Dean Hesterberg, PO Box 7619, North Carolina State University, North Carolina State University, Department of Soil Science, Raleigh, NC 27695-7619 and Michael Vepraskas, NC State University, Dept. of Soil Science, Box 7619, Raleigh, NC 27695-7619.
Redox potential is one of the master variables along with pH that most affects the mobility and biological availability of elements in soils. Redox processes occur in spatially-localized zones of soil as clearly indicated by redoximorphic features - i.e., zones of iron depletion and accumulation. This paper will focus on soil chemical, morphological, and physical properties that lead to heterogeneous redox processes. Most redox processes are driven by microbes relying on organic residues and organic matter as energy sources, and any of several common soil elements as electron acceptors. As an example, depletion of iron oxides around plant roots indicate that the roots provide a readily-metabolizable source of organic carbon that leads to reductive dissolution of iron oxides and diffusion of Fe(II) from the rhizosphere. In other cases, the macroporous nature of root channels leads to preferentially drainage of water, and rapid oxidation of Fe(II) as it diffuses to and precipitates as insoluble Fe-oxide minerals. These and other examples indicate that spatially-heterogeneous redox processes depend on coupling of macro- and micro-biological processes related to carbon metabolism, chemical processes related to mobility of elements under different redox conditions, and hydrological processes involving water drainage and diffusion of gases and dissolved ions. An understanding of these processes at different length scales can provide insights on preferential dissolution, transport, or immobilization of plant nutrients and potentially-toxic substances in soils.