The Influence Mechanisms Of Wetland Types/Emergent Plants On The Microbial Communities In Freshwater Wetland Sediments

Wetland is one of the most important nature ecosystems on the earth,which has an important impact on maintaining the global ecological balance as well as human survival and production activities.As a distinct component of wetland ecosystem,sediment is the source or sink of water nutrients and pollutants.Sediment microbes in wetlands are the key biological pathway for material transformation and migration,hence affecting the entire wetland ecosystems.The physiochemical properties of sediments vary widely in different wetland types,which might lead to the differences of microbial communities in sediments,and this effect might be related to wetland plants.Because of the rhizosphere effect of wetland emergent plants,the plant roots produce a sediment microenvironment different from the non-rhizosphere sediments,which might also lead to the difference of sediment microbial communities.Studying the influence mechanisms of wetland types and emergent plants on the sediment microbial communities could be helpful to understand the ecological function distribution and detoxification mechanisms of microbial communities in freshwater wetland ecosystems,which is conducive to the improvement of purification capacity of wetland ecosystems.In freshwater ecosystem,the water level of marsh wetland and constructed wetland is relatively shallow,while that of river wetland and lake wetland is relatively deep.The difference of water level between deep-water wetland and shallow-water wetland has a significant impact on the growth of emergent plants and the oxygen availability in sediments,which could further lead to the difference of the structure and ecological function of sediment microbial communities between deep-water wetland and shallow-water wetland.The rhizosphere of wetland plants is suitable for the growth and reproduction of sediment microorganisms,which makes the abundance and metabolic activity of rhizosphere sediment microorganisms much higher than those of non-rhizosphere sediment microorganisms.Thus,it could lead to the differences of microbial community structure and ecological function between rhizosphere and non-rhizosphere sediments of wetland plants.However,the studies on the influence mechanisms of wetland types and emergent plants on the microbial communities in freshwater wetland sediments are not comprehensive.In this study,we used 16S rRNA gene sequencing and community metabolic prediction to compare sediment bacterial communities and predicted metabolic genes between shallow-water wetland(marsh wetland and constructed wetland)and deep-water wetland(river wetland and lake wetland)in summer and autumn in Dongping Lake Basin,and combined with the physiochemical properties of the water and sediment to analyze the influence mechanisms of wetland types on the microbial communities.The large difference in growth status of emergent plants between shallow-water and deep-water wetlands might be an important reason for the influence of wetland types on sediment microbial communities.To further study the effects of emergent plants on the microbial communities in wetland sediments,a subsurface-surface-flow constructed wetland—Zhaoniu River Constructed Wetland(ZRCW)in northern China was selected as the research site.We combined 16S rRNA gene sequencing and microbial community metabolism prediction to compare the sediment microbial communities and predicted metabolic genes between the rhizosphere and non-rhizosphere of emergent plants[Phragmites australis(Cav.)Trin.ex Steud.(Phr),Typha angustifolia L.(Typ),and Cyperus alternifolius L.(Cyp)]in the summer and autumn of ZRCW.And the environmental factors that play important roles in rhizosphere interactions,such as physiochemical properties of sediments and root exudates,were determined.Nitrogen cycle is one of the cores of element cycles in wetland ecosystem,which has an impact on other nutrient cycles.Influenced by rhizosphere effect,the abundance and activity of microorganisms related to nitrogen cycle are usually different between rhizosphere and non-rhizosphere.In order to further explore the influence mechanisms of emergent plants on sediment microbial communities related to nitrogen cycle,this study selected ZRCW as the research site.And sediment culture control experiment,high-throughput sequencing of functional genes,and qPCR technology system were combined to compare the activity,community structure,and functional gene abundance of the two important microbial communities involved in nitrogen transformation—ammonia oxidation and nitrogen fixing communities between rhizosphere and non-rhizosphere of emergent plants(Phr,Typ,and Cyp)in summer and autumn in ZRCW.And the root exudates and physiochemical properties of sediments were determined.The results showed that in autumn of Dongping Lake Basin,the deep-water wetlands had significantly higher community richness than the shallow-water wetlands,which was mostly related to sediment pH,while there was no significant richness difference in summer.Affected by rhizosphere effect,the composition of bacterial communities differed between the shallow-water wetlands and the