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Environmental fate & pathways

Biodegradation in soil

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Key value for chemical safety assessment

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Testing is waived on the basis that the chemical safety assessment according to Annex I indicates that it is not necessary to conduct the soil simulation test. Simulation test (soil) is not considered necessary because:

    • The risk characterisation ratios (RCRs) for the aquatic and sediment compartment, based on the assumption that the silanol hydrolysis product is not biodegradable, are <1.
    • The substance is used under highly controlled conditions and, therefore, exposure of the aquatic environment, and thus the sediment compartment, is low
    • The silanol hydrolysis product has a low log Kow value and, therefore, exposure of the soil compartment is expected to be low.

Standard biodegradation testing is not applicable to the inorganic hydrolysis product ammonia.

Ammonium is an important intermediate in the assimilation of nitrogen from the soil by plants. Nitrogen is present in the soil largely in the organic form and is unavailable to plants. Microbial processes must mineralise it. As nitrification is an energy-yielding process, the rates of conversion are rapid, so that ammonium rarely accumulates in soil while bacteria are active. Organic nitrogen compounds are reduced to ammonium, which is converted to nitrite (NO2–) byNitrosomonasand then to nitrate byNitrobacter. Most plants can assimilate the ammonium ion, but it is usually oxidised to the nitrate ion, the most common form of mineralised nitrogen in soil, which may be assimilated by plants as well.

(Environment Canada, Health Canada 2001).

In general, natural ammonia levels in soil are very low (<1 mg/kg) due to the rapid conversion of ammonium to nitrite byNitrosomonasspecies and then to nitrate by

Nitrobacterspecies in the temperature range 0–35ºC (Henry 1995, cited in Environment Canada, Health Canada, 2001).

Environment Canada, Health Canada, 2001. Canadian Environmental Protection Act, 1999. PRIORITY SUBSTANCES LIST ASSESSMENT REPORT. Ammonia in the Aquatic Environment. Environment Canada Health Canada. February 2001