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In the IMOA soils project, 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 11 toxicity tests was performed after spiking with Na2MoO4 (5 plant assays: root elongation for barley and shoot yield for Oilseed rape, Red clover, Ryegrass and Tomato; 3 invertebrate assays: reproduction for Enchytraeus crypticus, Eisenia andrei and Folsomia candida; and 3 microbial assays: nitrification, glucose induced respiration and mineralisation of plant residues). For the invertebrates, ecotoxicity tests were also conducted on an OECD artificial soil. Additionally, 3 soils were aged outdoors after spiking with sodium molybdate. After 6 and 11 months, subsamples were collected and the 10 ecotoxicity tests (same as above, without barley root elongation assay) were conducted on these soils.

The total number of 113 dose-response curves (i. e. 10 soils * 11 assays + 3 invertebrate assays in OECD control soil) yielded in total 82 useful EC10 and 4 NOEC data (when no reliable EC10 was available because the EC10 was below the lowest tested concentration). For the other 27 dose-response-curves, no reliable EC10 or NOEC could be derived because there was either already an effect at the smallest dose tested (unbounded LOEC, in 5 plant dose-response curves) or no effect at the largest dose tested (unbounded NOEC, in 12 microbial and 10 invertebrate dose-response curves). All data were based on added mg Mo/kg dw soil.

A four-step calculation method was used for the derivation of the terrestrial PNEC for molybdenum. A HC5,50% was derived using a species sensitivity distribution determined using a log-logistic distribution. A lab-to-field factor (L/A factor) of 2 was then applied to the HC5 to account for the effect of spiking on the toxicity of Mo in soil organisms. The background concentration was added in order to obtain a PNEC based on a total mg Mo/kg dw soil. The final step was the normalization of the PNEC to different soil properties (pH and log clay content being the driving soil property for plant toxicity, and clay being the driving soil property for micro-organisms and invertebrates).

The PNECsoil ranges from 4.9 - 353 mg Mo/kg dw soil depending on the organic carbon and clay of the soil. The values of 10.5 and 9.9 mg Mo/kg dw are put forward as generic, reasonable worst case PNEC values for Mo for arable and grazing land, respectively.

A detailed overview of all the terrestrial toxicity data that were identified for molybdenum is given in the Background Document "Environmental effects assessment of molybdenum", which is attached in the technical dossier in IUCLID Section 13.

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