Effects Of Carbon-iron Coupling On N₂O Emissions From Reddish Paddy Soil

Nitrous oxide（N₂O）is a gas that plays a vital role in the greenhouse effect,and paddy soil is an important source of N₂O emissions from farmland soil.In paddy soil,redox reactions take place alternately,and both organic carbon and iron oxide can affect N₂O production through biotic and abiotic reactions.However,there are few studies on the effect of organic carbon and iron oxide on N₂O emission from paddy soil.In this study,the effects of carbon-iron coupling on N₂O emissions flux and microbial community were investigated by indoor culture experiments in red paddy soil.The denitrifying functional genes（nir S,nir K,nos ZI,and nos ZII）were quantified by q PCR,to study the correlation between the N₂O emissions flux,soil inorganic nitrogen,iron content,p H,soluble organic carbon,and denitrification function genes,and reveal the mechanism of carbon-iron coupling driving N₂O emission,providing the theoretical basis for the establishment of reasonable N₂O emission reduction measures in paddy soil.The main results are as follows:1.Glucose significantly inhibited N₂O emission from red paddy soil,ferrihydrite significantly promoted N₂O emission when ammonium fertilizer was applied,and carbon-iron coupling inhibited N₂O emission significantly（P<0.05）.The inhibition range of glucose on N₂O was 60.1%-84.6%.When nitrogen fertilizer was applied,the increasing range of ferrihydrite N₂O was 26.7%-79.5%.The N₂O emissions of CFe and CNFe treatments were 5.4 mg N kg^-1and 6.8 mg N kg^-1,respectively,which were significantly lower than that of CK treatment(15.6 mg N kg^-1)and N treatment(25.7mg N kg^-1).Therefore,the combination of glucose and ferrihydrite reduced the emissions of N₂O.2.Carbon-iron coupling had no significant effect on total nitrogen loss in red paddy soil,but reduced the proportion of N₂O in total nitrogen loss significantly,and increased the proportion of N₂.The proportions of N₂O-N in CFe and CNFe treatments were 7.2%,respectively,while the proportions of CK and N treatments were 21.6%and 21.1%,respectively.Under the same condition for carbon and iron,the cumulative N₂O emissions of ammonium treatment(6.4 mg N kg^-1)were significantly lower than that of nitrate treatment(18.1 mg N kg^-1),and the cumulative N₂O emissions increased to 11.9 mg N kg^-1after 10%C₂H₂inhibition of complete denitrification（P<0.05）.The results showed that carbon-iron coupling promoted"complete denitrification"of paddy soil and promoted the conversion of N₂O into N₂.3.The addition of glucose decreased the contents of ammonium and nitrate nitrogen and increased the content of Fe²⁺in paddy soil.Carbon-iron coupling increased the abundance of denitrifying functional genes（nir K,nos ZI,and nos ZII）,which was conducive to the denitrification effect.Carbon-iron coupling decreased the nitrate-nitrogen content in paddy soil rapidly to a low level in the early stage of culture,accompanied by the rapid production of N₂O.The content of Fe²⁺of CFe and CNFe treatments increased continuously in the early stage of culture,reaching a maximum of 1350.4 mg kg^-1and 2245.0 mg kg^-1,significantly higher than that in the carbon-free treatment,indicating that glucose as electron donor promoted the reduction of ferrihydrite.Carbon-iron coupling increased the abundance of nir K,nos ZI and nos ZII denitrifying genes by 125.7%,69.3%,and 50.1%,respectively,and nir K and nos ZI reached a significant level（P<0.05）.Therefore,organic carbon as an electron donor promotes the dissolution of Fe²⁺,and the carbon-iron coupling is beneficial to complete denitrification and reduce the emissions of N₂O.4.Glucose and ferrihydrite are the main factors that affect soil microbial community structure.At the phylum level,both glucose and ferrihydrite increased the relative abundance of Proteobacteria by 6.1%and 4.1%,respectively.At the genus level,glucose increased the relative abundance of Subgroup＿6 by 38.6%and inhibited the relative abundance of SC-I-84,Gemmatimonas,and Candidatus＿Udaeobacter by15.8%,36.0%,and 20.8%,respectively.Ferrihydrite increased the relative abundance of SC-I-84 by 8.8%.Cluster analysis of bacterial community at the genus level showed that microbial community structure was influenced by glucose and ferrihydrite,with glucose more than ferrihydrite.In conclusion,the inhibition of N₂O in paddy soil by carbon-iron coupling was achieved through the chemical-biological coupling effect.Organic carbon reduced the content of inorganic nitrogen in paddy soil,promoted the dissolution of Fe²⁺,and the carbon-iron coupling promoted the complete denitrification process,and then inhibited the emission of N₂O.