Distribution And Oxidation Effect Assessment Of Comammox In The Rhizosphere Of Aquatic Plants In The Middle Reaches Of The Yangtze River

Nitrification is a rate-limiting process in the global nitrogen cycle.It was previously considered to be carried out in two steps: ammonia nitrogen was first oxidized to nitrite,and then oxidized to nitrate.In recent years,complete ammonia oxidizers(Comammox)have been found to complete two-step reactions independently,which has changed the traditional concept of nitrification.At present,there is still a lack of in-depth understanding of the existence and diversity of Comammox bacteria in the rhizosphere of aquatic plants and the influencing factors.In this paper,the rhizosphere of plants in the representative waters of the middle reaches of the Yangtze River was taken as the research object.The activities of ammonia-oxidizing bacteria(AOB),ammonia-oxidizing archaea(AOA)and Comammox were detected,and the diversity and abundance of Comammox were determined.The effects of different plant species on the abundance and community composition of ammonia-oxidizing microorganisms were revealed,and the understanding of nitrogen cycle was improved,which provided a new basis for controlling nitrogen pollution in water bodies.The main results are as follows:(1)This study confirmed that all three types of ammonia-oxidizing microorganisms can coexist in river,lake and pond sediments,and Comammox plays an important role in the nitrification process.Comammox had the highest contribution to nitrification in all plant regions,followed by AOB and AOA.Among the five representative submerged plants,the nitrification rate of ammonia-oxidizing microorganisms in Ceratophyllum demersum and Potamogeton wrightii was higher,which was conducive to water ecological restoration.At the same time,the total potential nitrification rate of rhizosphere was higher than that of non-rhizosphere sediments.(2)Clade A is the dominant species in the Comammox community in river,lake and pond ecosystems.Clade A.1 is enriched in the rhizosphere of submerged plants and is closely related to Candidatus Nitrospira nitrificans and Candidatus Nitrospira nitrosa.Clade A.2 belongs to Nitrospira sp.ST-bin1.Nitrate nitrogen and carbon-nitrogen ratio are important environmental factors that lead to the niche separation of the four branches of Comammox.Clade A.1 and Clade B are important branches to complete the nitrification process.Co-occurrence network analysis showed that the cooperative relationship in the Comammox community was much higher than the competitive relationship,and the connection was closer in the same subgroup.(3)High abundance of ammonia-oxidizing microorganisms were detected in the riparian zone of Houguan Lake,among which Comammox contributed the most to the nitrification process,followed by AOB and AOA.The distribution of abundance and activity in the land-water ecotone is: terrestrial plants are higher than emergent plants and higher than submerged plants.Alternanthera philoxeroides,Nelumbo nucifera and Ceratophyllum demersum are the species with the highest total nitrification rate among representative terrestrial plants,emergent plants and submerged plants,respectively.Their reasonable planting helps to remove nitrogen in the lakeshore.Clade A was dominant in Comammox community,Clade A.1 was closely related to Candidatus Nitrospira nitrificans and Candidatus Nitrospira nitrosa,while Nitrospira sp.ST-bin1,Nitrospira sp.ST-bin2 and Nitrospira sp.ST-bin4 were distributed in Clade A.2.Clade B was closely related to the outside world.(4)Spearman correlation analysis of Houguan Lake showed that the abundance and activity of ammonia-oxidizing microorganisms were negatively correlated with p H and water content,and positively correlated with total carbon and total nitrogen.Nitrate nitrogen only showed a significant positive correlation with the abundance of AOA and Comammox clade B,which was an important factor causing the niche separation of ammonia-oxidizing microorganisms.In addition,the nitrification rate of Comammox was significantly positively correlated with the abundance of Clade B,that is,Clade B contributed more to the ammonia oxidation process.It is worth noting that although ammonia-oxidizing microorganisms have a direct substrate competition relationship,their abundance and activity showed a significant positive correlation in the riparian zone of Houguan Lake.Changes in spatial location lead to changes in various physical and chemical factors such as water content,nitrate nitrogen,total carbon and total nitrogen,which will affect the Comammox community structure in soil.The co-occurrence network analysis showed that there were more positive cooperative relationships than negative competitive relationships in the community.There is more cooperation within the branch and more competition between branches.For rhizosphere soil,it has higher richness,diversity and evenness than Comammox in non-rhizosphere,and the internal connection of branches is closer and more bacteria are cooperative.