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The effects of environmental concentration silver nanoparticles (ecAgNPs) on microbial communities and the nitrogen cycling in river sediments remain largely uncharacterized. As a fundamental component of sediments, Fe(III) can interact with AgNPs and participate in nitrogen transformation processes. N 2 O is an important intermediate in nitrogen transformation processes and can be a potent greenhouse gas with significant environmental effects. However, the impacts of the co-existence of AgNPs and Fe(III) on microbial communities and N 2 O emission in river sediments are still unclear. In the present study, mesocosm experiments were conducted to assess the changes of microbial communities and N 2 O emission in response to the co-existence of AgNPs and environmental concentration Fe(III). Our results revealed that the microbial community diversity and N 2 O emission in river sediments responded differently to ecAgNPs (0.05 mg/kg) and high-polluting concentration AgNPs (hcAgNPs, 5 mg/kg), which was further regulated by the environmental concentration Fe(III) (1 mg/g and 10 mg/g). After ecAgNPs treatments, a marked increase was observed in microbial diversity compared to hcAgNPs treatments, regardless of the Fe(III) concentration in the sediment. The β-NTI index indicated that AgNPs had stronger impacts on phylogenetic distance of bacterial communities in sediments containing 1 mg/g Fe(III) than that containing 10 mg/g Fe(III). In sediments containing 1 mg/g Fe(III), ecAgNPs did not affect N 2 O emission, but hcAgNPs significantly inhibited the emission of N 2 O. However, in sediments containing 10 mg/g Fe(III), N 2 O emission was significantly stimulated upon exposure to ecAgNPs, but the inhibition effect of hcAgNPs was barely observed. Functional prediction and real-time PCR analyses indicated that AgNPs and Fe(III) predominantly affected N 2 O emissions by affecting the abundance of the nirK gene. Our results provide new insights into the ecological impacts of the co-existence of environmental concentration AgNPs and Fe(III) in altering microbial communities and nitrogen transformation functions in river sediments., (Copyright © 2018. Published by Elsevier B.V.)