Solid Phase Silver Sulfide Nanoparticles Contribute Significantly to Biotic Silver in Agricultural Systems

The current and continued influx of engineered nanoparticles (NPs) into the environment is significant, including the release of NPs that have been historically stored or retained in soils to various waterbodies. However, the reactivity and dynamic nature of NPs transformation processes are poorly understood due to the lack of long-term environmentally relevant experiments that accurately represent ecosystem complexity. Here, we established a two-year mesocosm system to quantify the relative reactivity of silver sulfide NPs using stable isotope tracers, with more recent 109Ag2S-NPs inputs to the 80 L water column (water-borne NPs, 141 mg) and historically stored Ag2S-NPs in soils (soil-borne NPs, 4.5 ± 0.3 μg g−1). Soil-borne NPs accounted for 59.4–92.1% of the Ag accumulation in the grain of rice Oryza sativa L. (31.4–112.4 μg kg−1), radish roots Raphanus sativus L. (106.2–396.7 μg kg−1), and rice borers Chilo suppressalis (21.5–30.7 μg kg−1), highlighting the significance of soil-borne NPs in agricultural ecosystems. Based on the measured soil-to-plant transfer factors, recommended concentrations of soil-borne NPs should be less than 2.4 μg Ag g−1 for rice growth and 0.7 μg Ag g−1 for radish growth to minimize human exposure to silver via consumption of these edible tissues. This work demonstrates that quantifying the reactivity of NPs transformation processes and different NPs fractions in the environment is not only important to accurately characterizing the risk of these materials but also to ensuring the safety and sustainability of agriculture.