deep-water wetlands,and the shallow-water wetlands had greater complexity of bacterial co-occurrence than the deep-water wetlands,suggesting that the influence of wetland types on sediment microbial communities might be related to the growth:status of wetland plants.Besides,the composition of both predicted metabolic genes and microbial communities was significantly affected by dissolved organic carbon(DOC)and dissolved oxygen(DO).The shallow-water wetlands exhibited high predicted gene abundances related to xenobiotic biodegradation possibly due to the high DO or DOC level.Compared with the shallow-water wetlands,most of the deep-water wetlands exhibited higher predicted gene abundances related to element(carbon,nitrogen,and sulfur)metabolism possibly due to the low DOC and DO levels in the freshwater basin.In ZRCW,influenced by the root exudates of emergent plants,the abundance of microbes in Phr rhizosphere sediments in summer was significantly higher than that of Typ and Cyp rhizosphere sediments and non-rhizosphere sediments.And the diversity and richness of microbial communities in rhizosphere sediments were significantly higher than those in non-rhizosphere.Influenced by rhizosphere effect,the microbial community composition differed between rhizosphere and non-rhizosphere of emergent plants,and microbial communities in rhizosphere sediments had greater complexity of bacterial co-occurrence than those in non-rhizosphere sediments.Besides,the composition of both predicted metabolic genes and microbial communities was significantly affected by total organic carbon(TOC)in sediments,TOC in root exudates,and oxidation-reduction potential(ORP).Because of the high content of organic carbon,the rhizosphere sediments exhibited a relatively high predicted gene abundance related to the xenobiotic biodegradation,nitrogen fixation,and nitrate dissimilation reduction.In addition,the high ORP level in rhizosphere sediments might also play a role in the xenobiotic biodegradation.Compared with rhizosphere sediments,non-rhizosphere sediments showed higher predicted gene abundances related to sulfate dissimilation reduction and methane metabolism possibly due to the low ORP level.Ammonia oxidation rate(AOR)was enhanced in the rhizosphere sediments compared with non-rhizosphere sediments.Based on correlation analysis,the variation of AOR between rhizosphere and non-rhizosphere sediments was attributed to the increase of ORP in rhizosphere sediments caused by root oxygen secretion.Acetylene inhibition experiment showed almost all nitrification activity was due to lithotrophic NH3 oxidation,thus,root exudates had no significant effect on AOR.AOR had a significant linear correlation with ammonia-oxidizing archaea(AOA)activity rather than ammonia-oxidizing bacteria(AOB)activity.However,the high AOA activity was not correlated with high AOA abundance,but instead mostly determined by specific AOA taxa 'Group I.la' cluster.Overall,emergent plant rhizosphere exhibited strong positive effects on AOR by altering community structure and abundance of the related microorganisms.Release of available oxygen in plant rhizosphere is vital for promoting ammonium oxidation.Nitrogen-fixation rate(NFR)was enhanced in the rhizosphere sediments compared with non-rhizosphere sediments.Based on correlation analysis,the variation of NFR between rhizosphere and non-rhizosphere sediments might be mainly due to the utilization of soil organic carbon compounds by diazotrophs,especially sugar-containing root exudates as carbon sources.Additionally,the increase of ORP in rhizosphere sediments caused by oxygen secretion from the roots of emergent plants might also be the reason for the.increase of NFR in rhizosphere sediments.Furthermore,NFR strongly correlated with diazotroph abundance,suggesting enhancement of diazotrophs abundance could contribute to NFR.Moreover,relative to Cyp and Typ,Phr should be prioritized for selection during constructed wetland construction due to the high concentrations of root exudates and increased oxygen-secretion ability.Overall,emergent plant rhizosphere exhibited strong positive effects on NFR by altering community structure and abundance of the related microorganisms.Release of available oxygen and carbon in plant rhizospheres is vital for promoting nitrogen fixation.This study revealed the influence mechanisms of wetland types and emergent plants on the sediment microbial communities.The response mechanisms of nitrogen cycle microorganisms in wetland sediments to emergent plants were further discussed.This study found that no matter between wetland types or between rhizosphere and non-rhizosphere,the communities of sediment microorganisms(including those related to nitrogen cycle)were different.And the influence of wetland types on sediment microbial communities might be related to the growth status of wetland plants.The contents of available oxygen and available carbon in sediments are the main factors affecting the structure and functional potential of sediment microbial communities.This study deepened the cognition of the distribution and influencing factors of the microbial communities in the freshwater wetland ecosystem,which is of great significance to the wetland ecosystem management.