Abstract: Human activity can cause accumulation of silver nanoparticles (AgNPs) in estuarine and coastal environments; however, the effects of accumulated AgNPs on nitrogen transformation remain unclear. This study investigated the effects of AgNPs on dissimilatory nitrate reduction to ammonium (DNRA) rates in Yangtze estuarine and intertidal sediments using exposure experiments with three different sizes of AgNPs (10 nm, 30 nm, and 100 nm) and three different AgNPs concentrations (0.1 mg/L, 5 mg/L, and 10 mg/L). The results showed that AgNPs inhibited DNRA rates of sediments with different salinity levels, but the inhibition did not increase significantly with time. AgNPs had relatively high inhibition in medium salinity areas depending on the physical properties of the respective sediments. Moreover, it was shown that both the size and concentration of AgNPs were important factors affecting their toxicity. When the concentration of AgNPs was ≤ 5 mg/L, the inhibition of 10 nm AgNPs on the DNRA rate was greater than those of 30 nm and 100 nm AgNPs; the strongest inhibition effects among different salinities reached 16.03% (at 0.2‰ salinity), 20.27% (at 8.0‰ salinity), and 15.36% (at 20‰ salinity). However, when the concentration of AgNPs was 10 mg/L, the inhibition of 30 nm and 100 nm AgNPs on the DNRA rate under the same salinities increased to 17.48%, 33.18%, and 26.45%, respectively; these values were larger than the inhibition of 10 nm AgNPs. There was no significant correlation between Ag⁺ concentration and the inhibition of AgNPs on the DNRA rate (p > 0.05), indicating the inhibitory effect of AgNPs on the DNRA rate could not be fully explained by the toxicity of Ag⁺. These results are of great significance to understand the influence of emerging pollutants on the estuarine nitrogen cycle.

Key words: silver nanoparticles, DNRA, size, salinity, Yangtze Estuary