Silver nanoparticles (AgNPs) are increasingly being incorporated into consumer products for their antimicrobial properties, and recent risk assessment scenarios suggest that a significant fraction of these AgNPs will enter terrestrial ecosystems through application of biosolids from wastewater treatment plants.  Given AgNPs antimicrobial activity, we examined their ecological effects by comparing plant, microbial, and biogeochemical dynamics between mesocosms that received no treatment (Control), amended with biosolids (Biosolid treatment), amended with biosolids containing 10nm silver nanoparticles (AgNP treatment), or biosolids amended with Ag⁺ (Ag⁺ treatment) to assess whether AgNPs and Ag⁺ have different effects.  Biosolids were added at 200g dry biosolid/mesocosm, a rate recommended by the US EPA to match plant N demand.  In both silver treatments, biosolid Ag content was enriched by 57µgAg g^-1 biosolid. Mesocosms were high-density polyethylene tubs planted with 3 individuals each of 5 species: Lobelia cardinalis, Lolium multiflorum, Carex lurida, Juncus effusus, Panicum virgatum, and Microstegium vimineum. For each treatment, there were six experimental replicates. 

Results/Conclusions

All treatments receiving biosolids stimulated plant growth compared to the Control treatment, but the AgNPs and Ag⁺ treatments had 22 and 31% less aboveground Microstegium biomass, respectively, relative to the biosolid only treatment. Similarly, Microstegium also exhibited a had a 34% lower photosynthetic rate in Ag⁺ relative to the biosolid only treatment, suggesting that the decrease in biomass may in part be driven by altered physiology. In contrast to the aboveground biomass data, there was a significant increase in root biomass in the top 1cm of soil in both the AgNP and Ag⁺ treatments relative to the Biosolids only treatment, suggesting that silver may have stimulated root production at the community level.

Microbial community composition, biomass, and activity were all altered by AgNP and Ag⁺ additions. Potential activity of the microbial extracellular enzymes sulfatase, leucine amino peptidase, and phosphatase were respectively 7, 32, and 34% lower in the Biosolids + AgNPs treatment. Corresponding to this decrease, microbial biomass was 20% lower in the Biosolids + AgNPs  treatment. Community composition of bacteria, as analyzed by DGGE and t-RFLP characterization of the 16S-rRNA gene was also distinct for treatments receiving Ag, suggesting this shift in activity and abundance was coupled to a change in community composition.