Pedosphere - Redox Conditions in Wetland Soils

Redox Conditions in Wetland Soils

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 N₂ (nitrogen), H₂S (hydrogen sulfide) and CH₄ (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 decomposition is incomplete.

The redox potential describes which way chemical reactions will proceed in oxygen deficient soils and controls the nutrient cycling in flooded systems. Redox potential, or 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 and will likely donate electrons to a part of the system that has a low concentration of electrons, or an oxidized environment, to equilibrate to the chemical gradient. The oxidized environment has high redox potential, whereas the 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 oxygen (O₂) there is in the system. The oxidizing environment accepts electrons because of the presence of O₂, which acts as electron acceptors:

O₂ + 4e- + 4H+ → H₂O

This equation will tend to move to the right in acidic conditions which causes higher redox potentials to be found at lower pH levels. Bacteria, heterotrophic organisms, consume oxygen while decomposing organic material which depletes the soils of oxygen, thus increasing the redox potential. In low redox conditions the deposition of ferrous iron (Fe2+) will increase with decreasing decomposition rates, thus preserving organic remains and depositing humus. At high redox potential, the oxidized form of iron, ferric iron (Fe3+), will be deposited commonly as hematite. By using analytical geochemical tools such as x-ray fluorescence (XRF) or inductively coupled mass spectrometry (ICP-MS) the two forms of Fe (Fe2+ and Fe3+) 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. This suggests 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 O₂ use SO₄2- as an electron acceptor and produce hydrogen sulfide (H₂S) and carbon dioxide in the process:

2H+ + SO₄2- + 2(CH₂O) → 2CO₂ + H₂S +2H₂O

The H₂S gas percolates upwards and reacts with Fe2+ and precipitates pyrite, acting as a trap for the toxic H₂S gas. However, H₂S is still a large fraction of emissions from wetland soils. In most freshwater wetlands there is little sulfate (SO₄2-) 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 (CH₄) and carbon dioxide (CO₂), which are released to the atmosphere. Methane is also released during the reduction of CO₂ by the same bacteria.