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Toxicity to soil microorganisms

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Microbial parameters (10 functional or structural endpoints) were reviewed but not further used by Jensen et al. (2007) in the PNECsoil assessment as originally proposed in Jensen et al. (2001). The EC10 values for microbial processes observed in the laboratory ranged from <8–793 mg aqueous LAS/kg soil DM. Microbial iron reduction was the parameter most sensitive to LAS (extrapolated value of 5 mg/kg; Jensen et al., 2001), but this endpoint was not considered relevant for aerobic agricultural soils because the extrapolation was not well justified and in conflict with experimental data conducted under more realistic conditions (field trials) later developed by Jensen et al. (2007). Furthermore, the lowest microbial effect concentrations had been observed in the case of dosing of aqueous LAS solutions, whereas in reality LAS enters soil in a sludge matrix, where bioavailability was shown to considerably mitigate toxic effects (Elsgaard et al., 2001; Gejlsbjerg et al., 2001). For example, ammonium oxidation, an important aerobic transformation process showed a lowest EC10 value of 14 mg/kg for aqueous dosing, versus 68 mg/kg for LAS in a sludge matrix (Elsgaard et al., 2001). In the latter study, microbial communities also showed a strong recovery potential (Schowanek et al. 2007).

In conclusion, terrestrial toxicity to soil micro-organisms is not needed considering the higher rate of sensitivity, and thus protection afforded by, studies on plants and invertebrates exposed to LAS under more realistic test conditions (field trials). In addition, LAS is readily biodegradable in water and was not toxic to activated sludge in an OECD 301B test. As LAS is not directly applied to soil and there is no reason to assume that the toxicity of LAS to terrestrial micro-organisms would deviate from their toxicity to aquatic micro-organisms, therefore testing of soil micro-organisms can be waived.

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