistent with that of total diazotroph community,contrary to that of abundant species.Climatic factors directly drove the abundance of diazotroph andα-diversity,while species interaction had the strongest direct driving effect on β-diversity.The above results reveal the distribution of diazotroph community in alpine grassland soil and its influencing factors,which can provide a theoretical basis for the maintenance mechanism of diazotroph diversity in alpine grassland and the control of grassland degradation.