Toxicity of TiO₂ nanoparticles on soil nitrification at environmentally relevant concentrations: Lack of classical...

Titanium-dioxide nanoparticles (TiO₂-NPs) are increasingly released in agricultural soils through, e.g. biosolids, irrigation or nanoagrochemicals. Soils are submitted to a wide range of concentrations of TiO₂-NPs depending on the type of exposure. However, most studies have assessed the effects of unrealistically high concentrations, and the dose-response relationships are not well characterized for soil microbial communities. Here, using soil microcosms, we assessed the impact of TiO₂-NPs at concentrations ranging from 0.05 to 500 mg kg^-1dry-soil, on the activity and abundance of ammonia-oxidizing archaea (AOA) and bacteria (AOB), and nitrite-oxidizing bacteria (Nitrobacter and Nitrospira). In addition, aggregation and oxidative potential of TiO₂-NPs were measured in the spiking suspensions, as they can be important drivers of TiO₂-NPs toxicity. After 90 days of exposure, non-classical dose-response relationships were observed for nitrifier abundance or activity, making threshold concentrations impossible to compute. Indeed, AOA abundance was reduced by 40% by TiO₂-NPs whatever the concentration, while Nitrospira was never affected. Moreover, AOB and Nitrobacter abundances were decreased mainly at intermediate concentrations nitrification was reduced by 25% at the lowest (0.05 mg kg^-1) and the highest (100 and 500 mg kg^-1) TiO₂-NPs concentrations. Path analyses indicated that TiO₂-NPs affected nitrification through an effect on the specific activity of nitrifiers, in addition to indirect effects on nitrifier abundances. Altogether these results point out the need to include very low concentrations of NPs in soil toxicological studies, and the lack of relevance of classical dose-response tests and ecotoxicological dose metrics (EC50, IC50…) for TiO₂-NPs impact on soil microorganisms.