Dissolution Dynamics and Accumulation of Ag Nanoparticles in a Microcosm Consisting of a Soil-Lettuce-Rhizosphere Bacterial Community

Assessment of chronic impact of metallic nanoparticles (NPs) in soil ecosystems is a necessity for ensuring safe and sustainable application. NPs affect plants and their associated microbial life, while the plants and their associated microbiota affect the NPs' fate. Here, we measured the available Ag pool (determined as diethylenetriaminepentaacetic acid-extractable Ag) in AgNP-amended sandy loam soil (1, 10, and 50 mg Ag per kg of soil) over a period of 63 d with and without lettuce. The associated impacts on soil pH, Ag accumulation in lettuce, and the responses of the rhizosphere bacterial community were determined. We found that the addition of AgNPs significantly increased the soil pH from 7.70 to 7.87 after a short-term (7 d) incubation. Noteworthily, the extractability of Ag in AgNP-amended soil was concentration-dependent and changed over time because of their continuous dissolution and uptake by plants. Ag uptake and upward translocation in lettuce positively correlated with the extractable Ag content in soil. Furthermore, a long-term (63 d) exposure to 50 mg/kg of AgNPs altered the structure and composition of the rhizosphere bacterial community potentially by regulation of bacterial groups associated with element (e.g., N and S) cycling and stress tolerance. In conclusion, our results demonstrated that the dynamic dissolution of AgNPs in sandy loam soil plays an important role in influencing the overall Ag bioavailability of the NPs in plants. The enhanced effects of AgNPs on the alterations in the rhizosphere bacterial community highlight that the long time-resolved dynamics of NP exposure should be taken into consideration for accurate ecological risk assessment of NPs in the soil ecosystem.