Wetland soils form in a variety of climates and parent materials. They represent a broad spectrum of morphological properties and taxonomic classes, and can be dominated by inorganic or organic materials. However, wetland soils have in common the condition of prolonged saturation. The term saturation refers to the condition of zero or positive soil pressure, during which water would flow into unlined auger holes and a high proportion of pore space would be filled with water. This condition favors certain physical, chemical, biological, and morphological tendencies that help to distinguish wetland soils from their upland counterparts.
3.1 Physical Properties of Wetland Soils
Saturation causes a number of physical changes in soils, including: 1) Softening of the soil material as a result of the weakening effect of water on the bonds holding soil particles together as stable aggregates. This physical effect can have several consequences: (i) root penetration by wetland plants is made easier and the soil is much easier to manipulate when wet, which are advantages that rice farmers have exploited for centuries, and (ii) trafficability of land is much poorer when flooded. 2) Flooding alters soil temperature by darkening soil color, thus increasing heat absorption. Increased water content also increases heat conductivity and stabilizes soil temperate to more constant value compared to upland soils. Saturated soils are often cooler at the surface as a result of evaporation. In wetlands located in cooler climates, the presence of water may prevent soil temperatures from going below 0oC. 3) Soil dry bulk density (weight of dry soil per unit volume) is usually decreased as a result of flooding. Typical bulk densities of upland and wetland soils range between 0.3 - 1.5, and 0.1 - 1.0 g cm3, respectively. This is due to high water absorption capacity of organic matter and destruction of soil aggregates.
3.2 Important Chemical and Biological Processes of Wetland Soils
Wetland soil chemistry is strongly influenced by the chemical reduction normally associated with saturation. Upland soils can be transformed into wetland soils as a result of excessive rainfall, rising water table, and high oxygen demand in the soil. Under these conditions, oxidized chemical species are reduced as a result abiotic and biotic processes. Similarly, improving the drainage of wetland soils can result in the oxidation of many of the reduced compounds in the soils either by chemical or biochemical reactions. The relative abundance of oxidized and reduced chemical species is therefore an indicator of the degree of wetness or anaerobic conditions in soils. The elements Fe and Mn are soil components that are particularly influenced by oxidation and reduction. Examples of Fe-bearing minerals likely to be most stable under the reduced conditions of wetlands include pyrite (FeS2), siderite (FeCO3), vivianite (Fe3(PO4)28H2O), and jarosite (K Fe3(SO4)2(OH)6). The Fe in these minerals is in reduced (Fe2+, "ferrous") form. Common Fe minerals in relatively oxidized soil environments include goethite, lepidocricite, (both FeOOH), and hematite (Fe2O3).
Redox potential is used to measure degree of soil wetness or intensity of soil anaerobic conditions. Analogous to pH (which measures H+activity), redox potential (Eh) measures electron (e-) activity in the soil. Redox potential is defined as the tendency of a pair of chemical species to undergo a transfer of electrons, with one species accepting electrons (reduction) and the other donating electrons (oxidation). Redox reactions involve oxidants, reductants, protons, and electrons, as shown below:
a (oxidants) + b H+ + n e- = c (Reductants) + d H2O
The following Nernst equation shows the thermodynamic relationship between redox potential (Eh) and the oxidation/reduction reactions.
where: Eh is redox potential, E0 is the standard electrode potential, R is the gas constant, T is the absolute temperature, F is the Faradays constant, n = number of electrons transferred, and [ ] is activity of chemical species in mol/L.
Redox reactions are critical because they regulate the types of microorganisms and fate processes of many chemical constituents in wetlands. The Eh values of wetland soils range from +700 mv to -300 mv. Negative values represents high electron activity, and intense anaerobic conditions typical of permanently waterlogged soils. Under these conditions, there is a low potential for transfer of electrons between oxidized and reduced species, due to lack of oxidized species, such as oxygen. Positive values represent low electron activity and aerobic conditions or moderately anaerobic conditions typical of wetlands in transition zone. Under these conditions, there is a greater potential for electron transfer, due to presence of oxidized species such as oxygen, nitrate, and oxidized forms of Fe and Mn.
An important redox-related process in wetland soils of marshes involves the formation and potential transformation of the mineral pyrite (FeS2). Saltwater marsh soils tend to have neutral pH and to support salt tolerant plants (Ponnamperuma, 1972). Pyrite formation occurs as a result of reduction of reduction of SO42- contained in seawater, high concentrations of Fe(II) in the sediments and rapid accumulation of organic matter that promotes reduction reactions.
