The isotope effect of denitrification in permeable sediments

Natural ratios of N-15/N-14 are commonly used to help constrain marine nitrogen budgets. This requires an understanding of the isotope effect (epsilon) 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 epsilon values published for denitrification in this sediment type. We undertook controlled column experiments to quantify the cellular (epsilon(cell)) fractionation factors for N and O isotopes of nitrate in permeable sediments collected from the Danish Kattegat. Values of epsilon(cell) were 18.1 +/- 1 parts per thousand for N and 14.2 +/- 0.8 parts per thousand for O during dissimilatory nitrate reduction, which is consistent with epsilon(cell) values determined in cohesive sediments and recently published epsilon(cell) 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 (epsilon(app)). Model simulations of benthic denitrification in a typical rippled sediment at different realistic environmental conditions showed an average net epsilon(app) of 2.7 +/- 1.3 parts per thousand and 2.9 +/- 1.0 parts per thousand depending on the inclusion or exclusion of nitrification. These results are similar to ranges of epsilon(app) reported in cohesive sediments, and support recent models which balance the global nitrogen budget. (C) 2014 Elsevier Ltd. All rights reserved.