Tack et al. 2006

Tack FMG, Van Ranst E, Lievens C, Vandeberghe RE. 2006. Soil solution Cd, Cu and Zn concentrations as affected by short-time drying or wetting: the role of hydrous oxides of Fe and Mn. Geoderma 137: 83-89.

These authors examined the effects of short-term changes in redox conditions on the interaction between trace heavy metals and oxides of iron and manganese in agricultural soils of Flanders. The expectation was that two weeks of water-saturated and hence reducing conditions would allow these iron and manganese oxides to dissolve, and then re-precipitate when soils were returned to oxidizing conditions at field capacity or dry conditions. This process of iron and manganese chemistry would dominate redox conditions in soil solution, and dominate the chemistry of trace metals, controlling the solubilities of the trace metals.

Contrary to expectations, short-term wetting and drying cycles did not push the chemistry of trace metals around to a significant degree. The authors state that they cannot distinguish between the competing explanatory hypotheses of very slow transformations of the expected type or that these processes occurred at the expected rates but reverted during the later stages of the incubations of soils at particular moisture levels.

In the conclusion, these authors describe the importance of periods of drying, where soil moisture levels fall far below field capacity. Unlike cycles of saturation and field capacity, dried soil has much higher oxygen levels, and the moisture shortage has important effects on soil microorganisms, and the local redox conditions in microhabitats. Occasional very dry periods have a disproportionate effect on heavy-metal chemistry in soils.

For my purposes, this paper was most useful for the clear description of the methods used to measure iron oxides in soils, from a distinctly geochemistry perspective. These authors clearly state that measurement of soil levels of “amorphous” and “crystalline” Fe(III)-oxides are based on operational definitions, as the measurements are based on not-entirely-specific dissolutions of particular fractions of soil iron with particular solutions. These dissolution methods are widely used, and it was useful to see the limitations of these methods clearly described.