Fe (OH)3 + e- + H+ Fe(OH)2+ H2O
SO42- + 6e- + 8H+ S + 4H2O
S + 2e- + 2H+ H2S
Fe(OH)2 + H2S = FeS + 2H2O
FeS + S = FeS2 (pyrite)
The drainage of pyritic marsh soils poses an environmental problem, because oxidation of pyrite to ferric hydroxide and sulfuric acid results in severe acidity (to pH less than 2). Bacteria involved in oxidizing FeS2are Thiobacillus ferroxidans and T. thioxidans.
The chemical and biological processes of wetland soils are strongly mediated by influences and adaptations of the living communities, as exemplified for the redox processes discussed above. The low-oxygen environment typical of wetlands inhibits aerobic microbial activity while stimulating activity of facultative- and obligate anaerobes, and favors growth of hydrophytic vegetation adapted to living under anaerobic conditions. The presence and types of wetland and aquatic vegetation suggests the degree of soil wetness and intensity of anaerobic conditions.
3.3 Organic Soils
Soils vary greatly in their natural organic matter content. However, organic matter tends to accumulate in wetland soils to a greater extent than upland soils because of a high rate of production relative to rate of decomposition (Mausbach and Richardson, 1994). Thick, dark, organic-rich surface layers consisting of slightly- to highly decomposed plant remains are therefore common for wetland soils. If these organic-rich layers are thick enough such that they essentially comprise the most important part of the soil zone, they are referred to as organic soils. The distinction between organic and mineral soils is an arbitrary one, in that depth- and organic-C-content must be specified as by a soil taxonomic system. The USDA soil taxonomy specifies an Order of organic soils - Histosols - which is defined as meeting specific depth requirements (in most cases, 40 cm or more) of organic soils material. Organic soil material, in turn, must contain at least 12% organic C if no clay is present, and up to 18% if 60% clay is present (i.e., proportional increase in organic C requirement with increasing clay content). If a layer consisting of organic soil material is thick, but not thick enough to qualify as a Histosol, then it may meet the criteria for a Histic Epipedons ("epipedon" refers to a diagnostic surface or near-surface horizon as defined within the USDA soil taxonomic system). Both Histosols and Histic Epipedons are almost exclusively restricted to wetlands. Most organic soils are formed from the accumulation of detritus from hydrophytic vegetation, and transformations of this material into stable humic substances and peat. The organic matter in organic soils is generally most highly decomposed near the soil surface.
The three most extensively-occurring Suborders of Histosols are distinguished based on the amount of identifiable plant material:
Saprists - about two thirds of the material well decomposed (muck) and < 1/3 of the plant material identifiable (peat);
Hemists - about 1/2 of the material is well decomposed and the other half contains identifiable plant material;
Fibrists - about 1/3 of the material is well decomposed and >2/3 of the plant material is identifiable.
A fourth Suborder, Folists, is quite restricted in occurrence. It commonly forms in thick deposits of decomposing leaves and is the only Suborder of Histosols not associated with wetland conditions.
Genetically, organic soils are distinguished from mineral soils by processes and conditions affecting the rate and duration of plant detrital accumulations. Long-term dominance by plants that produce abundant below-surface biomass under quiescent, continuously flooded conditions favors the formation of organic soils. Under this scenario, soils rapidly attain intense anaerobic conditions, with redox potential dropping to <-200 mV within a few days after flooding. Organic matter accumulation rates far exceed the oxidation rates, resulting in organic matter accumulation. For example, the sawgrass organic deposits in the Everglades of south Florida were formed about 5000 years ago, reaching depths of up to several meters.
Organic soils are far less extensive than mineral soils, but where they do occur they are commonly of considerable economic importance. Many have been artificially drained for agricultural purposes. Under drained conditions, aerobic biological oxidation of organic matter results in rapid rates of soil subsidence, with rates of about 3 cm/year in the Everglades (Stephens, 1969). At current rates of subsidence, land in the Everglades Agricultural Area will have peat depths less than 100 cm by the year 2000, making them less attractive for productive agriculture. The rate of soil subsidence can be reduced by implementing short-term (1-2 month) or long term anaerobic conditions, such as through flooding or high water table management strategies.
3.4 Mineral Soils
Wetland soils that do not meet the taxonomic requirements for organic soils (e.g., Histosols) are mineral soils. The chemical reduction that accompanies saturated conditions generally results in morphological features that are far more evident in mineral- than organic soils, due to the masking effect of organic matter for the latter. Reduction promotes dissolution of redox-sensitive Fe- and Mn oxides in soils, enabling their mobilization in accordance with principles of mass flow and diffusion unless they encounter an oxidized zone where they re-precipitate as oxides. This redox-induced redistribution provides visual evidence of saturation and of redox gradients within the soil matrix (Fig. 1A, 1B), since both Fe and Mn are strong coloring agents (shades of yellow, brown, red, and black) in their oxide forms. Gray-colored zones, in many cases, are attributable to reduction and commonly depletion of these metals.
