File:Biogeochemical cycle turn over.jpg

English: Schematics showing examples of ways (A) transport and (B) environmental change can serve as environmental controls on microbial populations involved in iron cycling. Part (A) depicts microbial reduction of ferric iron (Fe(III)) within an anoxic zone of an aquifer. The ferrous iron (Fe(II)) produced by iron reduction is relatively soluble. As such, some of it can be transported by groundwater flow to a discharge zone at the surface, where oxygen is available and iron oxidation can occur. The iron oxides shown in Figure 5.6 likely formed because of this very situation. Part (B) depicts the impact of seasonal variation in precipitation as a control on the water saturation and redox state of a wetland soil. During the wet season, water limits the oxygen supply at depth, allowing microbes to use ferric iron minerals as electron acceptors in the soil. During the dry season, the soil dries, exposing formerly anoxic portions of the soil to oxygen. As a result, ferrous iron (Fe(II)) produced in the wet season can then be oxidized.