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Abstract

Coastal zones receive high inputs of anthropogenic reactive nitrogen (N R ) compounds, yet open ocean concentrations of such compounds have not increased (Galloway et al., 2004). Therefore, coastal seas buffer the oceans from N R -inputs through microbially-mediated redox transformations. Benthic denitrification to N 2 gas is the dominant N R -loss process in these environments (Middelburg and Soetaert, 2004). Nitrification is another key N R -cycling process, as it is the aerobic conversion of ammonium – a product of remineralized organic matter – into nitrate, the substrate which denitrifying organisms reduce to N 2 gas. Understanding of nitrification rates in coastal sediments is limited and mostly based on studies from muddy, diffusive sediments. However, permeable sediments cover up to 70% of the continental shelf (Emery, 1968), and significant pore-water advection takes place in many of these sediments (Huettel et al., 2014). Nitrification can be enhanced by increased solute availability in such conditions, but understanding is limited due to the paucity of direct studies on sediments under advective pore-water flows. Therefore, we performed incubations with 15 N-labeled substrates under advective conditions to determine the ammonium oxidation, nitrite oxidation, and nitrification rates in sandy permeable sediments from a subtidal and intertidal site. We observed high rates of ammonium oxidation, nitrite oxidation, and nitrification in the subtidal sediments. These rates were comparable to a previous study in the same environment that found nitrification supporting high rates of N R -loss. We observed comparatively low rates of ammonium oxidation and nitrite oxidation in the intertidal sediments, and no nitrification was quantified. We also investigated the composition and abundance of the nitrifying community in all sediments using 16S rRNA CARD-FISH. There is significant uncertainty associated with these results. Nonetheless, they suggest that Nitrospira may be responsible for high nitrite oxidation rates in highly permeable subtidal sediments in the German Bight.