Aquatic macrophytes mitigate the short-term negative effects of silver nanoparticles on denitrification and greenhouse gas emissions in riparian soils.

Abstract Silver nanoparticles (AgNPs) are increasingly released into the aquatic environments because of their extensive use in consumer products and industrial applications. Some researchers have explored the toxicity of AgNPs to nitrogen (N) and carbon (C) cycles, but little is known about the role of aquatic plants in regulating the impact of AgNPs on these biogeochemical processes and related microorganisms. Here, two 90-day pot experiments were conducted to determine the effect of AgNPs on denitrification rates and greenhouse gas emissions in riparian wetland soils, with or without emergent plants (Typha minima Funck). As a comparison, the toxicity of equal concentration of AgNO3 was also determined. The results showed that AgNPs released a great quantity of free Ag+, most of which was accumulated in soils, while little (less than 2%) was absorbed by plant shoots and roots. Both AgNPs and AgNO3 could increase the soil redox potential and affect the growth and nutrient (N and phosphorus) uptake of plants. In soils with plants, there was no significant difference in denitrification rates and emissions of N2O and CH4 between control and AgNPs or AgNO3 treatments at all tested concentrations (0.5, 1 and 10 mg kg−1). However, low levels of AgNPs (0.5 mg kg−1) significantly enhanced CO2 emission throughout the experiment. Interestingly, in the absence of plants, a high dosage (10 mg kg−1) of AgNPs generally inhibited soil denitrification and stimulated the emissions of CO2, CH4 and N2O in the short-term. Meanwhile, the abundance of key denitrifying genes (nirS and nirK) was significantly increased by exposure to 10 mg kg−1 AgNPs or AgNO3. Our results suggest that emergent plants can alleviate the short-term negative effects of AgNPs on N and C cycling processes in wetland soils through different pathways.