The Production Of Nitrous Oxide And The Mechanisms Of Nitrate Reduction In The Riparian Zone Of Condtructed Wetland

N₂O is a gas with a strong greenhouse effect.It is also one of the substances that indirectly destroy the ozone layer,and its total global emissions are increasing year by year.This makes N₂O one of the main environmental concerns.The concentration of N₂O in the atmosphere is now 323 parts per billion,19%higher than before industrialisation.Studies have shown that terrestrial ecosystems account for two-thirds of all N₂O emissions and aquatic ecosystems one-third.Among them,China's wetland area occupies the first place in Asia,which is bound to cause a large amount of N₂O release.Wetland riparian zones as interfaces between waters and land exhibit unique ecological environment characteristics and ecological service functions,and is the key area of nitrogen cycle reaction.The mechanism of N₂O production and reduction in denitrification of constructed wetland ditch system was investigated,and the differentiation of microorganisms involved in this process and dissimilatory nitrate reduction to ammoniumprocess was studied.It is of great significance to enrich the theory of nitrogen cycle,optimize the structure and function of the riparian zones and reduce N₂O.This study from the space and time scale of constructed wetland ditch system,on the mechanism of N₂O production and reduction process and DNRA process,carryed out the experimental study,through the physicochemical analysis of the sediments,indoor cultivation,potential N₂O producing rate and potential denitrification rate measurement,combined with fluorescence quantitative PCR,a new generation of high-throughput sequencing technology and building clone library and other technical means.The following conclusions were drawn:1.The ditch edge was a hot area with a higher potential N₂O production rate and potential denitrification rate in the constructed wetland ditch system,showing strong spatiotemporal heterogeneity.The sediment in the ditch edge(PNA:0.96-8.9 ug N₂O g^-1h^-1,PDA:4.24-38.17 ug N₂O g^-1h^-1)was higher than that in the ditch center(PNA:0.04-1.0 ug N₂O g^-1h^-1,PDA:0.02-9.16 ug N₂O g^-1h^-1),and higher in summer than in winter.The denitrifying functional genes nir S and nir K?N₂O producers?and nos Z I and nos Z II?N₂O reducers?in the ditch edge were both higher than that in the ditch center.The abundance trend of nos Z II gene was significantly different from that of other functional genes in the two seasons Ammonium salt;moisture content and C/N played an important role in the denitrification process of ditch system.2.Denitrifying bacteria are chemoheterotrophic bacteria,and their abundance changes are sensitive to various environmental factors.The sediment samples of the constructed wetland in Shijiuyang had higher bacterial diversity,and the distribution of the dominant denitrifying bacteria showed a large amount of rare genera and a few common genera:Oryzomicrobium,Cupriavidus,Pseudogulbenkiania and Steroidobacter are the main dominant genera.Oryzomicrobium,Cupriavidus,Azospira,Steroidobacter and Thiothrix contributed significantly to the potential of N₂O production rate.Pseudogulbenkiania,Cupriavidus,Ardenticatena,Thermonema,Runella,and Hymenobacter promoted the potential denitrification rate.Nitrite and total organic matter were the main factors to explain the changes of denitrifying community.3.In the ditch system of Shijiuyang constructed wetland,the abundance of DNRA bacteria showed a trend that the sediment in the ditch center was higher than in the ditch edge,and there was a significant difference between samples in winter and summer.The richness of DNRA community in the center sediment of the ditch was higher than that in the ditch in two seasons.The dominant bacteria in DNRA community were Anaeromyxobacter and Caldilinea.Total organic matter,C/N and moisture content in environmental factors were the key factors affecting the abundance and community characteristics of DNRA bacteria in constructed wetland sediments.According to the above artificial wetland ditch system denitrifying N₂O production and reduction mechanism,and results of microorganisms involved in this process and the DNRA process,we proposed that measures including?1?increasing the length and area of constructed wetland in the riparian zones,?2?restoring vegetation in the ditch edge,?3?strengthening water to the riparian zones with the function of contact and strengthening the advantages of the abundance of microorganism and active measures would help to reduce the levels of N₂O formed in wetland ecosystem,and promote the DNRA process.