Soils form an important environment in wetland systems. A wide range of biogeochemical reactions take place in the different layers of soil. They are at the origin of some fundamental biodegradation processes.
The soil environment itself is divided in different parts presenting different characteristics. The deeper parts of wetland soil will be the main interest of this research.
This rapid overview presents the principal knowledge needed to understand the subject.

1. Soils conditions and Redox potential

Any kind of soil that is used for the construction of wetland systems will turn into an hydic soil (saturated soil where anaerobic conditions appear) after a short to long period of flooding and continuous anaerobic conditions.
The sketch below shows the Aerobic (it requires oxygen) and Anaerobic (oxygen-free) condition present in the different layers of wetland systems:

Aerobic, Anoxic and Anaerobic conditions in mature wetlands
(Kadlec et al. 1996)

 

Oxidation and reduction are chemical reactions that imply the transfer of protons and electrons between the molecules.
The redox potential E_h is the tendency of these chemical species to participate in such transfers. The sketch above shows how, in many wetland soils, the redox potential decreases with vertical depth into the sediments. This is due to the decreasing availability of free oxygen which is brought to the system by atmospheric diffusion at the top of the sediment layer.

It is then possible to identify aerobic conditions from the availability in dissolved oxygen, when E_h >300mV. Whereas, when E_h <-100mV, anaerobic conditions apply because of the absence of dissolved oxygen. Soils which are neither aerobic nor anaerobic are called anoxic soils (anaerobic and aerobic conditions present) (Kadlec et al. 1996).

2. Nitrogen

The nitrogen cycle is an important and complex process which involves soils. Nitrogen is one of the most limiting nutrients controlling the productivity of aquatic, terrestrial and wetland ecosystems:

Nitrogen transformations in wetlands
(Reddy et al. 2008)

All organisms require Nitrogen for basic processes of life. It can be found in nature in two forms: organic and inorganic nitrogen. For the purpose of this research only precise forms of inorganic nitrogen will be the main interest.

     • Ammonia/ammonium N (NH₃/NH₄⁺):

As it has already been recognised in previous studies, the inorganic forms of nitrogen: Nitrate NO₃^- and ammonium ion NH₄⁺ are some of the most common contaminants present in waste , fertilizer and contaminated land (Environment Agency 1996). It will be useful to identify these chemicals in groundwater contamination.

Attenuation of NH₄⁺ by sorption:

The process of sorption includes both adsorption and absorption of the contaminant phase from the porous media to the solid. This process has the effect of slowing the contaminant migration rate in the groundwater flow.
It is cation exchange that primarily controls sorption of NH₄⁺. This occurs at negatively charged mineral surfaces. For aqueous solutions with low to neutral pH, this exchange occurs principally on clay surfaces. Sorption to metal oxide surfaces also makes an important difference when pH values are above neutral (Buss et al. 2004)

Attenuation of NH₄⁺ by microbial activity:

The oxidation process of NH₄⁺ by certain bacteria is known as nitrification.
This process generally occurs under aerobic conditions and is usually in two stages. These stages are performed by different microorganisms, collectively called nitrifiers. Nitrosomonas oxidizes NH₄⁺ to nitrite as follows (Scholz et al. 2006):

NH₄⁺ + 1.5O₂ -> NO₂^- + H₂O + 2H⁺ + energy

Nitrobacter oxidizes nitrite to nitrates as follow (Scholz et al. 2006): NO₂^- + 0.5O₂ -> NO₃^- + energy
The fact that 3.3kg of O₂ is required to degrade one kg of NH₄⁺ shows that the nitrification process described above needs a continuous supply of oxygen (Buss et al. 2004).
Because of this reduced oxygen availability in wetlands the process of nitrification which is the most common, only occurs in aerobic water column, aerobic soil-floodwater interface and aerobic root zone.

Recent studies have shown that it is possible for NH₄⁺ to be oxidised to nitrogen gas under anaerobic conditions. That means it is possible for some of the anaerobic bacteria to use NH₄⁺ as their electron donor and gain energy trough the oxidation. Ammonium oxidation using nitrite as an electron acceptor is usually known as anammox and it is described as follows (Buss et al 2004):

5NH₄⁺ + 3NO₃^- -> 4N₂ + 9H₂O + 2H⁺ + energy
NH₄⁺ + NO₂^- -> N₂ + 2H₂O + energy

This reaction plays an important role in the nitrogen cycle.
In continental sediment layers, up to 67% of dinitrogen (N₂) was found to be produced from anaerobic ammonium oxidation with nitrate, versus only 33% due to denitrification.
No previous studies have reported results for wetlands (Reddy et al 2008).

