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Toxicity to soil microorganisms

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A 28 day EC50value of >6.5 mg/kg soil dw and an EC10value of >6.5 mg/kg soil dw (highest nominal concentrations tested) have been reported in an OECD TG 216 (Soil Microorganisms: Nitrogen Transformation Test) for the effects of the structurally related substance L3 on nitrate formation rate of soil microflora. Additionally, a28 day EC50value of >8.5 mg/kg soil dw and an EC10value of >8.5 mg/kg soil dw (highest nominal concentrations tested) have also been reported in an OECD TG 216 study for the effects of the structurally related substance L2 on nitrate formation rate of soil microflora.

The data indicate that there were no inhibitory effects on the soil micro-organisms. However, analysis of the test substance concentrations in both studies show that test material was lost at the very beginning of the test for L2 and by day three of the test with L3, with all concentrations below the limit of detection, therefore the micro-organisms would have been exposed to unquantifiable, minimal amounts of the test substance, if any at all.No dose-response relationship was apparent for L2, and in view of the <LOQ recoveries of test material, it is not appropriate to ascribe the observed effects to L2.

Apparent significant effects (positive, non-inhibitory) on nitrate concentration and nitrate transformation rate were observed in the study with L3 in the 0.41, 0.81, 1.6, and 3.3 mg/kg treated soils at day 28; replicates at equivalent test concentrations showed good repeatability. No dose-response relationship was apparent, and in view of the <LOQ recoveries of test material, it is not appropriate to ascribe the observed effects to L3.

The results of these tests cannot be used for derivation of PNECs and risk assessment.

L3 and L2 have very high Henry's Law Constants and losses through volatilisation during test media preparation and within the test system were anticipated. Because of this, measures were taken during the soil micro-organism test to minimise test substance losses.

An initial test was conducted with each substance using a closed bottle system. During this test, nitrate levels decreased significantly in all test vessels, including the control. The report authors explained that this was most likely due to the test vessels being sealed air tight, thus not supplying enough oxygen to allow the nitrifying bacteria to convert the alfalfa nitrogen source to NO3.Thus the study was repeated under a separate protocol, using a different test design.

During the definitive tests, the following measures were taken to prevent loss of test item, while allowing for air exchange:

- The test vessels were sealed with perforated parafilm, which allowed for air exchange and avoided loss in moisture. 

- All test substance concentrations were dosed using gastight Hamilton syringes in multiple spots underneath the soil.

- Soil was dosed at ~30% above the calculated maximum sorption concentration to allow for some losses through volatility.

Despite these measures, significant loss of test material was observed at a very early stage in both studies. It may therefore be concluded that the soil micro-organisms were not exposed to adequate concentrations of test substance, and the EC50and EC10values determined are not representative of actual exposure of the test organisms to the test substance.

Based on these experimental findings, the registrants believe it is not technically feasible to conduct an OECD TG 216 test for the registration substance on the basis that the test substance is too volatile to maintain adequate concentrations in the test system.