Essay: Phosphorus

Phosphorus is found in wetlands in inorganic (IP), also called orthophosphate, and organic (OP) form. These comes in particulate (PP) and dissolved (DP) form. Depending on the nature and usage of the land in the catchment area, the pools of IP and OP that enter streams, drainage system and the wetlands will differ in relative size (Dunne 2005). IP compounds are most often associated with oxides and hydroxides of iron, aluminum and calcium carbonate. Whereas OP typically consist of the P incorporated in the biota consisting of relatively rapid decomposable P compounds (nucleic acids, phospholipids, and sugar phosphates) and also more slowly decomposable organic P compounds. The latter being in the refractory pool (Reddy et al. 1999). Dissolved inorganic P (DIP), i.e. orthophosphates, are directly bioavailable, whereas the largest part of the OP pool must undergo mineralization to become bioavailable (Ibid).

P is associated with and transported mainly as particulate P (PP) in the runoff from fields with clay or silt dominated soil characteristics (James et al 2002, Dunne 2005, L??kanen et al, 2004)

Particulate P consists of orthophosphates PO43- (check) absorbed to soil particles and organic matter that stems from erosion of farmland in the wake of either irrigation, precipitation or snowmelt. PP enters the water bodies through surface runoff or through the drainage systems (Reddy et al. 1999).

As briefly mentioned the retention of P in wetlands, is attained by different mechanism and in combination of sedimentation, chemical sorption, i.e. adsorption and absorption, precipitation, microbial interactions, and uptake by vegetation (James et al. 2002, Kadlec & Knight 1996). The particulate P is retained by the physical process of sedimentation whereas the dissolved P is retained through both biological and chemical processes; i.e. uptake in biota and sorption to metal cations forming precipitates (James et al 2002, Reddy et al. 1999).
The term sorption covers both adsorption and absorption is particularly a process of interest in the matrix basin. The process of sorption onto and into the wood chips can be seen as a two-step process, with a rapid phase of adsorption onto the surface and a slower phase of diffusion into the solid phase (Reddy et al. 1999, Kadlec 2005).

Retention in the wetland sediment depends on physical and chemical nature of the soil or sediment. In order for P to be retained here the IP must be in direct contact with the adsorbent either by contact with settling particles in the water column or by diffusion into the sediment. For the latter to occur, water column concentration must be higher than that of the pore water in the sediment (Lijklema, 1993, Reddy et al., 1999) This means that when water column P concentration is low, during e.g. dry periods, the sediment retained P can be released from the pore water to the water column (Reddy et al. 1999).

The size of the wetland in relation to its catchment area (Awetland /A catcment) has proven to affect P retention efficiency, because it to some degree affects the hydraulic load (Reddy et al, Kadlec & Knight 1996). In general the P retention of wetlands is inversely proportional to the P loading, i.e. the higher the load, the lower the retention effectiveness. Though it is important to bear in mind that a number of other factors plays a role individually as well as combination effects (Van der Falk 2012). The net retention of wetlands is in general reported, as either specific or relative retention, i.e. kg P / acre / year or percentage of load retained.

It is known that the water velocity plays a significant role in sedimentation processes, due to the settling time of particles. Water velocity is decreased by vegetation and different physical structures of the wetland resulting in the settling of particulate matter and the P that is adsorbed by it. Through these dynamics the accumulated sediment can prove to be a long term sink for P and other nutrients. A number of factors, such as wind-driven mixing and bioturbation and gas ebullitions can resuspend the particulate matter from the sediment and possibly release the P bound to it (Reddy et al. 1999, Van der falk 2012).

Biogeochemical cycling of P and the retention and release in wetlands is complex, though basically a sedimentary cycle (Van der falk 2012)

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