Intensive and extensive nitrogen loss from intertidal permeable sediments of the Wadden Sea

Nitrogen (N) loss rates were determined in permeable sediments of the Wadden Sea using a combination of stable N isotope incubation experiments and model simulation approaches during three seasons. Three different incubation methods that employed the isotope pairing technique were used: intact core incubations simulating either (1) diffusive or (2) advective transport conditions and (3) slurry incubations. N loss rates from core incubations under simulated advective transport conditions exceeded those rates measured under diffusive transport conditions by 1-2 orders of magnitude, but were comparable to those observed in slurry incubations. N loss rates generally showed little seasonal and spatial variation (207 ± 30 μmol m^-2 h^-1) in autumn 2006 and spring and summer 2007. Utilizing an extensive time series of nutrient concentrations and current velocities obtained from a continuous monitoring station, nitrate and nitrite (i.e., NO^-_x) flux into the sediment was modeled over a full annual cycle. Fluxes were sufficient to support the experimentally derived N loss rates. Combining the measured rates with the modeled results, an annual N removal rate of 745 ± 109 mmol N m^-2 yr^-1 was estimated for permeable sediments of the Wadden Sea. This rate agrees well with previous N loss estimates for the Wadden Sea based on N budget calculations. Permeable sediments, accounting for 58-70% of the continental shelf area, are an important N sink and their contribution to the global N loss budget should be reevaluated.