Responses Of Soil Microbial Community To Simulated Nitrogen Deposition In A California Grassland

The increasingly serious global nitrogen deposition problem derived from anthropogenic activities dramatically impacted various ecosystems.Semi-arid grassland,one of the most crucial terrestrial ecosystem types,is very vulnerable to environmental disturbances.Nitrogen depositionis threatening the ecological balance of grasslands,as previous studies showed that nitrogen deposition can increase plant biomass or decrease plant biodiversity.Since soil microorganisms are the major engines driving soil biogeochemical cycles and played a vital role in maintaining the nutrition circulation and energy flow in soil ecosystem,responses of soil microbial community to atmospheric nitrogen deposition will undoubtedly influence the soil biogeochemical cycles.However,those responses remained largely unclear due to the complexity of soil microorganisms or the limitation of research methods.Using the high-throughput MiSeq sequencing and a microarray named GeoChip,we conducted a long-term?14 years?nitrogen deposition experiment in a semi-arid grassland,California,US,and investigated the responses of the taxonomic and functional compositions of soil microbial communities.We found that:?1?Microbial community taxonomic composition were significantly altered and different taxa showed different response strategies,increasing the relative abundance of Bacteroidetesand Gammaproteobacteria,and decreasing therelative abundance of Planctomycetesand Deltaproteobacteria.?2?Long-term nitrogen deposition also changed the functional gene compositions of soil microbial community.The relative abundance of genes associated with labile carbon degradation were increased,while those associated with recalcitrant carbon degradation were decreased,indicating that long nitrogen deposition can accelerate the decomposition of labile carbon.The relative abundance of amoA gene associated with nitrification were significantly increased by nitrogen deposition.Consistently,the nitrifying enzyme activity was also increased and showing a positive correlation with amoA gene abundance.?3?Nitrogen addition significantly increased the dominant plant biomass,soil total carbon,soil total nitrogen and nitrite,suggesting that the short-term effects of nitrogen deposition was maintained over time.Microbial taxonomic and functional gene compositions were linked to different environmental factors.Soil pH play a vital role in driving themicrobial taxonomic composition,while soil organic matter was closed correlated with both microbial taxonomic and functional gene compositions.?4?Responses of microbial community to long-term nitrogen deposition were influenced by other global change factors?warming,elevated CO₂ and elevated precipitation?,as elevated precipitationcaninfluence the taxonomic and functional gene compositions of microbial community,suppressing the responses of genes associated with carbon degradation and nitrogen cycling to nitrogen deposition.To our knowledge,this is the first study to adapt both MiSeq and GeoChip to investigate the responses of soil microbial taxonomic and functional gene compositions to atmospheric nitrogen deposition,providing more clues for researches about responses of grassland ecosystems to nitrogen deposition.