Concentration-dependent responses of soil bacterial, fungal and nitrifying communities to silver nano and micron particles.

The growing use of silver nanoparticles (AgNPs) is likely to result in increased environmental contamination. Although AgNPs have been reported to affect microbial communities in a range of ecosystems, there is still a lack of information concerning the effect of low concentrations of AgNPs on soil microbial community structures and functional groups involved in biogeochemical cycling. In this study, the concentration-dependent effects of AgNPs and silver micron particles (AgMPs) on bacterial and fungal community structures in an agricultural pastureland soil were examined in a microcosm-based experiment using enzyme analysis, molecular fingerprinting, qPCR and amplicon sequencing. Soil enzyme processes were impacted by Ag contamination, with soil dehydrogenase activity reduced by 1 mg kg-1 of AgNPs and AgMPs. Soil urease activity was less susceptible, but was inhibited by ≥ 10 mg kg-1 AgNPs. The significant (P ≤ 0.001) decrease in copy numbers of the amoA gene by 10 mg kg-1 AgNPs indicated that archaea ammonia oxidisers may be more sensitive to AgNP contamination than bacteria. Amplicon sequencing revealed the bacterial phyla Acidobacteria and Verrucomicrobia to be highly sensitive to AgNP contamination. A broad reduction in the relative abundance of Acidobacterial genera was observed, with the exception of the genus Geothrix which increased in response to AgNP and AgMP amendment. Broad tolerance to Ag was observed among the Bacteriodetes, with higher relative abundance of most genera observed in the presence of AgNPs and AgMPs. The proteobacterial genus Dyella was highly tolerant to AgNPs and AgMPs and relative abundance of this genus increased with Ag concentration. Soil fungal community structure responded to both AgNPs and AgMPs, but the nanoparticle had an impact at a lower concentration. This study demonstrates that pastureland soil microbial communities are highly sensitive to AgNP amendment and key functional processes may be disrupted by relatively low levels of contamination.