Effects Of Nitrogen Fertilization And Cutting On Soil Microbial Communities In Hybrid Elephant Grass(Guimu No.1)-Planted Field And Associated Underlying Mechanisms

As an important component of terrestrial ecosystems,soil microorganisms play vital roles in regulating material and energy flow in the ecosystem and maintaining the structure and function of ecosystems.The composition of soil microbial communities has been recognized as a sensitive indicator for nutrient cycling,health status and service of terrestrial ecosystems.In grassland,management practices such as fertilization and cutting can exert great influence on soil physicochemical and biological properties,as well as on forage biomass and quality.Here,we investigated the effects of nitrogen fertilization and cutting on soil microbial biomass,community structure,and diversity in forage grassland(Guimu No.1)using Illumina Miseq high-throughput sequencing.Furthermore,in combination with environmental factor and network analyses,the underlying mechanisms for changes in soil microbial community structure following different management practices were discussed.Main results are as follows.1.Nitrogen fertilization had no influence on soil organic carbon(SOC),total nitrogen(TN)and total phosphorous(TP),but did significantly affect soil p H.No significant influence of cutting frequency and intensity on soil basic properties.Nitrogen fertilization did not cause significant variation in soil microbial biomass carbon(MBC),while had negative effect on soil microbial biomass nitrogen(MBN).Cutting frequency exerted weak influences on MBC and MBN,while cutting intensity affected MBN,as indicated by highest MBN concentration in medium cutting intensity treatment(i.e.15 cm).In addition to SOC and TN,TP was another important factor controlling MBC in the forage grassland.2.The distribution of soil microorganisms at the species level showed that soil bacterial and fungal communities in Guimu No.1 forage grassland were dominated by several species with high abundance.Soil bacteria consisted of 444 genus,which could be clustered into 304families,219 orders,101 classes,and then 42 phyla,while soil fungi consisted of 298 genus,which could be clustered into 159 families,82 orders,31 classes,and then 11 phyla.Bacillus,mycetozoan,chloroflexi and bacteroid were the dominant bacterial communities,while Ascomycota,basidiomycetes,and zygomycetes were predominated in soil fungal communities.The relative abundance of soil bacteria and fungi at the phylum level did not significantly respond to different management practices in the Guimu No.1 forage grassland.3.Analysis of the?-diversity of soil bacterial and fungal communities demonstrated that nitrogen fertilization and cutting had no significant influences on the Shannon index of soil bacterial and fungal communities,but significantly affected the species richness of soil bacteria and fungi.Both the species richness of soil bacteria and fungi were highest in A3B3C1 treatment,i.e.nitrogen fertilization at 1000 kg hm^-2yr^-1,cutting frequency at 3times per year cutting intensity at 5 cm.The results suggested the responses of the species richness of soil bacteria and fungi to management practices follow the intermediate disturbance hypothesis.The result of?-diversity based on Bray-curtis distance matrix showed that nitrogen fertilization and cutting frequency and intensity greatly affected the structure stability of soil bacterial community.4.Total nitrogen(TN)and available nitrogen(AN)were major drivers for distribution of bacterial communities at the phylum level,while available nitrogen,p H and microbial biomass nitrogen(MBN)were principle drivers at the operational taxonomic unit(OUT)level.Basidiomycetes was negatively correlated with p H,total phosphorous(TP)(P<0.05),while zygomycetes was positively correlated with total potassium(P<0.05).Furthermore,the distribution of fungal communities at the phylum level was mainly controlled by microbial biomass phosphorus(MBP)and TN,while MBN,TP and AN mainly determined fungal communities at the OTU level.5.Through the establishment of microbial molecular ecology network model,we showed that nitrogen fertilization and cutting frequency and cutting intensity changed the quantity and size of network modules,as well as changed the distribution of modules in the network,key species in the module,and their interactions.Increases in nitrogen fertilization and cutting frequency resulted in a more complicated bacterial network.The resistance of bacterial network to disturbance was strong in the treatment of cutting at 25 cm.In contrast,there was no clear trend for responses of fungal network to management practices.Fungal networks under the treatments of no fertilization and cutting frequency at 3 times per year were relatively more sensitive to environmental disturbance,and was more resistant to disturbance under the treatment of cutting intensity at 5 cm.6.Based on the comprehensive ecological effects of plant growth,soil properties,and soil microbial community,these treatments were clustered into 4 groups through Euclidean distance and Ward method,i.e.,undevlopment of plant growth and soil environment,improverment of plant growth,improvement of soil environment,and coordinated development of plant and soil environment.Especially,the comprehensive ecological effects of A2B1C2 treatment(i.e.,500 kg·hm^-2·a^-1fertilization and once cutting on 15 cm above ground)was the best.