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Toxicity to soil microorganisms

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A read-across approach was used from data for soluble Pb salts without concern on the toxicity of the accompanying anion. All reliable toxicity data selected for calculation of the PNEC were derived from soils spiked with soluble lead salts (such as lead chloride, lead acetate and lead nitrate). All results were based on elemental Pb concentrations, allow-ing straight comparison of the results and read-across to other Pb substances. Soil microbial process tests evaluate the native soil microbial communities exposed to the substance. From the database of lead toxicity to micro-organisms, there are 30 individual high quality NOEC/EC10 values evaluating 5 functional parameters representing the N-cycle (i.e. denitrification, N-mineralization and nitrification) and the C-cycle (basal res-piration and substrate-induced respiration). The selected NOEC and EC10 values on functional parameters vary from 97 (for basal respiration) to 7880 mg Pb/kg dw (for potential nitrification rate). Additionally, 23 NOEC or EC10 values are selected for 8 different enzyme assays, ranging from 132 mg Pb/kg dw for dehydrogenase to ≥ 20720 mg Pb/kg dw for phosphatase. The ecological relevance of reduced enzymatic activities is unclear and, therefore these data are not used for PNEC calculation, but only presented as supporting evidence. The results for the potential nitrification rate (PNR) are preferred above data for the substrate induced nitrification (SIN). Theoretically, PNR is a more sensitive endpoint compared to SIN because the sensitivity of the latter can be ob-scured by substrate limitation.

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