Sorption and the Process of Nutrient Cycling

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Sorption is a process that results in the association of a liquid or a gas phase with a solid material. The solid particle that takes up another material is referred to as the sorbent, while the material that is taken up is called the sorbate. When the sorbate adheres to a 2 dimensional surface the process is called adsorption and occurs onto the surface of minerals through van der Waals, dipole-dipole interactions, and electrostatic charges (Hemond and Fechner-Levy, 2000). Absorption occurs into a 3 dimensional surface. For example, a non-polar chemical may be absorbed by organic particles such as humic substances by displacing the weakly held water molecules within the humus (Schwarzenbach, 1993).

Sorbent particles have a high surface area due to their small size (<1 μm diameter) and consequently, a greater proportion of their atoms being located at their surface (Langmuir, 1997). The high surface area gives small particles unique surface properties relative to their larger counterparts with an otherwise similar physical structure. Due in part to their small size they have a high numbers of unsatisfied surface charges which enables them to act as sorbents for dissolved compounds in the soil solution. These small particles have higher solubilities. Colloid sized particles for example, do not settle out of solution and are capable of transporting sorbed material through the soil solution or in surface waters (Langmuir, 1997).

The partitioning of chemicals between the solid and liquid phases is an important determinant of the bioavailability and movement of nutrients in forest ecosystems. Structurally identical molecules will behave differently if they are associated with a solid surface as opposed to being dissolved i...

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...terial (Baldock and Skjemstad, 2000). The organic matter discussed in these studies is relatively low in nitrogen, however they illustrate an important role of sorption reactions in increasing the turnover rates of nutrients including nitrogen in the soil.
Oxidation of NH4+ to NO3- may alter the sorption reactions taking place, especially in soils without variable charges. As a cation, NH4+ is better able to persist in the soil while NO3- is easily leached in soils with a CEC. When a soil becomes water logged its pH increases. The change in pH could potentially alter the sorption capacity of the soil especially in soils with pH dependent charges.
Ammonia volatilization from urea is influenced by urease enzymes sorbed to soil colloids, accounting for 79-89% of the urease activity compared to microbial urease (Ardakani et al, 1975; Paulson and Kurtz, 1969).

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