The process of gray-color formation in mineral soils is sometimes referred to as "gleying" or gleization". Gray coloration corresponds to "low chroma" (e.g., <2) designations using the conventional Munsell notation for color characterization. A soil horizon dominated by redox-related gray colors is given the subordinate designation "g" (e.g., Btg). High chroma colors of various hues (e.g., brown or red) in zones intermixed with gray-colored zones are usually attributable to the oxidation, and subsequent concentration, of these metals. Over time, these zones of concentration may harden to form nodules and concretions. However, such hardened features, unless they have gradual or diffuse boundaries with the surrounding matrix, are indicative of relict rather than contemporary saturation. Features that reflect current wet-dry cycles tend to remain soft.
Historically, the patterns of gray in zones of periodic saturation were called "gray mottles". Gray coloration was used as a diagnostic criterion for saturation in both formal (e.g., in soil taxonomy) and informal (e.g., soil interpretations) contexts in soil science. However, soils scientists recognized that gray coloration is not always the result of chemical reduction. For example, gray colors are sometimes "inherited" from light-colored parent material. Also, carbonates and clean sand grains tend to impart gray colors to soils. Hence, a new terminology was sought which would more explicitly and precisely convey the interpretations of saturation and reduction. Such a terminology was developed by the Committee on Soils with Aquic Moisture Regimes (Bouma, 1991). Interpretive morphological terms recommended by the committee were formally adopted for use in a revision of USDA soil taxonomy (Soil Survey Staff, 1992). Collectively, they are referred to as redoximorphic features (Fig. 1A, 1B).
Redoximorphic features are now identified in standard soil descriptions, and include the following (Vepraskas, 1992):
1. Redox concentrations: Bodies interpreted as redox-related concentrations of Fe and Mn.
a. Nodules and concretions: Partially hardened bodies. Concretions have concentric layers in cross section, while nodules have a uniform internal fabric.
b. Masses: Soft bodies with reddish or brownish colors.
c. Pore linings: Zones of Fe and/or Mn accumulation along pore surfaces, as inferred from coloration.
2. Redox depletions: Bodies of low chroma (gray colors) corresponding to zones where (I) Fe and Mn oxides have been depleted through reduction, and in some cases, (ii) clay has been depleted by mobilization due to the loss of the oxide cements.
3. Reduced matrices: Soil matrices where low chroma is the result of chemical reduction of Fe, but not total depletion of Fe. A color change resulting from Fe oxidation occurs within 30 minutes.
Mineral soils, like organic soils, can have thick, dark surface horizons attributable to organic matter accumulation. These horizons may consist entirely of mineral soil material (i.e., below the organic C requirement for organic soil material). For example, many soils of semi-arid prairie regions (e.g., Mollisols, USDA taxonomic system) have such surface horizons (called Mollic Epipedons) which are attributable to the climax grass vegetation rather than wetness per se. Alternatively, mineral soils can have organic soil material thick enough to qualify as Histic Epipedons. The distinction between the Mollic Epipedon and a Histic Epipedon, both dark and thick, can be important in wetland delineation (see section 3.5) because the former is not necessarily a wetland indicator while the latter is a very reliable one. However, dark mineral surface horizons can be indicators of wetness in conjunction with immediately underlying redoximorphic features. In humid climates, for instance, surface horizons tend to be thicker and darker in wetlands.
3.5 Soils and Wetland Delineation: Hydric Soils Concept
Increased awareness of the important ecological role of wetlands has stimulated policies promoting wetland preservation. It has hence become necessary to delineate wetlands from uplands in a consistent and scientifically-sound fashion for jurisdictional purposes. Soils are one of the three key components conventionally used to establish wetland boundaries, the others being vegetation and hydrology (Cowardin et al., 1979). Each component is assessed independently. The soil assessment is particularly critical because soils are the most stable of the three components; e.g., vegetation can be quickly altered and hydrology is sensitive to seasonal climatic fluctuations. The influences of prolonged saturation on soil properties, as previously discussed, are the crux of wetland soil delineation. Recent efforts have been made to document and catalogue soil properties that are most consistently associated with wetland vegetation and hydrology.
"Wetland soils" could simply be defined as soils that occur in wetlands. However, this would be a circular definition if we intend to use soil characteristics as independent criteria in jurisdictional wetland delineation. Delineating wetland soil boundaries requires the specification of soil criteria that document the types of saturated conditions associated with wetland ecosystems. The term "hydric soils" is commonly used in jurisdictional language to apply to soils that meet such criteria. The USDA-NRCS, in conjunction with the National Technical Committee for Hydric Soils, has defined hydric soils as "... soils formed under conditions of saturation, flooding, or ponding long enough during the growing season to develop anaerobic conditions in the upper part" (Federal Register, July 13, 1994). The growing season is the period of the year when soil temperature and moisture are most favorable for microbial activity, and hence anaerobiosis if the soil is saturated. Hydric soils, as defined, would tend to be wet even during the season when evapotranspiration loss is at a maximum.
