Wetland soils are widely diverse. They are found from the arctic to the tropics. They can be mineral or organic, seasonal or year-round, marine or freshwater. The one thing they all have in common is that, for at least part of the year, they are saturated with water. This saturation has a significant impact on the soil's characteristics such as the biota, chemistry, and physics. However, over the past century more than half of all the wetlands in the United States have been drained for agriculture and other uses such as construction. When the soils are drained the characteristics are drastically changed. This paper is an attempt to describe the changes in artificially drained soils and to consider a few of the consequences of these changes.
The physical properties of saturated soils vary somewhat from wetland to wetland but are characterized by certain processes. One is the interaction of the soil with the watertable. Three patterns of possible groundwater flow have been considered: water could flow into the saturated areas from the surrounding area (discharge), making the saturated area the focal point; water could flow through swamps because of local relief (flow-through); or water could flow from the saturated zone into surrounding areas (recharge) possibly due to differential water use by plant communities or pumping (Crownover et al, 1995). There can also be vertical exchange of water between the groundwater and saturated soil. For example, capillary effects pull water upward into the soil from the water table. Besides the vertical and horizontal flow of water, the area of the soil taken up by water is important. Wetland soils are either saturated or nearly saturated so that much of the pore space is taken up by water. Air plays a much more minor role in saturated soils than in unsaturated soil.
The high water content of these soils causes the chemistry to be primarily reducing rather than oxidizing as it is in most other soils. Most of the reactions are mediated by biological activities. Such a chemical environment means that the rate of decomposition of organic matter is relatively slow. The reduced carbon in the organic matter of saturated soils is the source of energy and electrons to drive the redox reactions (Schipper et al, 1994). Under slightly reducing conditions, the process of denitrification breaks down nitrate (NO3-) into N2 "through intermediates including nitrite (NO2) and N2O" (McBride, p. 265). Under strongly reducing conditions, some of the N2 is transformed into NH4- through plant enzyme-catalization in a process called nitrogen fixing. Reducing conditions also change sulfate (SO42- ) into H2S a noxious smelling gas. Much of the hydrogen sulfide is dissolved and dissociated in the water where the sulfide interacts with Fe2+ form iron sulfides. In more moderately reducing conditions the iron can be contained in siderite (Fe CO3). In both instances the solubility of iron is greatly reduced...