The isotope effect of denitrification in permeable sediments

Natural ratios of 15N/14N are commonly used to help constrain marine nitrogen budgets. This requires an understanding of the isotope effect (ε) associated with nitrogen fixation and denitrification. Permeable sediments cover 70% of the continental shelf and are suggested to represent an important sink for fixed nitrogen, yet there are no ε values published for denitrification in this sediment type. We undertook controlled column experiments to quantify the cellular (εcell) fractionation factors for N and O isotopes of nitrate in permeable sediments collected from the Danish Kattegat. Values of εcell were 18.1 ± 1‰ for N and 14.2 ± 0.8‰ for O during dissimilatory nitrate reduction, which is consistent with εcell values determined in cohesive sediments and recently published εcell values for pure cultures at environmentally relevant nitrate concentrations. A diagenetic model was formulated to estimate the net transmission of this isotope effect to the overlying water under realistic advective flow (εapp). Model simulations of benthic denitrification in a typical rippled sediment at different realistic environmental conditions showed an average net εapp of 2.7 ± 1.3‰ and 2.9 ± 1.0‰ depending on the inclusion or exclusion of nitrification. These results are similar to ranges of εapp reported in cohesive sediments, and support recent models which balance the global nitrogen budget.