The characteristic features of wetland soils are the result of the activity of anaerobic soil bacteria. Both obligate and facultative anaerobic bacteria exist in the saturated zones of many soils. Legally, a soil is designated as a wetland soil when these characteristic wetland features are present at or near the surface of the soil.
When soil is saturated and air is excluded from soil pores, anaerobic bacteria dominate the soils microbic community. What do these microbes do that aerobic microbes cannot do? The main difference is that they use substances other than oxygen molecules (O2) as their electron acceptors. When most organisms respire, they transfer electrons from one molecule to another, in a chain. Each time an electron is transferred, energy is released that is used by the organism for its life processes. Ultimately, that electron is transferred to an oxygen molecule. When O2 is no longer available, the series of electron transfers stop. The aerobic organism will die if O2 is not supplied.
Anaerobic organisms do not use O2 as their electron acceptors. There are a number of substances that they will use as a substitute for O2, including (among others) nitrate (NO3-), ferric iron (Fe3+), manganese (Mn2+), sulfate (SO42-) and carbon dioxide (CO2). (See related section: Biogeochemistry.)
Since Fe3+ is quite prevalent in most soils, anaerobic bacteria add an electron to it and thereby reduce it to Fe2+ (ferrous iron). Iron oxides containing the Fe3+ form of iron are yellow or red in color and are insoluble in water. Fe2+ forms are colorless and water soluble. Saturated soils are therefore gray or white in color, while dryer soils are yellow or red. Areas of the subsoil that are mottled with gray, yellow and red are areas with a fluctuating water table (alternating between saturated and unsaturated conditions). The presence of gray colors or mottles near the surface is an indicator of a wetland soil. Black manganese nodules are also sometime present in wetland soils, and serve as another indicator.
Anaerobes also reduce SO42- to H2S (hydrogen sulfide) producing the characteristic rotten egg odor of certain wetlands (especially the Sulfaquents and Sulfhemists of the salt marshes). Anaerobic bacteria are also less efficient than aerobic bacteria at decomposing organic matter. As a result, wetland soils tend to have higher organic matter contents than unsaturated soils. Often, the surface horizons are black or, indeed, entirely composed of organic matter.
Wetlands are the habitat of many unique and economically important plants and animals. However, equally important to the well being of the environment are the processes that occur in wetland soils.
There are three ways that wetland soils may filter contaminants from water:
In addition to water filtration, wetlands are important for flood control. As mentioned above, wetland soils are usually high in organic matter, if they are not largely composed of it. Soil organic matter absorbs many times its weight in water, like a spongehence the spongy feel when one walks through a wetland. Failure to maintain wetlands in a relatively natural state, especially in flood plains, will result in greater losses when flooding occurs.
Further information on wetland soils can be found at: http://www.ftw.nrcs.gov/soils_data.htm. For more information on federal wetland laws, link to Federal Regulations or visit the FEDLAW website at: http://www.legal.gsa.gov.
R. Scharf, SCDNR Land, Water, and Conservation Division