Effects of ferric iron reduction and regeneration on nitrous oxide and methane emissions in a rice soil

A laboratory soil slurry experiment and an outdoor pot experiment were conducted to study effects of ferric iron (Fe(III)) reduction and regeneration on nitrous oxide (N2O) and methane (CH4) emissions in a rice (Oryza sativa L.) soil. The anoxic slurry experiment showed that enhancing microbial Fe(III) reduction by ferrihydrite amendment (40 mol Fe g−1) transitionally stimulated N2O production and lowered CH4 production by 16% during an initial 33-day incubation. Increased regeneration of Fe(III) through a 4-day aeration period in the Fe-amended slurry compared to the control slurry reduced CH4 emission by 30% in the subsequent 15-day anaerobic incubation. The pot experiment showed that ferrihydrite amendment (63 μmol Fe g−1) stimulated N2O fluxes in the days following flooding. The Fe amendment suppression on CH4 emission was obscured in the early season but became significant upon reflooding in the mid- and late-seasons. As a result, seasonal CH4 emission in Fe-amended pots was 26% lower than the control with a single 2-day drainage and 69% lower with a double 2-day drainage. The reduction in CH4 emission upon reflooding from the Fe-amended pots was mainly attributed to the increased Fe(III) regeneration during drainage showing a mechanism of Fe(III) regeneration in mitigating CH4 emission by short-term drainage in flooded soils.