Impact of Ag₂S NPs on soil bacterial community - A terrestrial mesocosm approach

Soils might be a final sink for Ag₂S nanoparticles (NPs). Still, there are limited data on their effects on soil bacterial communities (SBC). To bridge this gap, we investigated the effects of Ag₂S NPs (10 mg kg^-1 soil) on the structure and function of SBC in a terrestrial indoor mesocosm, using a multi-species design. During 28 days of exposure, the SBC function-related parameters were analysed in terms of enzymatic activity, community level physiological profile, culture of functional bacterial groups [phosphorous-solubilizing bacteria (P-SB) and heterotrophic bacteria (HB)], and SBC structure was analysed by 16S rRNA gene-targeted denaturing gradient gel electrophoresis. The SBC exposed to Ag₂S NPs showed a significative decrease of functional parameters, such as β-glucosidase activity and L-arginine consumption, and increase of the acid phosphatase activity. At the structural level, significantly lower richness and diversity were detected, but at later exposure times compared to the AgNO₃ treatment, likely because of a low dissolution rate of Ag₂S NPs. In fact, stronger effects were observed in soils spiked with AgNO₃, in both functional and structural parameters. Changes in SBC structure seem to negatively correlate with parameters related to phosphorous (acid phosphatase activity) and carbon cycling (abundance of HB, P-SB, and β-glucosidase activity). Our results indicate a significant effect of Ag₂S NPs on SBC, specifically on parameters related to carbon and phosphorous cycling, at doses as low as 10 mg kg^-1 soil. These effects were only observed after 28 days, highlighting the importance of long-term exposure experiments for slowly dissolving NPs.