Hydric soils

Vepraskas MJ, Faulkner SP. Redox chemistry of hydric soils. In Richardson JL, Ve-praskas MJ, editors. Wetland Soils Genesis, Hydrology, Landscapes, and Classification. Boca Raton CRC Press, Taylor Francis Group 2001. pp. 85-105. 

Trinitrotoluene (TNT) is the most commonly used energetic compound and found in the soils at U.S. Army installations [16,17], Military grades of TNT contain up to 8% DNTs (2,4-dinitrotoluene and 2,6-dinitrotoluene) as manufacturing impurities, and TNT is often degraded to DNT in hydric soils (i.e., wet anaerobic environments) [18,19], Other compounds are also generated when TNT is degraded (e.g., 2-amino-4,6-dinitrotoluene [2-ADNT], 4-amino-2,6-dinitrotoluene [4-ADNT], 1,3,5-trinitrobenzene [TNB], and others). Contamination from TNT, DNTs, and their environmental breakdown products is known to persist in soil for years [20],... 

Hydric soils (saturated soil conditions exhibiting temporary or permanent anaerobiosis)... 

Definition Wetlands are lands transitional between terrestrial and aquatic systems where the water table is usually at or near the surface or the land is covered by shallow water. For purposes of this classification, wetlands must have one or more of the following three attributes (i) at least periodically, the land supports predominately hydrophytes, (ii) the substrate is predominately undrained hydric soil, and (iii) the substrate is nonsoil and is saturated with water or covered by shallow water at some time during the growing season of each year. Source Cowardin et al. (1979). U.S. Department of Interior—Fish and Wildlife Service. This definition places emphasis on all three major attributes of wetlands, and is difficult to apply as it requires a comprehensive study of the site. The boundaries identified by this criteria are much more reliable than the EPA definition. 

The National Technical Committee for Hydric Soils (NTCHS) has developed the following criteria for hydric soil identification (USDA NRCS, 2006) ... 

The Committee on Wetlands Characterization, Water Science and Technology Board, National Academy of Sciences, developed a reference definition for wetland that stands outside the interests of any private or public agency (Lewis, 1995). A wetland is an ecosystem that depends on constant or recurrent, shallow inundation or saturation at or near the surface of the substrate. The minimum essential characteristics of a wetland are recurrent, sustained inundation or saturation at or near the surface and the presence of physical, chemical, and biological features reflective of recurrent, sustained inundation or saturation. Common diagnostic features of wetlands are hydric soils and hydrophytic vegetation. These features will be present except where specific physicochemical, biotic, or anthropogenic factors have removed them or prevented their development. ... 

Although this definition primarily focuses on uplands, in a broader sense, it does include soils that undergo periodic or continuous flooding. Depending on scientific disciplines and ecosystems, soils saturated with water are often called flooded soils, wetland soils, waterlogged soils, and marsh soils. Soil scientists have used terms such as flooded soils, waterlogged soils, and paddy soils. Ecologists refer to these systems as wetland soils. Now, wetland soils have been defined as hydric soils. 

In upland/drained soils, oxidized forms of chemical species dominate the system, whereas in wetland soils reduced forms dominate the system (Figure 3.6). During flooding or in the absence of molecular oxygen, the oxidized forms are converted into reduced forms, through several microbially mediated catabolic processes. Under drained conditions, many of the reduced forms are converted into oxidized forms through chemical and biological processes. Presence of reduced forms indicates soil wetness or anaerobic soil conditions, which are used as indicators of hydric soil identification. 

By definition, all wetlands have hydric soils. However, land areas with hydric soil features may not support all functions of wetlands. With a broader definition, wetland soils can be divided into the following major groups ... 

Many of the hydric soil indicators were developed for delineation purposes. During the development of these hydric soil indicators, observations were concentrated near the edge of the wetlands and in the interior of wetlands therefore, there are wetlands that lack any of the approved hydric soil indicators in their wettest portions. Delineators and other users of the hydric soil indicators should concentrate their observation efforts at the wetland edge when these conditions are suspect. 

The following list of indicators was obtained from the USDA NRCS (2006) report on field indicators of hydric soils. Field indicators of hydric soils are grouped as follows ... 

Wetlands are usually defined by hydric soils, the presence of vegetation adapted to wet conditions, and hydrology. There are several definitions of wetlands, each stressing different characteristics and wetland values. 

List the major criteria developed by the National Technical Committee for hydric soil identification. 

List some important field indicators used in the identification of hydric soils. 

Richardson, J. L. and M. J. Vepraskas. 2001. Wetland Soils. Lewis Publishers, Boca Raton, FL. 417 pp. USDA NRCS. 2006. In G. W. Hurt and L. M. Vasilas (eds.) Field Indicators of Hydric Soils in the United States, Version 6.0. USDA NRCS in cooperation with the National Technical Committee for Hydric Soils, Ft. Worth, TX. 

Redox potential can be nsed as an operational parameter for characterizing hydric soils for the presence or absence of rednced substances, and the degree of soil anaerobiosis. The Eh values obtained should be used in the context of related soil physicochemical properties such as soil pH and hydrologic conditions. 

Canse mottling and gleying, which serve as indicators of hydric soils Oxidizes toxic organic contaminants... 

Unique characteristics of ferromanganese nodules and associated oxidation-reduction reactions have been used by soil scientists as morphological indicators to help identify hydric soils (see Chapter 3). These characteristics are termed by soil scientists as redoximorphic features however, various terms such as redox concentrations, redox depletions, and reduced matrix are synonymously used for the oxidation-reduction of iron and manganese and their respective concentrations. We prefer not to define these characteristics as redoximorphic features because oxidation-reduction reactions not only involve iron and manganese but also a range of elements that support biotic communities in the biosphere. 

The concentrations of humic and fulvic acids vary considerably from soil to soil, but are closely related to the total organic matter content in the soil. Soil organic matter in well-drained soils ranges from as little as 1.0 wt% or less in coarse-textured soils (Psamments) to more than 6.5 wt% in prairie grassland soils (e.g., Hapludolls) (Soil Survey Staff 1975). Poorly drained mineral soils (aquic suborders), including hydric soils in wetlands, often... 

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