Registration Dossier

Ecotoxicological information

Toxicity to soil microorganisms

Currently viewing:

Administrative data

Link to relevant study record(s)

Description of key information

A total of 10 topsoils with contrasting properties that may affect the toxicity of Mo in soil were collected and on each of these soils a series of 3 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, 3 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

For the microbial assays a total of 18 individual high quality EC10 values (for 3 different microbial assays) are selected for the PNEC derivation. These values range from 10 mg added Mo/kg dw (for the glucose induced respiration assay in soil 3) to 3840 mg added Mo/kg dw (for the substrate induced nitrification assay in soil 10). 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 3 aged soils (soil 4,5 and 6), 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 2 of the 3 aged soils at the highest concentrations left in these soils. In one soil (soil 6), 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.