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Ecotoxicological information

Toxicity to soil microorganisms

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Description of key information

A total of ten topsoils with contrasting properties that may affect the toxicity of Mo in soil were collected and on each of these soils a series of three microbial assays (nitrification, glucose induced respiration and mineralisation of plant residues) were performed after spiking with sodium molybdate.

The microbial toxicity assays resulted in 18 individual high quality EC10 selected for PNEC derivation. The EC10 values ranged from 10 to 3840 mg added Mo/kg dry weight soil.

Additionally, three soils were aged outdoors after spiking with sodium molybdate. After 6 and 11 months, subsamples were collected and the same ecotoxicity tests as above were conducted on these soils. In general, this showed that long-term equilibration of Mo in soils decreases its toxicity to micro-organisms. The results of the toxicity in aged soils are used to determine a Leaching/Ageing factor to correct for the effect of spiking on the toxicity of Mo in soil organisms.

Key value for chemical safety assessment

Additional information

The assessed endpoints in tests with soil micro-organisms were nitrification and respiration. A total of 18 individual high quality EC10 values (for three different microbial assays) were retained for the PNEC derivation. These values range from 10 mg added Mo/kg dw (for a glucose induced respiration assay) to 3840 mg added Mo/kg dw (for a substrate induced nitrification assay). All data are based on added nominal Mo concentrations in soil. A dose confirmation showed an average recovery of 103% for the spiked soils for the nitrification assay and 98% for both the glucose induced respiration and plant residue mineralisation assays. The substrate induced nitrification, glucose induced respiration and plant residue mineralisation assays yielded reliable, bounded NOEC values for 8,6 and 4 soils out of 10, respectively. In all the other soils, no toxic effect was observed at the highest dose tested (10000 mg added Mo/kg dw).

For the three aged soils the comparison of Mo in freshly spiked and 11-month aged soils show that long-term equilibration of Mo in soil generally decreases its toxicity to micro-organisms. No toxicity was observed in two of the three aged soils at the highest concentrations left in these soils. In one soil toxicity was still observed in the 11-month aged soil for the glucose induced respiration and plant residue mineralisation assays.

The results of the toxicity in aged soils is used to determine a leaching/ageing factor to correct for the effect of spiking on the toxicity of Mo in soil organisms (see next section of this document).