Soil Nutrients,Enzyme Activities And Bacterial Diversity In Root Zone Under Four Re-Vegetated Plant Species In The Riparian Zone Of The Three Gorges Reservoir

Species replacement has a potential impact on the soil geochemical cycle.It is of great significance to study the dynamic changes of soil ecosystem from the perspective of root-soil-microbe interactions after vegetation restoration in the Three Gorges Reservoir(TGR).In this study,the rhizosphere and bulk soils of Cynodon dactylon,Hemarthria altissima,Taxodium distichum and Salix matsudana were collected between155—175 m elevation of the vegetation restoration demonstration base of Ruxi River to explore the differences of nutrient contents and enzyme activities between rhizosphere and bulk soils to clarify the growth adaptabilities and nutritional utilization strategies.At the same time,the diversity of root associated bacterial community was also investigated by using the high throughput sequencing method,with the aim to identify symbiotic bacterial flora that would be beneficial to the stress resistance of plants growing in the harsh ecosystem of the TGR.The results showed that:1)Suitable plant species have enhanced the transformation efficiency of rhizosphere nutrients in different ways,so as to improve their adaptability to the soil environment in the TGR.Compared with bulk soil,root activities lead to significant differences in nutrient contents in the rhizosphere.Among them,SOC,TN and AN were enriched in rhizosphere to a certain degree(R/S>1),while the changes of phosphorus and potassium were not consistent in different plant species.Generally speaking,the two herbaceous plants(Cynodon dactylon and Hemarthria altissima)had more reasonable nutrient regulations in the rhizosphere,which achieved more effective nutrient distribution and supply than that two woody species in the TGR.2)Sucrase,urease and acid phosphatase all processed positive rhizosphere effects(R/S>1)in these four plant species,that was,the transformation and decomposition of mineral elements(carbon,nitrogen and phosphorus)in rhizosphere could be improved by root exudation of all these four plant species,further to activate their relevant circulating enzyme.Considering the different root structure and physiological characteristics of herbaceous and woody plants,the rhizosphere effect of invertase and acid phosphatase in two herbaceous plants were more obvious,and the activation of urease was more significant in two woody plants.3)There were stronger interactions among the activation mechanisms of carbon,nitrogen,phosphorus and potassium in the rhizosphere of four suitable plant species in the riparian zone of the TGR to maintain a balanced nutrient ratio.SOC had a significant positive correlation with pH value,TN,AP and sucrase;but all these indexes were significantly negatively related with TK.At the same time,invertase showed a significant positive correlation with acid phosphatase,but a significant negative correlation with urease.4)The differences of temperature and plant growth situation between different sampling time resulted in significant differences in soil nutrient availability and soil enzyme activity.Under the background of climate change,the altitude effect of each element was weakened.Therefore,in this study,the temporal dynamic changes of each element in different elevations were relatively consistent.Overall,compared with T1(May),SOC content in four suitable plant soils decreased first and then increased,TN content increased significantly,while AP and AK content decreased significantly in T3(September).At the same time,in the T1,T2(July)and T3,the activities of three enzymes(sucrase,urease and acid phosphatase)decreased first and then increased in the TGR.5)The results of high-throughput sequencing showed that there was no significant difference in the bacterial ?-diversity indices(Simpson index,Shannon index,Chao index,and coverage index)between the rhizosphere and bulk soils of four plant species,but they were significantly higher than those in the endophyte.At the same time,with the decrease of altitude,the ?-diversity indices of bacterial community in these four plant species decreased significantly.Overall,the community structure of root-associated bacteria in the rhizosphere and bulk soil was more similar compared with endophyte of these four plant species.At the same time,Proteobacteria,Acidobacteria,Chloroflexi,Actinobacteria and Bacteroidetes were the top five mostabundant bacterial phyla in this three compartments(rhizosphere soils,bulk soils and plant roots)of the four plant species with the ecological functions of nutrition absorption and disease suppression,their colonization was beneficial to the stress resistance of plants growing in this harsh ecosystem of the TGR.6)Plant root exudates reduced the competitive pressure of bacteria on limited resources to a certain extent in rhizosphere soil.Among them,Proteobacteria and Planctomycetes was significantly positively related,while Acidobacteria showed a positive correlation with Chloroflexi and Crenarchaeota.Actinobacteria and Planctomycetes was significantly positively related,while Bacteroidetes showed a significant negative correlation with Gemmatimonadetes and Firmicutes.In addition,the results of redundancy analysis showed that the relative abundance of Actinobacteria,Gemmatimonadetes and Firmicutes were positively correlated with phosphorus.The relative abundance of Crenarchaeota was positively correlated with soil nitrogen and organic matter,but the relative abundance of Bacteroidetes was negatively correlated with soil pH,TN,AN and SOC.