Miljøprojekt, 902 – The Influence of Sorption on the Degradation of Pesticides and other Chemicals in Soil

The Influence of Sorption on the Degradation of Pesticides and other Chemicals in Soil

5 Bioavailability

Bioavailability is a term frequently used when one describes the possibility of plants absorbing the chemical or the availability of the substance to soil organisms, the latter case often being coupled with toxicity. Bioavailability understood as availability to the microorganisms in the soil is very consistently used in literature f_OC using on bioremediation in which an increase in the microbial degradation of a chemical is wanted. The f_OC using on techniques for increasing bioavailibity in this context may probably be attributed to the fact that many bioremediation tasks have f_OC used on very hydrophobic substances such as the PAHs. Literature that describes the natural microbial degradation of pesticides in the soil environment only rarely makes use of the expression "bioavailability". Most publications phrase the problem – that the substances may be not be able to establish contact with the microorganisms – in different ways. E.g. formation of bound residues, the influence of the binding on the degradation, and microcavities in the soil where the chemical molecule can penetrate but where the space is too small for the microorganisms. Thus, the expression "bioavailability" can in principle be understood as the availability of the chemicals to microorganisms.

The amount of organic matter in the soil will generally increase the binding of the substances to soil particles unless it is a question of substances, such as glyphosate, which are chiefly bound to clay particles and where in several cases a reverse correlation has been shown between the binding of glyphosate and the amount of organic matter (Parfitt et al., 1995; Glass, 1987). The first reactions between foreign chemicals and soil are physically-chemically reversible interactions. However, it applies to many pesticides in soil that the sorption-desorption process is not completely reversible, and the possibility of desorption is often reduced the longer the chemical has been present in the soil (Lehmann et al., 1990; Barriuso et al., 1992). At the same time as the extractability of the pesticide decreases, the formation of bound residues increases. A large number of authors have described this connection in conceptual diagrams (Novak et al., 1995; Wauchope & Meyers, 1985; Scow & Hutson, 1992; Calderbank, 1989).

The amount of the organic matter in the soil has also another effect, as the presence of a large amount of organic matter in soil may be attributed to higher biological activity, as a large amount of degradable organic matter has been present on which the microorganisms could be propagated. A higher biological activity will often increase the transformation rate of not just the organic matter found naturally but also the added foreign organic substances. With this the organic matter may have two opposite effects in the soil. Mueller et al. (1992) showed that there was a positive linear correlation between the degradation rate for fluometuron and the content of organic matter in the soil and the microbial biomass while Simon et al. (1992) showed that such a correlation could not be shown for fenamiphos. Veeh et al. (1996) found a positive correlation between the degradation rate for 2,4-D and the number of microorganisms and asserted that it would be possible to find such a correlation for most pesticides with a low binding to the soil organic matter. Torstensson and Stenström (1986) measured the biological activity as a basic respiration rate and also studied the degradation of 2,4-D but in contrast to Veeh et al. (1996) found no clear correlation between the respiration rate and the degradation rate for 2,4-D. They found this correlation for linuron and glyphosate, however.

A very complex connection between the sorption of xenobiotic chemicals to the soil organic matter and the microbial degradation of the substances in the soil environment is more the rule than the exception. In order to throw light upon this connection, the development in the past few years has been in the direction of investigating the microbial degradation of chemicals in soil under circumstances as close to the real environment as possible. When the sorption is so significant as to whether the added chemical comes into contact with the degrading microorganisms, there will be a great difference in what is found when degradation experiments are carried out in soil that has a natural water content as opposed to experiments where the soil is suspended in water. Therefore, publications that describe degradation experiments with pesticides in aqueous suspensions are becoming very rare. At pesticide concentrations of more than 1 µg/g in soil, substances that can be degraded metabolically will often be degraded according to growth kinetics whereas at lower concentrations the degradation will often take place according to no-growth kinetics. A normal consumption of pesticides will lead to concentrations of 1 g/g or less in the upper soil layer. On the other hand, point source pollution with pesticides or industrial pollution with industrial chemicals may lead to higher concentrations. Therefore, one ought to be aware whether experiments are carried out at realistic concentrations.

While the aim is to carry out experiments under circumstances that resemble the natural ones, development has also gone in the direction of making a complete description of the mechanisms that control the degradation of the chemicals so that the influence of the sorption on this can be described.