Silver nanoparticles stabilized by humic substances adversely affect wheat plants and soil

More than 50% of engineered nanomaterials released into the environment contain silver nanoparticles (Ag-NPs). The mobility, bioavailability, and toxicity of engineered Ag-NPs are known to depend on their properties and the environmental conditions. However, almost nothing is known about the fate of naturally occurring Ag-NPs, which are formed during the reduction of Ag⁺ by natural organic matter, primarily humic substances (HSs). The aim of this work was to study the interaction of soils and plants with simulated natural Ag-NPs, i.e., Ag-NPs stabilized with HSs (Ag-HS-NPs). To reach this goal, Ag-HS-NPs were synthesized, and their sorption-desorption behavior on two contrasting soils (a mineral soil and one rich in organic matter) was evaluated, including alterations in the mineral composition of the soil solution. In parallel, the influence of Ag-HS-NPs on wheat seedling growth was estimated. Introduction of Ag-HS-NPs into the soils resulted in a 1.3- to 2-fold or greater increase in the concentration of many elements in the soil solution (Al, Cr, Cu, Fe, etc.), and this effect was more pronounced for the organic soil than for the mineral soil. To explain this effect, we hypothesized that this phenomenon was due to the partial dissolution of Ag-HS-NPs leading to the production of Ag⁺ that could be further reduced by soil organic matter, which was correspondingly oxidized. Therefore, the partial breaking of soil aggregates because of the decomposition of soil organic matter in the presence of Ag-HS-NPs could be expected. Plants treated with Ag-HS-NPs demonstrated a lower rate of water uptake, which decreased by over 81%. The shoot and root biomass decreased by 15–17% and by 13–15%, respectively. This study clearly demonstrates an underestimated hazard of Ag-NPs formed in nature in terms of their ability to adversely affect the environment.