Evaluation of the Response of Mentha spicata L. and Soil Microbiota to Silver Nanoparticles and the Associated Risk of Herbal Tea Contamination

The release of nano-sized metal particles into the environment, along with the associated risk of their migration through the food web, is linked to the widespread application of nanomaterials in industry and agriculture. In this study Mentha spicata L. plants were exposed to silver nanoparticles (1–2 nm) at concentrations of 1, 5, 10, 50, and 100 mg/L over a 28-day period, employing two distinct application methods: root application and foliar spraying. Silver content in plant tissues, soil samples, and herbal infusions prepared from exposed spearmint leaves was determined via atomic absorption spectrometry. In root-applied treatments, silver predominantly accumulated in the soil; however, translocation to the leaves was also detected, reaching a maximum concentration of 16.9 mg/kg in leaves at the highest exposure level (100 mg/L Ag nanoparticles). Silver nanoparticles significantly reduced the soil basal respiration rate compared to the control on the 7 and 30 days of incubation. When silver nanoparticles were applied to the foliage, accumulation in the leaves (5.04–140.4 mg/kg) was observed, alongside excretion through the root system into the rhizosphere (up to 2.41 mg/kg). Interestingly, silver nanoparticles enhanced photosynthetic efficiency in the plants by increasing β-carotene and chlorophyll content, and they also stimulated higher antioxidant activity compared to untreated plants. During brewing, silver extraction into the infusion was below 1% for soil-applied treatments but ranged from 22 to 98% for foliar applications. Risk assessment indicated that prolonged consumption of mint infusion containing silver concentrations exceeding 0.348 mg/L could pose adverse health effects.