The Impact of Iron on Soil N2O Production Depends on Oxygen Availability

Abstract:

The continuous increase of nitrous oxide (N₂O) abundance in the atmosphere is a global concern. Soils are both an important source and sink of N₂O, which is produced and consumed through biological processes including ammonia oxidation, heterotrophic denitrification, codenitrification, and through abiotic processes such as chemodenitrification. Iron is the most abundant element in the earth and is also the most prevalent redox-active metal in the biosphere. Its role in both chemical and biochemical reactions in N biogeochemistry cycling is well recognized. However, iron’s significance to N₂O production is poorly understood, especially under varying O₂ concentration. We examined N₂O production under different O₂ concentrations following amorphous iron (III) oxyhydroxide and ammonical N fertilizer additions in four soil slurries and two static soils (soil moisture was 50% of water holding capacity). Under 21% O₂, the addition of iron (III) significantly decreased N₂O production in all the soil slurries and static soils, while the opposite phenomenon was observed once the O₂ concentration became limited (≤3% in the soil slurry and ≤0.5% in the static soil). Our results show that the influence of iron on soil N2O production depends on O₂ availability, which is the dominant controller of N₂O production pathways. We hypothesize that under ambient O₂ conditions, iron can react with nitrite produced during ammonia oxidation, thus reducing the probability of NO₂^- being used by nitrifiers as electron acceptor in nitrifier denitrification. In contrast, under anaerobic conditions (O2<0.5%), less nitrite was detected in the presence of the iron addition. Under these conditions, iron may have inhibited N₂O reductase, or reduced iron (II) reacted with nitrite, both of which would lead to greater release of N₂O.These findings imply that management practices which focus on mitigating N₂O emission should avoid the application of iron-rich materials such as biosolids when anaerobic conditions might develop. In contrast, use of such materials could be a beneficial component of strategies to minimize N₂O emission in well-aerated systems.