The release of silver (Ag) nanoparticles (NPs) from the use of consumer products to the environment has raised concern about the risk to ecosystems because of its unpredictable toxicological impact to microorganisms in terrestrial environment. In this study, the impact of Ag chemical speciation (Ag(+) and Ag-NPs [50-nm uncoated and 15-nm polyvinylpyrrolidone (PVP)-coated Ag-NPs]) to soil nitrification kinetics was investigated using a batch soil-slurry nitrification method along with sorption isotherm and dissolution experiments. The results of nitrification potential (i.e., kinetic rate) suggest that Ag(+)/Ag-NPs, which strongly sorb in soils, suppressed the nitrification processes. Among each chemical species, the degree of suppression increased with increasing [Ag]total. Although ionic Ag (Ag(+)) species is known to exhibit greater antimicrobial effects than the solid state Ag species, such as Ag-NPs, in most studies, PVP-coated 15-nm Ag-NPs, however, more effectively suppressed the soil nitrification process than did Ag(+) under the same dose. Although several physicochemical-based toxicity mechanisms of dispersed Ag-NPs were discussed in pure culture and aquatic systems, it is not clearly understood how PVP-coated Ag-NPs could exhibit greater toxicity to nitrifying bacteria than Ag(+) in soils. In assessing the impact of Ag-NPs to microbial mediated processes (e.g., N cycles) in the terrestrial environment, it might be critical to understand the interactions and reactivity of Ag-NPs at the soil-water interface.