Denitrification and nitrous oxide effluxes in boreal, eutrophic river sediments under increasing nitrate load: a laboratory microcosm study

• Published in: Biogeochemistry 2008-12, Vol.91 (2-3), p.105-116
• Main Authors: Silvennoinen, Hanna, Liikanen, Anu, Torssonen, Jouni, Florian Stange, C, Martikainen, Pertti J
• Format: Article
• Language: English
• Subjects: Ammonium; Biogeosciences; Denitrification; Earth and Environmental Science; Earth Sciences; Earth, ocean, space; Ecosystems; Environmental Chemistry; Estuaries; Eutrophication; Exact sciences and technology; Fluvial sediments; Freshwater; Geochemistry; Isotope geochemistry; Isotope geochemistry. Geochronology; Life Sciences; Marine and continental quaternary; Marine ecosystems; Microbial activity; Nitrates; Nitrification; Nitrogen; Nitrous oxide; Oxidation; Oxygen; Oxygen consumption; Quaternary ammonium compounds; Rivers; Sediment load; Sedimentary geology; Sediments; Soil and rock geochemistry; Surficial geology; Temperature control; Water flow
• ISSN: 0168-2563; 1573-515X
• DOI: 10.1007/s10533-008-9262-z

Intact sediment cores from rivers of the Bothnian Bay (Baltic Sea) were studied for denitrification based on benthic fluxes of molecular nitrogen (N₂) and nitrous oxide (N₂O) in a temperature controlled continuous water flow laboratory microcosm under 10, 30, 100, and 300 μM of ¹⁵N enriched nitrate (NO₃ ⁻, ~98 at. %). Effluxes of both N₂ and N₂O from sediment to the overlying water increased with increasing NO₃ ⁻ load. Although the ratio of N₂O to N₂ increased with increasing NO₃ ⁻ load, it remained below 0.04, N₂ always being the main product. At the NO₃ ⁻ concentrations most frequently found in the studied river water (10-100 μM), up to 8% of the NO₃ ⁻ was removed in denitrification, whereas with the highest concentration (300 μM), the removal by denitrification was less than 2%. However, overall up to 42% of the NO₃ ⁻ was removed by mechanisms other than denitrification. As the microbial activity was simultaneously enhanced by the NO₃ ⁻ load, shown as increased oxygen consumption and dissolved inorganic carbom efflux, it is likely that a majority of the NO₃ ⁻ was assimilated by microbes during their growth. The ¹⁵N content in ammonium (NH₄ ⁺) in the efflux was low, suggesting that reduction of NO₃ ⁻ to NH₄ ⁺ was not the reason for the NO₃ ⁻ removal. This study provides the first published information on denitrification and N₂O fluxes and their regulation by NO₃ ⁻ load in eutrophic high latitude rivers.