Using iron metal rods to infer oxygen status in seasonally saturated soils

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The oxygen status in soils is fundamental to a number of earth-science processes including biological, physical and chemical interactions. Since oxygen is time consuming and often difficult to measure in soils, this research explores the use of zero-valent iron rods as a means of estimating oxygen content in soils. Zero-valent iron rods (polished nails) were placed in three soil toposequences and the surface coatings formed by oxidation were compared to measured oxygen concentrations taken every three weeks for one year. When water was present in the soils, the oxygen was measured as dissolved oxygen and in drained conditions it was reported as percent. Coatings on iron metal rods inserted in the soil appear to correlate well with specific oxygen ranges. Rods in soils with O2 concentrations below about 2 mg l- 1 do not develop bright (7.5YR 4/4 to 5/8) oxide/oxyhydroxide coatings, but instead formed black (10YR 2/1-2) coatings. Rods in soils with O2 concentrations between about 2% to 5% develop variegated bright (7.5YR 4/4 to 5/8) oxide/oxyhydroxide coatings indicating soil drainage and microsite differences in O2 concentrations. Rods in soils with O2 concentrations above about 5% with adequate moisture are almost completely coated with bright (7.5YR 4/4 to 5/8) iron oxide/oxyhydroxides. Mineralogy of oxide/oxyhydroxide coatings could not be established by XRD presumably due to the short-range order crystal structure of the metal oxide/oxyhydroxides. This method provides a simple and inexpensive means to qualitatively estimate the ranges of O2 status in soils. This research may have implications in hydric soil determination, horticultural and agricultural applications, as well as, remediation techniques.

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