Role of Chemodenitrification for N2O Emissions from Nitrate Reduction in Rice Paddy Soils

Atmospheric nitrous oxide (N2O) causes global warming and ozone depletion. Nitrate and nitrite reduction are the main sources for N2O emission in anoxic environments including both microbial (denitrification) and abiotic reactions (chemodenitrification), besides nitrification in oxic habitats. In flooded paddy soils, substantial concentrations of Fe(II) and nitrite are available, potentially triggering chemodenitrification. It is currently unknown to what extent chemodenitrification contributes to N2O emissions in such environments. We conducted anoxic microcosm experiments with two paddy soils that differ in natural Fe(II) and organic carbon content. We amended them with nitrite or nitrate and quantified N2O emissions. In sterilized soils, nitrite and not nitrate was abiotically reduced, pointing toward chemodenitrification. In microbially active soils, nitrate reduction was accompanied by nitrite accumulation, ammonium production, and N2O emission, implying the co-occurrence of denitrification, dissimilatory nitrate reduction to ammonium (DNRA), and chemodenitrification. N2O emissions from chemodenitrification accounted for 6.8-67.6% of the total N2O emissions, depending on the concentrations of Fe(II), nitrite, nitrate, and organic carbon, and the N2O emission rate from abiotic reactions was up to 2.4 mg N kg(-1) d(-1) Elevated Fe(II) levels in soils facilitated nitrite accumulation, chemodenitrification, and high abiotic N2O emission (up to 42.9%). In low organic carbon soil, more N2O was emitted by chemodenitrification in nitrite-amended setups (20.5% of total N2O emission) compared to nitrate-amended setups (6.8%). High organic carbon content in soils indirectly enhanced the proportion of abiotic N2O production (up to 67.6%), potentially favoring DNRA over denitrification, which decreased the biotic contribution to N2O formation. Our results suggest that chemodenitrification could be a significant contributor for N2O emissions in paddy soils via a complex network of biotic and abiotic processes involving C, Fe, and N biogeochemical cycling.