• Many wetland soils are characterized by a lack of oxygen induced by flooding.

• Oxygen diffusion in flooded soils is nearly 10000 times slower than in aerobic soils.

• Well aerated soils rapidly experience a decline in soil oxygen and redox potential when they are flooded.

• Some wetlands remain perpetually anaerobic.

• Reduced microbial activity and organic decomposition leads to accumulation of organic matter in wetland soils.

• Taxonomically, wetland soils are lumped as hydric.

• Hydric soils are those that in their undrained condition are saturated, flooded, or ponded long enough during the growing season to develop anaerobic conditions that favor the growth and regeneration of hydrophytic vegetation.

• Hydric soils fall in two categories:
  • Organic soils or histosols which typically contain at least 12 to 20% carbon (20 to 35% organic matter).

  • Mineral soils (entisols, ultisols, and inceptisols) that typically have less than 12 to 20% carbon.

• Table 5.1 shows a comparison of mineral and organic soils in wetlands.

Table 5-1. Comparison of mineral and organic soils in wetlands

	Mineral Soil	Organic Soil
Organic content	Less than 20-35%	More than 20-35%
Organic carbon	Less than 12-20%	More than 12-20%
pH	Usually near neutral	Acid
Bulk density	High	Low
Porosity	Low (45-55%)	High (80%)
Hydraulic conductivity	High (except for clays)	Low to high
Water-holding capacity	Low	High
Nutrient availability	Generally high	Often low
Cation exchange capacity	Low, dominated by major cations	High, dominated by hydrogen ion
Typical wetland	Riparian forest, some marshes	Northern peatland, southern swamps and marshes

PHYSICAL PROPERTIES OF HYDRIC SOILS

Mineral Soils
• Most constructed wetlands are initially dominated by mineral soils.

• Eventually, soils become organic soils.

• Clay soils may function as aquitards.

• The presence of clays may greatly increase treatment potential for conservative ions such as P and metals.

• Loamy soils consist of mixtures of clays, silts and sands and typically have excellent plant growth characteristics because of adequate nutrient holding capacity and high hydraulic conductivity.

• Sandy soils are less likely to bind chemical nutrients and may require fertilization.

• Wetland soils are light to dark grey in color if they are continuously saturated.

• They are light tan to brown in seasonally flooded areas.

• The grey color is called gleying and results from the presence of reduced iron compounds in a clay matrix.

  Organic Soils

• Histosols occur in wetland environments when the rate of organic matter formation is greater than the rate of decomposition.

• Under anaerobic conditions, decomposition slows down and organic soils develop.

• Organic soils are classified by their extent of decomposition into peat, muck, or mucky peat.

• Bulk density of organic soils tends to increase with decomposition.

• Organic soils cannot be classified by grain size because the act of drying destroys the physico-chemical structure.

• Even a slight degree of nonsaturation lowers the hydraulic conductivity of organic soils by two orders of magnitude.

• Organic soils are virtually undrainable; they retain a very high percentage of water.

• Organic soils are typically dark in color, ranging from black mucks to brown peats.

CHEMICAL PROPERTIES OF HYDRIC SOILS

Cation exchange capacity (CEC)
• Wetland soils have a high trapping efficiency for a variety of chemical constituents.

• Forces range from chemical bonding to physical dissolution.

• The combined phenomena are referred to as sorption.

• Cation exchange is the replacement of one ion by another.

• The humic substances contain large number of hydroxyl and carboxylic functions groups, which are hydrophilic and serve as cation binding sites.

• Other portions are nonpolar and hydrophobic in character.

• The result is the formation of micelles, which are groups of humic molecules with their nonpolar sections combined in the center and their negatively charged polar portions exposed on the surface of the micelle.

• Protons associate with these negatively charged particles to create electrical neutrality.

• Micelles are one form of Ligand capable of binding metal ions.

• The binding of a metal ion (M) to a ligand (L) to form a complex may be described by a chemical equation, for example:

2HL + M2+ <--> ML2 + 2H+

• The number of ligands per gram of dry soil is determined from the number of metal ions that can be sorbed by a fully protonated sample.

• This is the cation exchange capacity (CEC) of the material, usually measured in milliequivalents per gram.

• Peats hace CEC values of 1 to 1.5 meq/gr.

• For a heavy metal such as copper, this can translate to a large binding capacity, on the order of a few percent by weight.

• The pH of the soil has a large influence on the partitioning of the metal to the ligand, because excess hydrogen ions drive the reaction towards the ionic form of the metal.

• Drying destroys some of the sorptive capacity of the highly hydrated micellular chemical-physical structures.

Oxidation and reduction reactions
• Wetlands are ideal environments for chemical transformations because of the range in oxidation states that naturally occur in wetland soils.

• Free oxygen decreases rapidly with depth in most flooded soils because of the biological oxidation of microbes which consume organic matter in the soil, and also through chemical oxidation of reduced inorganic compounds.

