While a large increase in the commercial and home usage of silver nanoparticle (AgNP) products and technologies is expected in the future, few studies contribute to the risk assessment of AgNPs in terrestrial environments. In this study, the effect of AgNPs (total [Ag]: 1-100 mg kg^-1, 15-50 nm size with/without polyvinylpyrrolidone capping agent) on soil denitrifying bacteria was investigated via assessment of their physicochemical properties using bench-scale biogeochemical experiments, dissolution studies, and X-ray absorption spectroscopy. Although the effects on denitrification kinetics and pseudo-equilibrium end-points were variable among the AgNPs, the antimicrobial effect resides at the soil-water interface as sorbed-AgNP and -Ag(I) species. There is clear evidence of surface oxidation/oxidative dissolution of AgNPs that contribute to the ionic Ag toxicity. However, more than 90% of AgNPs remained insoluble as dispersed and/or aggregated AgNPs, which exhibited nanoparticle-specific toxicity.