Effects Of Nitrogen Addition On Soil Microbial Community In Grassland Ecosystem

Soil microorganism is an important component of terrestrial ecosystem.The increase of nitrogen supply can affect the structure and function of ecosystems,potentially affecting the soil microbial structure and diversity.As the largest terrestrial ecosystem,grassland is extremely vulnerable to global changes.However,a large number of grassland studies had found that the response of soil microorganisms to nitrogen addition were not consistent.In this study,nitrogen addition experiments were carried out in three types grasslands experimental platforms:Qinghai-Tibet alpine meadow,Inner Mongolia typical grassland and Inner Mongolia meadow grassland.The soil microbial biomass and diversity were analyzed by phospholipids fatty acid method.In order to understand clearly the effects of nitrogen addition on soil microorganism in grassland ecosystem,we supplemented a meta-analysis of 21 similar studies on the global grassland ecosystem.Furthermore,we used high-throughput sequencing technology to analyze soil bacterial and fungal communities in the Inner Mongolia typical grassland,to explore the effects of nitrogen addition on soil microbial community structure.Main results were as follows:?1?Nitrogen addition had no significant effect on soil microbial biomass,bacterial biomass,fungal biomass and actinomycete biomass in alpine grasslands,typical grasslands and meadow grasslands and had no significant effect on its diversity and community structure.Through the soil physical and chemical properties of three grassland types,we found that the soil moisture,soil organic carbon content and soil total nitrogen content were not significantly changed after nitrogen addition.Therefore,we speculated that the insensitivity of soil microbes to nitrogen addition may be related to the less change of soil characteristics.?2?The results of meta-analysis showed that nitrogen addition significantly increased the biomass of soil fungi and actinomycetes.The growth rate was 24.8%and 9.25%.When the nitrogen gradient was 0-5 g·m^-2·a^-1,the growth of fungi was significantly promoted by 37.35%.Nitrogen addition significantly increased soil microbial diversity,bacterial diversity and fungal diversity,and the growth rate were1.16%,2.43%and 2.55%.And when the nitrogen addition gradient was 0-5g·m^-2·a^-1,the diversity was significantly increased.The growth rate was about 2.04%.This study also found that there was no significant correlation between mean annual temperature or mean annual precipitation and soil microbial biomass response ratios.?3?High throughput sequencing results were found that the Acidobacteria,Actinobacteria and Proteobacteria were the dominant bacterial phyla in the typical steppe of Inner Mongolia,and nitrogen addition had no significant effect on the relative abundance of each bacterial phyla and the most genus.However,some rare genus were sensitive to nitrogen addition?P<0.05?.For example,with the increase of nitrogen addition gradient,the relative abundance of Ohtaekwangia,Jahnella,and Mizugakiibacter continuously increasing,and the relative abundance of Porphyrobacter,Rhodoplanes,Pseudomonas,and Acidobacterium showed a trend of increasing first and then decreasing.While the relative abundance of Roseomonas,Aggregicoccus,and Frateuria with the increase of nitrogen addition gradient showed a downward trend.The Ascomycota was the dominant fungal phyla in the typical steppe of Inner Mongolia,followed by Basidiomycota,Glomeromycota,Zygomycota,and Chytridiomycota.Our studies had shown that nitrogen addition had no significant effect on the relative abundance of each fungal phyla and the most genus,but had a significant impact on some rare genus of fungi.For example,the relative abundance of Devriesia and Cladophialophora were significantly lower than control treatment after high nitrogen addition?N10?.While the relative abundance of Thielavia increased gradually after nitrogen addition.As we can see,the control experiment showed that nitrogen addition had no significant effect on soil microbial community,which was different from the Meta analysis results in this paper.This indicated that the effect of nitrogen addition on soil microorganism was the result of multiple factors.Different studies were bound to produce different or even opposite results due to different factors,so the response of soil microorganisms to nitrogen addition was hard to predict.In addition,we found that the phylum and genus levels of soil microbial community had no significant response to nitrogen addition.This may be due to the complex changes in the level of bacterial or fungal species that cancel out the sensitivity of the phylum or genus to nitrogen addition.Therefore,future studies should strengthen the identification and analysis of bacterial and fungal species.