     • Nitrate NO₃^-:

The main process of NO₃^- reduction in wetlands soils is the process of denitrification which occurs at the end of the nitrogen cycle.
This process is described as the biological reduction of nitrite or nitrate to gaseous end products (nitrogen gas or nitrous oxide).
The actual process of denitrification occurs in the deep layers of soil where anaerobic conditions apply. Bacteria that are capable of anaerobic growth are called facultative bacteria and present the ability to use nitrogen oxides as alternate electron acceptors.
It is a distinct enzyme called nitrogen oxide reductas that catalyzes each step of the denitrification process.
Facultative bacteria regroup a wide range of bacteria. Not all of them are able to achieve the full reduction process. However, the name denitrifier is given to all organisms participating either totally or partially in the process (Reddy et al. 2008).

3. Phosphorus:

Phosophorus is the 12th most abundant element in the lithosphere.
Phosphorus load rapidly increased in soils and aquatic systems after use of heavy fertilizers during the industrial and green revolutions, when wetlands areas were drained for more agricultural land use. Wetlands can act either as sources or sinks of phosphorus(Reddy et al. 2008).

Phosphorus load and transport in watershed
(Reddy et al. 2008)

Phosphorus can occur in soluble or insoluble forms, organic or inorganic complexes. As opposed to the nitrogen gaseous cycle, the phosphorus cycle is rather sedimentary. The diagram that follows shows the phosphorus cycle in wetlands:

Phosphorus cycle in wetlands
(Reddy et al. 2008)

DIP: Dissolved Inorganic Phosphorus           DOP: Dissolved Organic Phosphorus
PIP: Particulate Inorganic Phosphorus           POP: Particulate Organic Phosphorusp
IP: Inorganic Phosphorus

     • Soluble Reactive Phosphorus (DRP or SRP respectively)

Particulate inorganic forms represent 90% of the phosphorus load in rivers and streams (Scholz et al. 2006).
This research program will include the determination of SRP concentrations as is generally the case in water quality measurements. By definition, SRP or DRP is the name given for biologically available orthophosphate (PO₄^3-, H₂PO₄^- or HPO₄^2-), which is its primary inorganic form.
The impact of the redox potential on DRP can be significant in wetland soils. DRP concentrations generally increase with reduced redox potential and decrease with increasing redox potential.

Contrary to NH₄⁺ the principal retention process of inorganic phosphorus in sediments is the process of Sorption (abiotic retention).
Sorption includes both adsorption and absorption processes. In chemical adsorption (irreversible process) chemical bonding is involved at the solid´s surface. Whereas, physical sorption is created by the surface tension of a solid, causing the retention of the molecule at the surface of the solid. This process is reversible. In the process of absorption, molecules or atoms of the aqueous phase may penetrate the solid phase.

When soluble inorganic phosphorus is added to soil systems, soil pore water concentration increases. To maintain equilibrium, phosphorus is rapidly fixed to soil surfaces by adsorption. It is the adsorption capacity of the soil that controls the phosphorus concentration in soil pore water. When these quick processes of adsorption are over and soil particles are saturated, the concentration of phosphorus increases in soil pore water. Sorption equilibrium is reached in the timescale of minutes or hours.
The figure below shows the two-step phenomenon of sorption described above:

Phosphorus exchange between solid phase and solution phase
(Reddy et al. 2008)

As it was said before, wetlands can also act as phosphorus source, releasing phosphorus retained in the different sediment layers.
A good example of this is the process of desorption (opposite of sorption), illustrated in the figure above, where phosphorus present on sufaces of solid phases is rapidly released and then slowly released from inside the solid phase.

As explained above, the majority of phosphorus retention is done by physical sedimentation. Phosphorus is then stored in the soil strata which become a buffer zone, preventing phosphorus release in freshwater and aquatic ecosystems.
However, Macrophytes (macroscopic aquatic plants) present in wetlands generally act as a nutrient pump extracting phosphorus components from the deepest sediment layers (Scholz et al.2006).
Previous studies on constructed wetlands in north-eastern Illinois (USA) show how harvesting these macrophytes at the end of the growing season can enhance phosphorus removal.