Insight Into The Mechanism Of Feammox From A Riparian Zone In Gonghu Bay, Taihu Lake

With the rapid development of China’s industry and agriculture,a number of nitrogen from industrial and agricultural wastewater has been discharged into the aquatic environment,making the accumulation of nitrogen nutrients in lakes,rivers and so on,resulting in eutrophication of water body,which have destroyed the structure and function of aquatic ecosystems.Microbial denitrogenation is one of important pathway to solve nitrogen pollution in water bodies.However,Feammox as a new process of microbial denitrogenation is less studied at present.Therefore,it is of great significance to explore Feammox process in riparian zone.In this thesis,in order to solve the increasingly serious problem of nitrogen pollution,we selected Feammox as the study object from riparian zone which are located in Qinshui river flowing into Gonghu Bay,Taihu lake,south of China.On one hand,field experiments explored the change law of Feammox in different soil layers and with or without vegetation cover.On the other hand,laboratory experiment analyzed the nitrogen removal mechanisms of Feammox.In this study,evidence of Feammox in riparian zone soils was demonstrated using isotope tracing technique and Membrane Inlet Mass Spectrometry（MIMS）.The abundance of Feammox bacteria and the microbial community diversity of riparian zone soils were analyzed by high-throughput sequencing technology.The transformation mechanism of nitrogen and iron were explored by the construction of NH₄⁺ and Fe（III）culture system.In this study,the main conclusions are as follows:（1）The N₂ production rates were detected using Membrane Inlet Mass Spectrometry by labeling ¹⁵NH₄⁺.The occurrence of Feammox were evidenced through detecting the ³⁰N₂ production in different soil layers and different vegetation cover soil.In this study,the detected Feammox rates were 0.25～0.29 mg kg^-1d^-1 in all soil layers.Moreover,Feammox rates in 5-10 cm soil layer were significantly higher than other soil layers（P<0.05）.On the other hand,the detected Feammox rates were 0.32～0.37 mg kg^-1d^-1 in vegetated soil significantly higher than that in un-vegetated soil(0.20 mg kg^-1d^-1,P<0.05),and the results indicated that the growth of vegetation can enhance Feammox process.（2）On one hand,the results of different soil layers showed that iron reduction rates in the ¹⁵NH₄⁺ treatments(0.22～0.26 g kg^-1d^-1)were obviously higher than that in the control treatments(0.20～0.23 g kg^-1d^-1,P<0.05),and the iron reduction rates were significantly correlated with ³⁰N₂ production rates（r = 0.864,P<0.01）.On the other hand,the results of vegetated or un-vegetated cover soil showed that iron reduction rates in the ¹⁵NH₄⁺ treatments(0.21～0.28 g kg^-1d^-1)were obviously higher than that in the control treatments(0.17～0.21 g kg^-1d^-1,P<0.05),and the iron reduction rates were significantly correlated with ³⁰N₂ production rates（r = 0.789,P<0.01）.These results indicated that the ¹⁵NH₄⁺ can enhance iron reduction,and iron reduction were significantly related to Feammox process.（3）The major phyla of iron reducing bacteria（FeBR）communities in riparian zones were Proteobacteria and Acidobacteria.The major genus of FeBR communities in riparian zones were Geobacter and Anaeromyxobacter.In the results of different soil layers,the abundance of FeBR in 0-10 cm soil layers significantly higher than 10-20 cm soil layers（P<0.05）.In addition,in the results of different vegetation cover soils,the abundance of FeBR in vegetated soil significantly higher than that in un-vegetated soil（P<0.05）.（4）The results of physicochemical characteristics in this study showed that richer iron soil have higher the abundance of iron reducing bacteria and Feammox rates.The Feammox rates were significantly correlated with Fe（Ⅲ）contents（r = 0.862,P<0.05）and TOC（total organic carbon）contents（r = 0.637,P<0.05）in different soil layers.In addtion,The Feammox rates were significantly correlated with Fe（Ⅲ）contents（r =0.740,P<0.05）and TOC contents（r = 0.849,P<0.05）in different vegetation cover soil.These results showed that the rich Fe（Ⅲ）contents or TOC contents can enhance the occurrence of Feammox process.（5）Construction of NH₄⁺ and Fe（Ⅲ）culture system,Nitrate（NO₃^-）,nitrite（NO₂^-）and Fe（Ⅱ）were detected in the effluent of reaction system during 90 days laboratory experiment,which indicated that Fe（Ⅲ）was reduced to Fe（Ⅱ）and NH₄⁺ was oxidized to NO₃^-and NO₂^-.In anaerobic environment,only Fe（Ⅲ）can be used as an oxidant and ammonia（NH₄⁺）as an reductant.Therefore,in the reaction system,the production of NO₃^-and NO₂^-were from NH₄⁺ oxidation,and the production of Fe（Ⅱ）was from Fe（Ⅲ）reduction.Ammonium oxidation and iron reduction（termed Feammox）occurred in the culture system.（6）In the stable stage of operation,0.355 mmol L^-1 NH₄⁺ and 2.117 mmol L^-1 Fe（Ⅲ）were involved in the reaction.The production of NO₃^-and NO₂^-were 0.195 mmol L^-1 and 0.009 mmol L^-1,respectively.From the perspective of nitrogen transformation,the amount of NO₃^-and NO₂^-were calculated according to the theoretical reaction equation.The consumption of NH₄⁺ was 0.204 mmol L^-1,which was less than the actual consumption of NH₄⁺.The result showed the occurrence of Feammox produced directly N₂.On the other hand,from the perspective of iron transformation,the amount of NO₃^-and NO₂^-were calculated according to the theoretical reaction equation.The consumption of Fe（Ⅲ）was 1.614 mmol L^-1,which was less than the actual value measured.This may also explain that part of the Fe（Ⅲ）participated in the process of Feammox produced directly N₂.