Insight Into The Process Of Mnammox From Agricultural Drainage Ditches In Taihu Lake

In recent years,the agricultural diffuse pollution in the Taihu Lake Basin has resulted in serious water pollution and eutrophication of rivers and lakes.The agricultural drainage ditches,used as farmland drainage channel,is the main place for nitrogen and phosphorus migration,and the interception of N,P,and other pollutants in the agricultural drainage ditches has become a key method to control agricultural diffuse pollution.Microbial denitrogenation has been widely concerned because of its high efficiency and economical characteristics.Anaerobic ammonium oxidation mediated by MnO₂?termed Mnammox?is a newly discovered microbial nitrogen removal pathway which has been rarely reported recently.Therefore,it has great theoretical and practical significance for agricultural diffuse pollution control and rivers and lakes nitrogen removal to research the Mnammox process.This thesis took the agricultural drainage ditches of Jiuli River in Taihu Lake Basin as the research place,and explored the process of Mnammox.We carried out field experiments to detect Mnammox rates and analyze the changes of Mnammox in different soil depths of agricultural drainage ditches at different temperatures using ¹⁵NH₄+ isotope tracing technique.The laboratory simulated reactor was used to conduct the experiment of Mn-reducting bacteria?MnBR?enrichment cultivation,and explore the mechanism of Mnammox.High-throughput sequencing technology was used to identify the abundance and community composition of Mnammox microorganisms in soil and microbial enrichment cultivations.The main conclusions were as follows:?1?Isotope tracer experiments:³⁰N₂ was detected in the treatments amended with ¹⁵NH₄+,and ¹⁵NH₄++MnO₂,and the ³⁰N₂ production rate in the treatments amended with ¹⁵NH₄++MnO2?0.31?0.79 mg·?kg·d?^-1?was significantly higher than that in the treatments amended with ¹⁵NH₄+only?0.25?0.67 mg·?kg·d?^-1?,which indicated that the addition of MnO₂ could promote the production of ³⁰N₂.The results proved the evidence of existence of Mnammox in agricultural drainage ditches soils.In this study,it had an obvious tendency that the ³⁰N₂ production rates increased significantly as the soil got deeper?P<0.05?.Besides,from the temperature point of view,Mnammox rates under low temperature?0.31?0.72 mg·?kg·d?^-1?were significantly lower than that under mesothermic temperature?0.40?0.79 mg·?kg·d?^-1?,indicating that temperature increase could promote Mnammox process.?2?The reduction rate of iron and manganese:The reduction rate of Mn????11.61?22.80 mg·?kg·d?^-1?and Fe????0.062?0.180 g·?kg·d?^-1?in all ¹⁵NH₄+treatments were both significantly higher than the control treatments?P<0.05?,indicating that the addition of ¹⁵NH₄+could promote Mnammox and Feammox.However,the reduction rate of Mn?IV?in all ¹⁵NH₄++ MnO₂ treatments increased further?13.44?34.22 mg·?kg·d?^-1?,while the reduction rate of Fe???greatly decreased?0.024?0.059 g·?kg d?^-1?in all ¹⁵NH₄++InO₂ treatments,indicating that the addition of MnO₂ might promote Mnammox but inhibit Feammox.These results provided strong evidence of the process of Mnammox.In addition,it was obvious that Nz,2 production rates had a strongly significant and positive correlation?r=0.813,P<0.0001;r=0.778,P<0.0001,respectively?with Mn???reduction rate.This conclusion further proved the occurrence of Mnammox in the agricultural drainage ditches.?3?High-through sequencing technology:Under low temperature,the abundance of the two main genus of MnBR,Geobacter and Geothrix,were 0.17?0.69%and 0.11?0.61%,respectively.While under mesothermic temperature,the abundance of the two main genus of MnBR,Geobacter and Geothrix,were 0.07?0.51%and 0.006?0.14%,respectively.The low relative abundance of MnBR in the soil indicated a low level of manganese concentrations in the soils?4?Physicochemical characteristics of soil:Under low temperature,a significant negative correlation had showed between TOC content and Mnammox rate?r=-0.958,P<0.05?,and under mesothermic temperature,a significant negative correlation also had showed between TOC content and Mnammox rate?r=-0.956,P<0.05?.It was indicated that excessive TOC in the soil might hinder the process of Mnammox?5?MnBR enrichment cultivation experiments:NO₂^-,NO₃^-,Mn₂⁺,and ³⁰N₂ were detected in the effluent and the concentrations were 2.25?7.71 mg·L^-1,0.012?0.766 mg·L^-1,1.76?3.22 mg·L^-1,and 0.61?4.25 mg·?L·d?^-1,respectively.The concentration of NH₄+ in the effluent?40 mg·L^-1?was lower than that in the influent?50 mg·L^-1?,and the average removal rate of NH₄+ and total nitrogen was 20%and 15%,respectively.The results comfirmed that Mnammox could occur in the presence of mirocbes.?6?Changes before and after MnBR enrichment cultivation:Before the cultivation,the most abundant MnBR at the phylum level in the soils were Proteobacteria and Firmicutes,with relative abundances of 26.23%and 0.77%,respectively.The major genera of MnBR were Geobactor and Geothrix,with relative abundances of 0.0705%and 0.0061%,respectively.After the cultivation,Proteobacteria was the dominant phyla of MnBR and the relative abundance was 49.35%,and the major genus of MnBR was Acinetobacter,with relative abundances of 26.26%.In addition,there were 20.26%of all the sequences which had not been identified,and they were likely to be the important MnBR that were not currently cultured.High-through sequencing results showed that microbial community structure changed significantly before and after cultivations.