Iron Oxidation Coupled Nitrate Reduction Process And Its N₂O Release In Red Soil Paddy Fields

The process of nitrogen transformation in soil is associated with many ecologic environment changes,Nitrous oxide（N₂O）released during the conversion process is also closely related to global warming and ozone layer destruction,it is the third largest greenhouse gas after CO₂ and CH₄.Moreover,studies have found that in addition to the microorganisms,chemical processes also play an important role in the process of soil nitrogen conversion.N₂O mainly comes from nitrification and denitrification,and denitrification is a transformation from nitrate to molecular nitrogen（N₂,N₂O,etc.）,which,together with nitrification,nitrogen fixation and mineralization,constitute the natural nitrogen cycle.Whereas the relative contribution of biological and chemical denitrification to N₂O emission remains unclear.Therefore,it is of great significance to understand the reaction mechanism of soil denitrification process and N₂O release effect under different nitrate supply conditions for the efficient utilization of nitrogen fertilizer and the control of N₂O emission.Iron as an active metal element in the earth’s crust,its continuous redox cycle is an important link with the geochemical cycle of other elements.The iron cycle controls the environmental processes such as the mineralization and denitrification of soil organic matter and the fixation of heavy metals,which contains ferrous oxidation and ferric reduction,it is a bond of soil nutrient cycle and pollutant transformation.Iron is often used as electron donor or acceptor in the process of chemical transformation of nitrogen in current research.And denitrification process of iron oxide coupled nitrate in paddy soil needs further study.The typical red paddy soil in south China（yingtan,Jiangxi）was selected for the research.By adding nitrate to the soil,we detected substrate and product changes in related processes and analysised them with molecular biology techniques,then we can understand the biochemical processes,the leading process and generation mechanism of N₂O emission.In addition,in order to provide the theoretical basis of chemical process for ferrous coupled nitrate reduction process,the effects of different environmental conditions（temperatures）on the chemical reaction between nitrite and Fe（II）were also studied from different perspectives.The main conclusions are as follows:Through the addition of nitrate（NO₃^-）in the microcosmic environment of paddy soil for 10 days,the related kinetic changes were observed through continuous sampling test.Results showed that the soil itself could provide sufficient Fe（II）to react with NO₃^-,NO₃^-was mainly consumed by the denitrification pathway.No obvious nitrate dissimilation reduction pathway was observed in the kinetic results.When the system produces NO₂^-,the ability of NO₃^-and NO₂^-to oxidize Fe（II）may be greater than the reduction reaction of Fe（III）.Meanwhile,a kinetic model is established to obtain the reaction rate and discuss the relative contribution of NO₃^-reduction and other processes under the presence of iron in the soil.The main dominant microorganism enriched in nitrate-reducing Fe（II）oxidation system is Anaerolineaceae uncultured＿bacterium＿c＿SBR2076,Geobacteraceae,Planctomycetaceae.Through the addition of nitrite（NO₂^-）in microcosmic environment of soil,the reaction kinetics of iron redox,nitrite reduction,production of N₂O and other processes was observed.We hypothesized Fe-N biochemical denitrification,microbial Fe（III）reduction and organic nitrogen mineralization were existed in this system.In this chapter,the ability of microorganisms to reduce Fe（III）was greater than that of NO₂^-chemical action to oxidize Fe（II）.After the addition of NO₂^-for 10 days,enrichment of Geobacteraceae and Bacillaceae in the soil was obvious.Soil N₂O emission is closely related to excessive accumulation of NO₂^-.Temperature not only affects soil microbial metabolism and decomposition of organic matter,it will also affect the chemical reaction itself.Different temperature conditions affect the nitrite-reducing Fe（II）oxidation rate,the production of N₂O and the formation of minerals,but make no difference to isotope fractionation.The nitrite-reducing Fe（II）oxidation reaction is common in farmland soil,and it will influence the C,N cycling in soil.These above results not only revealed the metabolism mechanism of nitrate-reducing Fe（II）oxidation and the N₂O emissions in the paddy field,but provided theoretical basis for other processes related to the nitrate-reducing Fe（II）oxidation in the soil.