• This decline of free oxygen is measured as an increasingly negative electric potential between a standard platinum electrode and the concentration of oxygen in the soil.

• This measure of electric potential is called reduction-oxidation potential or redox potential (Eh).

• When Eh > 300 mV, conditions are aerobic because dissolved oxygen is available.

• When Eh > -100 mV, conditions are anaerobic because there is no dissolved oyxgen.

• The intermediate condition (near zero DO) is termed anoxic.

• Oxidation and reduction are chemical transformations involving the movement of electrons between molecules.

• These transfers frequently result in striking differences between the chemical properties of the molecules that are being oxidized or reduced.

• A generalized oxidation-reduction reaction can be described as two half reactions.

• The first half reaction involves the gain or acceptance of electrons by an oxidized molecule to become a reduced molecule (reduction)

OX1 + ne- = RED1

• The second half reaction involves the loss of electrons by a reduced molecule to become an oxidized molecule (oxidation)

RED2 - ne- = OX2

• When an oxidant OX and a reductant RED are combined in a saturated soil, the complete oxidation-reduction reaction is of the following form:

OX1 + RED2 = OX2 + RED1

• As soils become increasingly reduced, chemicals other than free oxygen provide electrons for further reduction.

• Aerobic soils have Eh range from 400 to 700 mV (pH = 7).

• Wetland soils have Eh from - 300 (strongly reduced) to 700 mV (well oxidized)

• Oxygen depletion occurs at +320 to +340 mV.

• Nitrate reduction (denitrification or ammonification) may begin before complete oxygen removal, and is complete at about +220 mV.

• Manganese reduction (manganic to manganous) occurs at about +220 mV.

• Iron (ferric to ferrous) is reduced at about +120 mV.

• Sulfate is reduced to sulfide at about -150 mV.

• Carbon dioxide is reduced to methane (methanogenesis) from -250 to - 300 mV.

• Figure 5.3 shows the time sequence of oxidation-reduction in newly flooded soils.

• Figure 5.4 shows a typical depth profile for oxidation-reduction reactions in a lightly loaded wetland.

• Treatment wetlands often have wastewaters with higher oxygen demands exerted by both carbonaceous and nitrogenous compounds.

• This causes a greater depletion of electron acceptors such as oxygen, nitrate, sulfate, and iron in both water column and underlying soil.

• The redox potential in treatment wetlands is typically lower than that of natural wetlands.

pH
• Aerobic soils have pH varying from 3 to 10.

• Following flooding, pH in wetlands may decline due to aerobic decomposition liberating carbon dioxide into the intersticial water.

• This initial swing is followed with a trend toward pH neutrality.

• This is the result of ferric iron reduction under flooded soil conditions.

• In some organic histosols, pH may remain very low, even following long periods of flooding.

• This is due to the slow oxidation of organic sulphur compounds resulting in the production of sulfuric acid and the presence of humic acids.

Microbial soil processes
• Important transformations of nitrogen, iron, sulfur and carbon result from microbial processes.

• The microbial processes are affected by redox potential and pH.

• Organic nitrogen is transformed to ammonia nitrogen by mineralization.

• Mineralization is the organic matter decomposition through the action of aerobic and anaerobic bacteria.

• Ammonia is converted to nitrite and nitrate by nitrification.

• Nitrate can be transformed to nitrous oxide and nitrogen gas through denitrification.

• Nitrogen gas (N₂) can be transformed to organic nitrogen by bacterial fixation in aerobic and anaerobic soils.

• Bacteria can transform reduced iron and manganese to oxidized forms by chemosynthesis, using oxygen as an electron acceptor.

• Sulfate can be reduced to sulfite by anaerobic bacteria in wetlands.

• The sulfate serves as an electron acceptor in the absence of free oxygen at low redox potentials.

• Organic carbon is degraded to carbon dioxide by respiration under aerobic conditions and by fermentation under anaerobic conditions.

• Greater amounts of energy are released by microbial respiration.

• In fermentation, organic matter serves as the electron acceptor, forming acids and alcohols.

• Methane is formed in wetlands (methanogenesis) by bacteria using carbon dioxide as electron acceptor at very low redox potentials.

Wetland algae and macrophytes
• Organic matter accumulation in wetlands is a direct result of fixation of carbon from the atmosphere.

• In some low nutrient wetlands, oxidation may result in no net accumulation of organic matter.

• Wetland macrophytes modify soil texture, hydraulic conductivity, and water chemistry.

• The top layer of soil contains the roots of emergent macrophytes.

• These occupy the top 20 or 30 cm of soil.

• Density of roots and rhizomes are in the range of 1000 to 200 gr/m².

• Root decay leaves detritus behind.

• The live and dead root mat provides stability and strength to low-density organic soils such as peats.

• Without roots, there is no bearing strength in wetland histosols.

• Steep gradients in most chemical compounds occur in the root zone.

• This is attributed to the extraction of nutrients by the plants (by osmosis) into their root system.

• Top soils are a vital and integral part of the biogeochemical cycle.