text: Nutrient cycling in lakes and freshwater wetlands depends heavily on redox conditions. Under a few millimeters of water, heterotrophic bacteria metabolize and consume oxygen. They therefore deplete the soil of oxygen and create the need for anaerobic respiration. Some anaerobic microbial processes include denitrification, sulfate reduction and methanogenesis and are responsible for the release of N (nitrogen), HS (hydrogen sulfide) and CH (methane).  Other anaerobic microbial processes are linked to changes in the oxidation state of iron and manganese.  As a result of anaerobic decomposition, the soil stores large amounts of organic carbon because the soil carbon sponge stays intact.
  The reduction potential describes which way chemical reactions will proceed in oxygen deficient soils and controls the nutrient cycling in flooded systems. Reduction potential is used to express the likelihood of an environment to receive electrons and therefore become reduced. For example, if a system already has plenty of electrons (anoxic, organic-rich shale) it is reduced.  In a system, it will likely donate electrons to a part that has a low concentration of electrons, or an oxidized environment, to equilibrate to the chemical gradient. An oxidized environment has high redox potential, whereas a reduced environment has a low redox potential.
  The redox potential is controlled by the oxidation state of the chemical species, pH and the amount of O there is in the system. The oxidizing environment accepts electrons because of the presence of O, which acts as an electron acceptor:
  This equation will tend to move to the right in acidic conditions. Higher redox potentials are found at lower pH levels. Bacteria, heterotrophic organisms, consume oxygen while decomposing organic material.  This depletes the soils of oxygen, thus decreasing the redox potential. At high redox potential, the oxidized form of iron, ferric iron (Fe), will be deposited commonly as hematite. In low redox conditions, decomposition rates decrease and the deposition of ferrous iron (Fe) increase.
  By using analytical geochemical tools such as X-ray fluorescence or inductively coupled mass spectrometry the two forms of Fe (Fe and Fe) can be measured in ancient rocks therefore determining the redox potential for ancient soils. Such a study was done on Permian through Triassic rocks (300–200 million years old) in Japan and British Columbia. The geologists found hematite throughout the early and middle Permian but began to find the reduced form of iron in pyrite within the ancient soils near the end of the Permian and into the Triassic. These results suggest that conditions became less oxygen rich, even anoxic, during the late Permian, which eventually led to the greatest extinction in Earth’s history, the P-T extinction.
  Decomposition in anoxic or reduced soils is also carried out by sulfur-reducing bacteria which, instead of O use  as an electron acceptor and produce hydrogen sulfide (HS) and carbon dioxide in the process:
  The HS gas percolates upwards and reacts with Fe and precipitates pyrite, acting as a trap for the toxic HS gas. However, HS is still a large fraction of emissions from wetland soils. In most freshwater wetlands there is little sulfate () so methanogenesis becomes the dominant form of decomposition by methanogenic bacteria only when sulfate is depleted. Acetate, a compound that is a byproduct of fermenting cellulose is split by methanogenic bacteria to produce methane (CH) and carbon dioxide (CO), which are released to the atmosphere. Methane is also released during the reduction of CO by the same bacteria.