Identification And Controlling Factors Of Feammox In Groundwater System Of Typical Alluvial-lacustrine Plain

With the development of social economy and the growth of population density,groundwater nitrogen pollution has become a global environmental problem,and it is urgent to carry out research on the reduction of nitrogen load in groundwater.anaerobic ammonium oxidation coupled to iron reduction（Feammox）is a newly discovered natural attenuation process of nitrogen in addition to denitrification and anaerobic ammonium oxidation.Feammox has been widely studied in various surface environmental media such as forest soil,paddy soil,hyporheic zone,wetland sediment,and river sediment.However,there is still a lack of multi-method identification and verification of the Feammox in groundwater system,and the understanding of the rate and controlling factors of the Feammox in groundwater system is also very limited.This research focuses on the Feammox in groundwater system of the alluvial lacustrine plain,taking the Poyang Lake Plain in the middle reaches of the Yangtze River as a typical area.The methods of hydrogeochemistry,H/O/C/N stable isotopes,organic matter characterization（3D fluorescence spectroscopy and ultraviolet-visible absorption spectrum）had been used to identify the spatial distribution,sources and main control processes of nitrogen in groundwater.The occurrence of Feammox and its main influencing factors in groundwater had been identified by using hydrogeochemistry,N/Fe isotope tracer,and microbial community/functional characterization methods.Finally,the rate of Feammox was quantified by indoor isotope labeling combined with acetylene inhibition culture experiments,and its controlling factors were revealed,which enriched the understanding of the natural attenuation process of nitrogen in groundwater system.The main results and insights obtained are as follows:（1）The differences in topography,geology,and hydrogeology in the study area led to differences in the distribution,sources,and main control processes of nitrogen in groundwater.The groundwater in Jiangbei Plain at the north of Yangtze River has reducing conditions,mainly the occurrence of ammonium.The ammonium mainly originates from the strong degradation of nitrogen-containing organic matter up to methanogenic stage,and the abundant humic substances can act as electron shuttles to promote Feammox.The concentrations of nitrate in groundwater are extremely low,and the denitrification rate is high.The groundwater in the Ganjiang River Delta at the south of Yangtze River has fluctuant conditions from weakly reducing to oxidizing.The ammonium originates from the coupled input of human activities and organic matter degradation.Feammox is also a potential process controlling the decay of ammonium.Nitrate in groundwater of this area is mainly distributed in stronger oxidation environment,and nitrate originates from soil nitrogen,manure or domestic sewage.Moreover,the denitrification rate is low and the process of reoxidation of nitrite to nitrate occurs at the same time.（2）Further sampling of typical areas（that is,areas where Feammox may occur）,the results show that suitable hydrogeochemical conditions,simultaneous enrichment of ¹⁵N-NH₄⁺and ⁵⁶Fe,higher abundances of Acidimicrobiaceae bacterium A6,and the positive correlation between the abundance of Feammox bacteria and the concentrations of Fe（Ⅲ）prove the occurrence of Feammox in the groundwater of Poyang Lake Plain.Geothrix and Rhodobacter may be directly or jointly involved in Feammox in groundwater.Under anoxic and slightly acidic conditions,Fe（Ⅲ）promoted the ammonium oxidation involving AOA,and AOA may play an important role in Feammox.The anaerobic ammonium oxidation process may take place with NO₂^-or NO₃^-produced by Feammox bacteria and AOA as reactants.Fe（Ⅲ）concentrations,as an important environmental factor,affected the abundances of nitrogen cycle-related functional genes and the compositions of ammonium-oxidizing and iron-reducing microorganisms.p H was also the main environmental factor affecting the abundance of AOA and Feammox microorganisms.The occurrence of the Feammox in groundwater was closely related to geological backgrounds such as red soil rich in iron and aluminum,weakly reducing conditions,and human activities such as upstream mining wastewater,urban domestic sewage,and industrial wastewater.（3）The results of the sedimentary indoor marking experiments of the water-containing layer show that the Feammox rate in the groundwater system in the Ganjiang River Delta is 0.63～0.93 mg N kg^-1 d^-1,and the nitrogen loss caused by the Feammox process in groundwater system of Ganjiang River Delta is about 29.9～44.1 kg N ha^-1 y^-1.The differences in the microbial community structure related to ammonium-oxidizing and iron-reducing was the main reason of spatial differences of Feammox rates.The amounts of ferric iron available to microorganisms was an important factor affecting the rates of Feammox in groundwater system of Poyang Lake Plain.The microbial community structure was significantly different before and after culture,and the abundances of six iron-reducing bacteria,including Geobacter,Dechloromonas,Clostridium＿sensu＿stricto＿12,Pseudomonas,Anaeromyxobacter and Geothrix,increased significantly,but only Clostridium＿sensu＿stricto＿12 and Anaeromyxobacter participate in Feammox in groundwater system of Poyang Lake Plain.p H,Fe（Ⅲ）and NH₄-N were the most significant environmental factors affecting the microorganisms related to ammonium-oxidizing and iron-reducing.The innovation of this thesis lies in:taking the lead in identifying and verifying the occurrence of Feammox in groundwater system by comprehensively using N/Fe stable isotope tracers and microbial characterization methods,and revealing the external and internal drivers of its occurrence;taking the lead in quantifying the occurrence of Feammox in the groundwater system,and the rates of Feammox and its controlling factors are revealed.