kinetics of denitrification in permeable sediments

• Published in: Biogeochemistry 2013-05, Vol.113 (1-3), p.563-572
• Main Authors: Evrard, Victor, Glud, Ronnie N, Cook, Perran L. M
• Format: Article
• Language: English
• Subjects: Ammonium; ammonium nitrogen; anaerobic conditions; Anoxia; Biogeochemistry; Biogeosciences; Canals; carbon; coasts; Cohesive sediments; Denitrification; Earth and Environmental Science; Earth Sciences; Earth, ocean, space; Ecosystems; Electrons; Environmental Chemistry; Exact sciences and technology; Kinetics; Life Sciences; Marine and continental quaternary; Marine ecosystems; marine sediments; microbial communities; nitrate nitrogen; Nitrates; nitrogen; Organic carbon; oxidation; Oxygen; Oxygen consumption; Permeability; Sand; Sediment core samples; Sediments; Surficial geology
• ISSN: 0168-2563; 1573-515X
• DOI: 10.1007/s10533-012-9789-x

Permeable sediments comprise the majority of shelf sediments, yet the rates of denitrification remain highly uncertain in these environments. Computational models are increasingly being used to understand the dynamics of denitrification in permeable sediments, which are complex environments to study experimentally. The realistic implementation of such models requires reliable experimentally derived data on the kinetics of denitrification. Here we undertook measurements of denitrification kinetics as a function of nitrate concentration in carefully controlled flow through reactor experiments on sediments taken from six shallow coastal sites in Port Phillip Bay, Victoria, Australia. The results showed that denitrification commenced rapidly (within 30 min) after the onset of anoxia and the kinetics could be well described by Michaelis–Menten kinetics with half saturation constants (apparent Kₘ) ranging between 1.5 and 19.8 μM, and maximum denitrification rate (Vₘₐₓ) were in the range of 0.9–7.5 nmol mL⁻¹ h⁻¹. The production of N₂ through anaerobic ammonium oxidation (anammox) was generally found to be less than 10 % of denitrification. Vₘₐₓ were in the same range as previously reported in cohesive sediments despite organic carbon contents one order of magnitude lower for the sediments studied here. The ratio of sediment O₂ consumption to Vₘₐₓ was in the range of 0.02–0.09, and was on average much lower than the theoretical ratio of 0.8. As a consequence, models implemented with the theoretical ratio of 0.8 are likely to overestimate denitrification by a factor of ~3. The most likely explanation for this is that the microbial community is not able to instantaneously shift or optimally use a particular electron acceptor in the highly dynamic redox environment experienced in permeable sediments. In contrast to previous studies, we did not observe any significant rates of oxic denitrification.