Dose response OECD 216 studies for agrochemicals

The nitrogen transformation test, as described by Organisation for Economic Co-operation and Development (OECD) test guideline 216, forms the basis of the current European risk assessment to assess the impacts of pesticides on the soil microbiome. This guideline contains both a two-dose threshold approach and a dose response study design, for use with agrochemicals and non-agrochemicals respectively. Recent proposals have suggested the use of a dose response study design for agrochemical risk assessment. However, differences in the principles underpinning the threshold and dose response study protocols including the method of endpoint calculation, study length and interpretation of stimulatory versus inhibitory responses, necessitate careful consideration of how to appropriately perform and interpret dose response OECD 216 studies for pesticides. Our study explores these considerations, through the conduct of dose response OECD 216 nitrogen transformation tests on four agrochemicals (dicyandiamide, nitrapyrin, dinoseb acetate and a fludioxonil metabolite). A comparison of four potential methods for endpoint calculation revealed this has little impact on the ecotoxicological inference derived from dose response OECD 216 studies. Hormetic responses were present for two of the four compounds tested, and for these compounds, the system had not reached a steady state over the 28-day study period. Test extensions beyond the 28-day test period defined in the dose response study may therefore need to be considered, as is currently implemented in the two-dose threshold approach. Furthermore, as dose response studies allow for consideration of the full ecotoxicological response over a range of concentrations, we suggest that dose response OECD 216 studies for agrochemicals should focus on inhibitory and not stimulatory responses. Finally, our work has shown that dose response OECD 216 studies are possible for agrochemicals, and their use offers a sensible approach to improving the European soil microbial risk assessment.