Counterion selection for ion controls of CuO-nanoparticles is important for assessing soil biological responses

Environmental evaluations of metal nanoparticles (NP) rely on metal ion controls to distinguish between effects of the metal NP and its dissolution products. However, the coordinating or counter anion used in experimental controls may potentially influence biotic indicators used in ecotoxicology and soil health monitoring, compromising the ability to detect real nanoparticle effects and confounding interpretation of metal NP impacts. Using the example of copper oxide (CuO) NP, we demonstrate for the first time that depending on the anion used in the metal ion control (CuCl2 versus CuSO4), differing and even opposite conclusions may be drawn for CuO NP effects on a key microbiological indicator (enzyme activities) in environmental samples (soils). Moreover, this effect was specific to environmental conditions (soil management system) and indicator type (enzyme class), raising important methodological and interpretive implications for assessments of CuO NP impacts on soils. Our findings imply that assessments of soil health impacts of metal NP should consider multiple coordinating anion controls for a given metal, especially when the specific counterion is known to impact the biotic indicator (e.g., nutrient ions).