The Mechanism Of Ferrous Iron Oxidation Coupled To Nitrate Reduction In Paddy Soils

Nitrate reduction generates not only gas products like nitrous oxide?N₂O?but also ammonium.It is an important nitrogen transformation process,which affects both nitrogen?N?nutrient and N₂O emission in soils.In flooded paddy soils,nitrate reduction is the main nitrogen transformation process.Iron is an abundant element in the Earth's crust and a redox active metal,and its redox cycling mediates many biogeochemistry processes in the environment.Ferrous iron[Fe???]oxidation coupled to nitrate reduction readily happens in flooded paddy soils,which could have a significant influence on the N transformation process and N₂O emission.Investigating the process of Fe???oxidation coupled to nitrate reduction in paddy soils will provide the theoretical bases for the decrease of soil N loss,the enhancement of utilization efficiency of N fertilizers,the mitigation of N₂O emission and the alleviation of greenhouse effect and ozone depletion.In this study,we selected two paddy soils?Soil 1 with low organic carbon and low iron content and Soil 2 with high organic carbon and high iron content?;dealt the soils with treatments such as flooded condition,nitrate fertilizer amendment,sterilization,glucose and acetate addition,exogenous Fe???addition,acetylene?C₂H₂?inhibition and anoxic incubation;analyzed the content of nitrate?NO₃^-?,nitrite?NO₂^-?,ammonium?NH₄⁺?,Fe???and dissolved organic carbon?DOC?;combined the data of abundance of denitrifying genes to explore the mechanism of Fe???oxidation coupled to nitrate reduction.The main results were as follows:1.N₂O emission in flooded paddy soils mainly derived from denitrification.Fe???could affect denitrification process through electron donation.Fe???oxidation in Soil 2donated more electrons to denitrification than did that in Soil 1.The electron contribution from Fe???oxidation to denitrification in this study reached as high as 30%.Compared with Soil 1,the high concentration of Fe???in Soil 2 facilitated complete denitrification and lowered N₂O emission.The result suggests that Fe???oxidation could regulate the N₂O emission from denitrification in paddy soil.2.In the sterilized soils,Fe???was oxidized by NO₂^-but not by NO₃^-,indicating that NO₃^-reduction coupled to Fe???oxidation was driven by microorganisms.The electron contribution from Fe???oxidation to NO₂^-reduction in Soil 1 and Soil 2 were 50.5%and50.7%,respectively.In these contributed electrons,86.9%and 87.6%stemmed from chemodenitrification,demonstrating that abiotic pathway outcompeted biotic pathway in the reaction of NO₂^-with Fe???.The contribution of abiotic reaction to N₂O emission from NO₂^-reduction were respectively 20.5%and 42.9%in Soil 1 and Soil 2,implying that high concentration of Fe???could facilitate N₂O emission from chemodenitrification.The main products of NO₂^-and NO₃^-reduction in Soil 1 were N₂O and N₂,while it was NH₄⁺in Soil 2.N₂O emission from NO₃^-reduction in Soil 1 was higher than that in Soil 2,but the contribution of chemodenitrification to N₂O emission from NO₃^-reduction in Soil2 was higher than that in Soil 1.This implies that Fe???oxidation in the soil with high iron content bears an important influence on N₂O emission from chemodenitrification.3.Nitrate fertilizer enhanced N₂O emission and NH₄⁺content in paddy soils,demonstrating the cooccurrence of denitrification and dissimilatory nitrate reduction to ammonium.Nitrate fertilizer decreased Fe???concentration in the soils without glucose and acetate addition,indicating that the process of NO₃^-reduction coupled to Fe???oxidation happened.Nevertheless,nitrate fertilizer had no effect on Fe???concentration in the soils with glucose and acetate addition,which was attributed to that the added organic carbon as an electron donor competed with Fe???for electron acceptor like NO₃^-and thus inhibited Fe???oxidation.The addition of organic carbon decreased N₂O emission from Soil 1,but had no obvious effect on N₂O emission from Soil 2.Moreover,the addition of organic carbon enhanced methane?CH₄?emissions from paddy soils because of the supplement of substrate for methanogenesis.Nitrate fertilizer inhibited CH₄ emissions from paddy soils,which was related with nitrate-dependent CH₄ oxidation.4.Exogenous Fe???addition facilitated N₂O emission from paddy soils,which ascribed that exogenous Fe???addition enhanced chemodenitrification and inhibited microbial N₂O reduction.C₂H₂ treatment increased N₂O emission from paddy soils,but didn't completely block N₂O reduction,and just retarded the consumed rate of N₂O.It implies that the inhibition of C₂H₂ on N₂O reduction was not permanent but temporary.The abundance of nosZ gene in Soil 2 was much lower than that in Soil 1,and exogenous Fe???addition decreased the abundance of nosZ gene in Soil 1,suggesting that high concentration of iron may inhibit the activity of microorganisms carrying nosZ gene.Exogenous Fe???addition lowered the abundance of narG and nirK genes via decreasing soil pH and dissolved organic carbon?DOC?.Nevertheless,the effect of exogenous Fe???addition on the abundance of nirS and nosZ genes in the two paddy soils were different,which may related with the soil properties such as organic carbon and iron content.In the paddy soils with both exogenous Fe???and C₂H₂ treatments,the abundance of 16S rDNA,narG,nirK,nirS and nosZ genes were all enhanced.This could be that exogenous Fe???changed the soil environment,increased the chance for microorganisms to utilize C₂H₂and thus facilitated the growth and generation of microorganisms.In summary,Fe??? oxidation coupled to nitrate reduction could influence N transformation and N₂O emission in paddy soils.The reaction between NO₃^-and Fe???was mainly driven by microbial process,and the reaction between NO₂^-and Fe???was dominant by chemical process.Organic carbon competed with Fe???for electron acceptor like NO₃^-during the nitrate reduction.Exogenous Fe???lowered the abundance of narG and nirK genes,but the influence of exogenous Fe???on the abundance of nirS and nosZ genes in the two soils were different.The abundance of nosZ genes in Soil 2 was much lower than that in Soil 1,implying that high concentration of Fe???might inhibit the activity of microorganisms carrying nosZ gene.The result of contributions of biotic and abiotic processes to N₂O emissions from paddy soils gave us the enlightenments:1)in the soils with high Fe content,chemodenitrification makes an important contribution to N₂O emission;2)in the soils with low organic carbon,microbial denitrification is the main source of N₂O emission.