The USDA hydric soils definition serves as a guideline for local technical limits used in jurisdictional wetlands delineation. However, the practical delineation of hydric soils generally requires some indirect assessment and professional judgement based on readily-observable indicators associated with near-surface saturation that can be verified directly in the field. Field indicators of hydric soils have evolved through collective observations and consensus, and have been formally catalogued by the USDA-NRCS (USDA-NRCS, 1996) (Fig. 1C, 1E).
The USDA hydric soil indicators are specified as applying to (i) all soils, (ii) sandy soils, and (iii) loamy and clayey soils. Some are applicable to all USDA Land Resource Regions (USDA, 1981), while others are restricted to certain regions. Most of the indicators reflect pronounced organic matter accumulation at the surface, immediately underlain by redoximorphic features or other local subsurface features consistently indicative of saturation and anaerobiosis.
Delineation of hydric soils requires intensive soil investigation at wetland boundaries, since the interior areas are commonly wet enough that the hydric status is indisputable (i.e., the soils in the wettest areas have almost certainly developed "... anaerobic conditions in the upper part"). The uncertainty of hydric soil identification generally increases with proximity to the upland boundary, increasing the challenge of consistent delineation. The indicators that discriminate most effectively at the boundary are therefore the most useful. Soils examination generally requires only a shallow excavation (e.g., about 50 cm) using a tiling spade, since evidence for near-surface saturation is being sought.
Detailed interpretive "hydric soils criteria" have been developed by USDA-NRCS (Federal Register, February 24, 1995) for the purpose of identifying soils series that could potentially be hydric soils. Series meeting the criteria are placed on a national Hydric Soils List (USDA, 1991). These hydric soils criteria encompass taxonomic and interpretive criteria linked with individual soil series within the USDA-NRCS soils data base. The criteria are not used in field delineation of hydric soils. The occurrence of a soil series on the USDA Hydric Soils List means only that a soil fitting the series in the field is potentially hydric. In effect, hydric- and nonhydric soils could both fit the same series, since hydric soil status per se is not a series criterion.
Bouma, J. 1991. ICOMAC circular 11, March 15, 1991. Final Report of the International Committee for the Classification and Management of Wet Soils, Agric. Univ., Wageningen, The Netherlands.
Cowardin, L.M., V. Cater, F.C. Golet, and E.T. LaRoe. 1979. Classification of wetlands and deepwater habitats of the United States. FWS/OBS-79/31. Washington, DC: U.S. Fish and Wildlife Service.
Federal Register, July 13, 1994. Changes in hydric soils of the United States. Washington, D.C.
Federal Register, February 24, 1995. Hydric soils of the United States. Washington, D.C.
Mausbach, M.J., and J.L. Richardson. 1994. Biogeochemical processes in hydric soil formation. p. 68-127. In Current topics in wetland biogeochemistry, Vol. 1, Louisiana State University, Wetland Biogeochemistry Institute.
Ponnamperuma, F.N. 1972. The chemistry of submerged soils. Adv. Agron. 24:29-96.
Soil Survey Staff, 1996. Keys to soil taxonomy. 7th ed. USDA-NRCS. U.S. Govt. Printing Office, Washington, D.C.
Snyder, G.H. 1987. Agricultural flooding of organic soils. pp. 63. Bulletin 870. University of Florida, IFAS.
Stephens, J.C. 1969. Peat and muck drainage problems. J. Irrig. Drainage Div. Proc. Am. Soc. Civil Eng. 95:285-305.
U.S. Department of Agriculture (USDA). 1981. Land resource regions and major land resource areas of the United States. USDA-SCS Agric. Handbook 286. U.S. Govt. Printing Office, Washington, D.C.
U.S. Department of Agriculture (USDA). 1991. Hydric Soils of the United States, Third Edition. Miscellaneous Publication Number 1491. Washington, DC: Soil Conservation Service.
United States Department of Agriculture, Natural Resource Conservation Service (USDA-NRCS). 1998. Field indicators of hydric soils in the United States. Hurt, G.W., P.M. Whited, and R.F. Pringle. Version 4.0. Fort Worth, TX.
Vepraskas, M.J. 1992. Redoximorphic features for identifying aquic conditions. Tech. Bull. 301. North Carolina Agricultural Research Service, North Carolina State University, Raleigh.