The Controls of Iron and Oxygen on Hydroxyl Radical (•OH) Production in Soils

Hydroxyl radical (center dot OH) is produced in soils from oxidation of reduced iron (Fe(II)) by dissolved oxygen (O-2) and can oxidize dissolved organic carbon (DOC) to carbon dioxide (CO2). Understanding the role of center dot OH on CO2 production in soils requires knowing whether Fe(II) production or O-2 supply to soils limits center dot OH production. To test the relative importance of Fe(II) production versus O-2 supply, we measured changes in Fe(II) and O-2 and in situ center dot OH production during simulated precipitation events and during common, waterlogged conditions in mesocosms from two landscape ages and the two dominant vegetation types of the Arctic. The balance of Fe(II) production and consumption controlled center dot OH production during precipitation events that supplied O-2 to the soils. During static, waterlogged conditions, center dot OH production was controlled by O-2 supply because Fe(II) production was higher than its consumption (oxidation) by O-2. An average precipitation event (4 mm) resulted in 200 mu mol center dot OH m(-2) per day produced compared to 60 mu mol center dot OH m(-2) per day produced during waterlogged conditions. These findings suggest that the oxidation of DOC to CO2 by center dot OH in arctic soils, a process potentially as important as microbial respiration of DOC in arctic surface waters, will depend on the patterns and amounts of rainfall that oxygenate the soil.