Fate of copper in soil: effect of agrochemical (nano)formulations and soil properties

Abstract

Nanotechnology may be used to design more performant copper agrochemicals. Assessing the benefits and risks of the novel nano-products requires a good understanding of their fate in soil relative to other forms of copper. This study compares nine forms of Cu including nano and conventional Cu-based formulations, as well as their salt or bulk equivalents. Readily available Cu was extracted with 0.01 M CaCl₂ over a period of 29 days in three soils with contrasting properties. For eight out of nine formulations, available Cu decreased over time indicating that complexation of Cu with soil phases was the process determining the availability of Cu. Freshly suspended CuO nanoparticles were the only form where available Cu increased over time, indicating that dissolution was limiting availability. The complexation/dissolution kinetics were mainly determined by the soil pH, whereas both soil pH and OC% determined the equilibrium concentration of Cu in the soil solution, in line with results from equilibrium modelling. Overall, the fraction of readily available Cu was very low in all cases, reaching <1% of the Cu added within a couple of weeks. Our results also show that the soil moisture content at the time of Cu amendment can greatly affect the measured available Cu. According to the Cu formulation, amending dry soil resulted in up to 100% more readily available Cu than amending wet soil (pre-incubated at 50% of its maximum water holding capacity for two weeks). Overall, we show that different Cu formulations and soil pre-incubation regime can significantly impact the short term availability of Cu, which is essential to consider when conducting impact or efficacy assessments of (nano)agrochemicals.