Clément et al. 2005

Clément J-C, Shrestha J, Ehrenfeld JG, Jaffe PR. 2005. Ammonium oxidation coupled to dissimilatory reduction of iron under anaerobic conditions in wetland soils. Soil Biology and Biochemistry 37: 2323-2328.

These authors observed an unexpected chemical reaction involving the accumulation of both nitrite (NO2-) and ferrous iron (Fe(II)) under anaerobic conditions. They investigated this phenomenon further, and propose a chemical reaction in wet soils in which ammonium is oxidized under reducing conditions by transferring electrons to Fe(III), generating NO2- and Fe(II).

Nitrite does not usually accumulate in soils. These authors suggest that under normal conditions, it is consumed at least as fast as it is produced, but their experimental conditions included inhibition of denitrification, allowing nitrite to build up to detectable levels. Other oxidizers besides Fe(III), such as Mn(IV), were not detected in soil samples and were not included in the experiment.
The proposed chemical pathway is thermodynamically feasible at pH 7, though it appears to rely on goethite as the ferric iron source; from my understanding of dissimilatory iron reduction (e.g. Lovley 1991), I would expect strongly crystalline forms of iron oxide such as goethite to be highly resistant to such destructive forces, and the iron source in the systems (natural and experimental) described here to be amorphous ferric oxides instead. But the underlying chemistry appears plausible to me.

One unexpected aspect of this short communication was the authors’ use of a Dionex ion chromatography system, apparently very similar to the device I will be using to analyze root exudates. Additionally, this paper discusses the “ferrous wheel”, a memorable name for chemical cycling of iron between valencies, as an established hypothesis; I need to track down the origins of this term and learn its importance regarding my own attempts to relate measured Fe(III) contents to redox conditions.