Effects of Carbon and Temperature On the Development of Reducing Conditions: A Mesocosm Study.

The factors necessary for a soil to become reducing include 1) a saturated soil environment, 2) oxidizable organic carbon, 3) microbes adapted to a reducing environment, and 4) temperatures warm enough for microbial activity. The relationship between soil saturation, carbon content, temperature, and length of time required for a soil to become reducing, is not clearly understood. This study focused on the effects of organic carbon content and temperature on the time required for saturated soils to become reducing. Surface horizons of soils were sampled along a transect gradient from well drained soils high on the landscape to very poorly drained soils low on the landscape, containing a wide range in organic carbon content. Soils were sampled from two different sites and were analyzed separately. Soils were dried and homogenized before being placed in one liter mesocosms. To each was added  five platinum electrodes, one KCl salt bridge with a calomel reference electrode, and two mini IRIS tubes.  Mesocosms were saturated and placed in incubators and were maintained at four temperatures: 2, 6, 10, and 14 degrees Celsius. Reducing conditions were documented using three methods: Eh, alpha alpha dipyridyl on soil porewater, and IRIS tubes. The initial onset of reducing conditions was identified by Eh measurements plotting below the Eh-pH line defined by the National Technical Committee on Hydric Soils  line or by a positive reaction of alpha alpha dipyridyl dye on two consecutive days. Time required for reducing conditions to occur was compared with the organic carbon content and temperature, in order to identify trends.