The potential benefits and limitations of different test procedures to determine the effects of Ag nanomaterials and AgNO3 on microbial nitrogen transformation in soil
Background: The procedure described in The Organisation for Economic Co-operation and Development (OECD) test guideline (TG) 216 is used to assess the effects of chemicals on microbial nitrogen transformation in soil, and the results are considered in regulatory risk assessments. We investigated the suitability of this method to characterize the effects of two silver nanomaterials and a soluble silver salt. We applied three different test procedures: (i) nitrogen transformation using the complex organic nitrogen source lucerne meal (OECD TG 216), (ii) nitrogen transformation using the inorganic nitrogen source (NH4)2SO4 (following OECD TG 216), and (iii) ammonium oxidation (ISO 15685). The results were compared with substrate-induced respiration (OECD TG 217). Results: The standard nitrogen transformation test using lucerne meal suggested that the test materials had no effect, whereas significant effects were identified with the other two test procedures. The absence of effects with lucerne meal probably reflected the sorption of silver ions to the additional organic nitrogen source, thus reducing its bioavailability, or blocking the silver nanomaterial oxidation sites by sorption of organic matter. Conclusions: This common test used in the context of chemical registration is therefore unsuitable for the detection of potential effects caused by silver nanomaterials and soluble silver salts because it can yield false negative results. We instead recommend the use of an inorganic nitrogen source. The observed effects were not specific to nanomaterials. The time course of the effect in the nitrogen transformation test based on (NH4)2SO4 and the potential ammonium oxidation test varied according to the test substance, indicating different kinetic behaviors of ion release.