Iron oxidation affects nitrous oxide emissions via donating electrons to denitrification in paddy soils

Paddy soils are important source of nitrous oxide (N2O), which production is mainly regulated through redox processes and electron transfer. Ferrous iron [Fe(II)] oxidation coupled to denitrification is ubiquitous in paddy soils, which could affect N2O production via donating electrons to denitrification. To clarify the effects of Fe(II) oxidation on denitrification and N2O emissions, a flooding experiment was conducted in two paddy soils with contrasting Fe(II) levels. The soil with high Fe(II) concentration emitted less N2O than did the other soil with low Fe(II) concentration. Nitrate addition decreased Fe(II) concentration and stimulated N2O production in both soils, suggesting that Fe(II) oxidation is coupled to denitrification. The stoichiometry of electron transfer between nitrate reduction and Fe(II) oxidation demonstrated that the percentage of electrons contributed by Fe(II) to denitrification accounted for 16.2% and 32.9%, and the ratios of the electrons donated by Fe(II) to the electrons accepted by nitrate for N2O production were 43.7% and 130.7% in the two soils with low and high Fe(II) concentration, respectively. The ratio beyond 100% implies that the electrons donated by high Fe(II) concentration exceed the electron demand for N2O production, which lead to the further reduction of N2O to N2. In conclusion, Fe(II) oxidation coupled to denitrification affects N2O emissions via electron donation, and Fe(II) in a high concentration bears great potential for efficient denitrification and low N2O emissions from paddy soils.