Soil‐specific outcomes in the OECD 216 Nitrogen Transformation Test.

This article focuses on evaluating the Organisation for Economic Co‐operation and Development (OECD) 216 Nitrogen Transformation Test, which assesses the impact of agrochemicals on soil nitrogen transformation. An analysis of 465 legacy OECD 216 studies revealed two distinct control soil nitrate concentration patterns: a "rise" response (consistent nitrate increase) and a "dip" response (initial nitrate decline followed by recovery). The dip response, linked to low nitrogen availability in soils and the use of organic nitrogen sources like lucerne meal, compromises the test's reliability by distorting nitrate formation rate calculations and potentially leading to misleading ecotoxicological conclusions. Experimental work demonstrated that replacing lucerne meal with an immediately available inorganic nitrogen source (ammonium sulfate) or extending the test period to include a seven-day priming phase can alleviate the dip response. The authors recommend these amendments to improve the consistency and robustness of OECD 216 studies, thereby enhancing confidence in ecotoxicological risk assessments based on this test guideline.