Silver (Ag+) reduces denitrification and induces enrichment of novel nirK genotypes in soil


  • Present address: Mats Johansson, The National Food Administration, Box 622, SE-751 26 Uppsala, Sweden.

  • Editor: Elizabeth Baggs

Correspondence: Sara Hallin, Department of Microbiology, Swedish University of Agricultural Sciences, Box 7025, SE-750 07 Uppsala, Sweden. Tel.: +46 18 67 32 09; fax: +46 18 67 33 92; e-mail:


The use of silver ions in industry to prevent microbial growth is increasing and silver is a new and an overlooked heavy-metal contaminant in sewage sludge-amended soil. The denitrifying community was the model used to assess the dose-dependent effects of silver ions on microorganisms overtime in soil microcosms. Silver caused a sigmoid dose-dependent reduction in denitrification activity, and no recovery was observed during 90 days. Dentrifiers with nirK, which encodes the copper nitrite reductase, were targeted to estimate abundance and community composition for some of the concentrations. The nirK copy number decreased by the highest addition (100 mg Ag kg−1 soil), but the nirK diversity increased. Treatment-specific sequences not clustering with any deposited nirK sequences were found, indicating that silver induces enrichment of novel nirK denitrifiers.