The Role of Inorganic Redox Systems in Controlling Reduction in Paddy Soils

Abstract

The unique water regime of paddy soils affects the reactivity of inorganic redox systems which usually remains inactive in drained soils. Oxygen, the inorganic nitrogen compounds, and sometimes manganese compounds, can undergo oxidation-reduction reactions in typically well drained soils, but other redox systems that are more difficult to reduce than these ordinarily are not involved in redox reactions. Thus inorganic ferric oxyhydroxide compounds, and especially sulfate and carbon dioxide do not normally undergo microbial reduction in well drained soils. Under excess water conditions in paddy soils, however, the supply of oxygen into the soil is curtailed and facultative anaerobic microorganisms and strict anaerobic microorganisms use these oxidized systems as electron acceptors and convert them to reduced forms. These reduc?ions are prevented as long as the soil is porous enough to allow entry of oxygen from the atmosphere since the reduction of ferric iron, sulfate and carbon dioxide cannot take place in the presence of oxygen. The major effect of flooding on reduction processes is to saturate the pore space of the soil with water, thereby stopping gaseous diffusion of oxygen into the soil.