Responses Of Microbial Communities To Changes Of Sediment Size Along A Channel At A Small Watershed In Highland Region Of Loess Plateau

Microorganisms are the most widely distributed and numerous organisms on the earth.They drive soil nutrient cycles,plant growth and biogeochemical processes.Soil is the habitat of microorganisms,about 90%of which live in soil sediments.Under soil erosion conditions,30-100 PG of soil is redistributed every year,and 70%-90%of the eroded sediments are redistributed in the downstream of the watershed.The quantity,quality,redox reaction conditions,sediments'sizes and distribution have significant effects on the composition and diversity of soil microorganisms.Therefore,it is of great significance to strengthen the study of soil microbial community in eroded sediments for further understanding the material cycle of terrestrial ecosystem.The main channel of Wangdonggou small watershed is selected as the research object.The watershed area is 8.3 km~2,located in Wangdong village,Changwu County,Xianyang City,Shaanxi Province.The tableland,slopeland and channel account for about 1/3 each,which is a typical representative of the Loess Plateau.In this study,6 sample sits with 5replicates were selected in the main channel of the Wangdonggou watershed.6 sites were separated into three treatments,namely"upstream","midstream",and"downstream".A total of 30 sediment samples(deep 10 cm)were collected.In this study,after the fractionation(>250?m,63-250?m and<63?m)of every treated sediment sample(Settling Tube Apparatus),16S r RNA and ITS high-throughput sequencing were performed on the sediment samples before and after fractionation.The study was carried out in the following three aspects:(1)The characteristics of sediment composition and nutrient distribution along the channel;(2)The composition and diversity of microbial communities along the channel;(3)Variations of soil microbial communities in three fractions along a channel bottom and their influencing factors.The main results were as follow:(1)There were significant spatial differences(P<0.05)in the contents of silt and clay along the channel,among which the content of silt was always the highest(about 66%),but the content decreased by 9.3%.The content of clay increased by 7.3%(P<0.05),and the content of sand had no difference(about 5%).The contents of SOC,TN,Olsen-P(except TP)were positively correlated with the clay content(P<0.05).After the fractionation,it was found that the nutrient contents(except TP)of 3 fractions(>250?m,63-250?m and<63?m)were positively correlated with the percentage of 63-250?m,and negatively correlated with the percentage of>250?m(P<0.05).(2)Bacterial and fungal communities had significant differences in different locations along the channel.In the up-,mid-and down-stream of the channel bottom,the relative abundances of bacteria and fungi in the sediment were different at the level of phylum,class and order:the relative abundances of?-Proteobacteria(eutrophic bacteria)increased by 120%significantly along the channel,while Cytophagales(oligotrophic bacteria)decreased by 6.6%(P<0.05).The relative abundance of Eurotiomycetes(eutrophic fungi)increased by 17.8times along the channel,while Microbotryomycetes(oligotrophic fungi)decreased by 98.6%(P<0.05).In addition,the richness and diversity of bacterial and fungal communities in the upper stream of the channel were the largest,and the network structures were the most complex.(3)After the fractionation,there were significant differences in bacterial communities'relative abundances,?-diversity indexes and community structures along the channel in 3different-size sediments:the relative abundances of Proteobacteria,Firmicutes and Bacteroidetes in>250?m and 63-250?m sediments changed significantly,while Firmicutes,Deinococcus-Thermus and Acidobacteria changed significantly in<63?m sediments(P<0.05).Bacterial communities in every sediment were gradually transformed from oligotrophic bacteria(Actinomycete)to eutrophic bacteria(Proteobacteria).In addition,the bacterial communities in 3 sediments clustered clearly in different positions of the channel bottom,and the?-diversity indexes decreased along the channel(the?-diversity indexes in63-250?m particle size were largest than other sediments).(4)After the fractionation,there were significant differences in fungal communities'relative abundances,?-diversity indexes and community structures along the channel in 3different-size sediments:the relative abundances of Dothideomycetes(eutrophic fungi)increased significantly,while Tremellomycetes(oligotrophic fungi)decreased significantly(P<0.05).In addition,the fungal communities in 3 sediments clustered clearly in different positions of the channel.The?-diversity indexes in>250?m and 63-250?m sediments increased,while the figures for<63?m sediment decreased along the channel(the?-diversity indexes in>250?m particle size were largest than other sediments).(5)The community structure and distribution of bacteria and fungi were related to the sediments'size composition along the channel.From up-to down-stream of the channel bottom,the sizes of sediments of the same size also changed,influencing the composition of bacterial and fungal communities.63-250?m sediments were more suitable for the growth of bacterial communities in channel,while fungal communities preferred the environment in>250?m sediments.The variation of the size composition of sediment along the channel is an important factor,leading to the variation of the microbial communities.Under the background of soil re-distribution,it is not only necessary to consider factors,such as substrates,moisture,and plant cover when exploring the structure and composition of microbial communities,but also the changes in the influence of sediment composition on the microbial communities.This study could help understand the soil re-distribution process and the material cycle in terrestrial ecosystems under eroded landform conditions.