Temperature effects on iron reduction in a hydric soil

For soils to be considered hydric, they must demonstrate both saturation and anaerobic conditions in the upper part of the soil. Although several technologies are available for monitoring of soil water tables, documentation of reducing conditions is more problematic. This has led to recent interest in the use of IRIS (indicator of reduction in soils) tubes. IRIS tubes are lengths of PVC pipe coated with ferrihydrite paint, which are inserted into the soil to document reducing conditions. Observations from preliminary studies led us to postulate soil temperature will affect the degree and rate of reduction and removal of ferrihydrite from the IRIS tubes. To quantify the impact of soil temperature on ferrihydrite reduction and removal from IRIS tubes, a time series experiment was designed such that IRIS tubes were placed into a hydric soil for time periods ranging from 7 to 28 days, between February and June, 2004. Water tables were monitored daily, Eh was measured at depths of 10, 20, 30, 40 and 50, cm on a weekly basis, and soil temperature was recorded every 4 hours. Removal of ferribydrite from IRIS tubes was not constant during periods of anaerobiosis and was related to soil temperature. At temperatures below 2 degrees C, there was essentially no measurable ferrihydrite reduction, even though soil redox potentials fell within the Fe(II) stability field. As soil temperatures increased between 2 and 8 degrees C, the quantity of ferrihydrite reduction increased with time. At soil temperatures between 8 and 20 degrees C, substantial (35% to 45%) ferrihydrite paint was reduced and removed from IRIS tubes within 7 days. For a given temperature, there was greater reduction of ferrihydrite in zones closer to the soil surface, presumably due to higher amounts of oxidizable organic matter in near surface horizons.