Journal cover Journal topic
Biogeosciences An interactive open-access journal of the European Geosciences Union
Biogeosciences, 14, 285-300, 2017
http://www.biogeosciences.net/14/285/2017/
doi:10.5194/bg-14-285-2017
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
19 Jan 2017
The fate of fixed nitrogen in marine sediments with low organic loading: an in situ study
Stefano Bonaglia1,a, Astrid Hylén2, Jayne E. Rattray1, Mikhail Y. Kononets2, Nils Ekeroth2,3, Per Roos4, Bo Thamdrup5, Volker Brüchert1, and Per O. J. Hall2 1Department of Geological Sciences and Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden
2Department of Marine Sciences, University of Gothenburg, Gothenburg, Sweden
3Calluna AB, Nacka, Sweden
4Center for Nuclear Technologies, Technical University of Denmark, Roskilde, Denmark
5Department of Biology and Nordic Center for Earth Evolution, University of Southern Denmark, Odense M, Denmark
acurrent address: Department of Geology, Lund University, Lund, Sweden
Abstract. Over the last decades, the impact of human activities on the global nitrogen (N) cycle has drastically increased. Consequently, benthic N cycling has mainly been studied in anthropogenically impacted estuaries and coasts, while in oligotrophic systems its understanding is still scarce. Here we report on benthic solute fluxes and on rates of denitrification, anammox, and dissimilatory nitrate reduction to ammonium (DNRA) studied by in situ incubations with benthic chamber landers during two cruises to the Gulf of Bothnia (GOB), a cold, oligotrophic basin located in the northern part of the Baltic Sea. Rates of N burial were also inferred to investigate the fate of fixed N in these sediments. Most of the total dissolved fixed nitrogen (TDN) diffusing to the water column was composed of organic N. Average rates of dinitrogen (N2) production by denitrification and anammox (range: 53–360 µmol N m−2 day−1) were comparable to those from Arctic and subarctic sediments worldwide (range: 34–344 µmol N m−2 day−1). Anammox accounted for 18–26 % of the total N2 production. Absence of free hydrogen sulfide and low concentrations of dissolved iron in sediment pore water suggested that denitrification and DNRA were driven by organic matter oxidation rather than chemolithotrophy. DNRA was as important as denitrification at a shallow, coastal station situated in the northern Bothnian Bay. At this pristine and fully oxygenated site, ammonium regeneration through DNRA contributed more than one-third to the TDN efflux and accounted, on average, for 45 % of total nitrate reduction. At the offshore stations, the proportion of DNRA in relation to denitrification was lower (0–16 % of total nitrate reduction). Median value and range of benthic DNRA rates from the GOB were comparable to those from the southern and central eutrophic Baltic Sea and other temperate estuaries and coasts in Europe. Therefore, our results contrast with the view that DNRA is negligible in cold and well-oxygenated sediments with low organic carbon loading. However, the mechanisms behind the variability in DNRA rates between our sites were not resolved. The GOB sediments were a major source (237 kt yr−1, which corresponds to 184 % of the external N load) of fixed N to the water column through recycling mechanisms. To our knowledge, our study is the first to document the simultaneous contribution of denitrification, DNRA, anammox, and TDN recycling combined with in situ measurements.

Citation: Bonaglia, S., Hylén, A., Rattray, J. E., Kononets, M. Y., Ekeroth, N., Roos, P., Thamdrup, B., Brüchert, V., and Hall, P. O. J.: The fate of fixed nitrogen in marine sediments with low organic loading: an in situ study, Biogeosciences, 14, 285-300, doi:10.5194/bg-14-285-2017, 2017.
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Short summary
Understanding nitrogen (N) cycling mechanisms in the ocean is crucial for improving ecosystem management. Here we study N processes by in situ lander and isotope tracer techniques in – so far overlooked – sediments with low organic loads. Denitrification and anammox are the main N transformation processes. However, we demonstrate high contribution of dissimilatory nitrate reduction to ammonium, which recycles a major portion of fixed N to the water column and sustains primary production.
Understanding nitrogen (N) cycling mechanisms in the ocean is crucial for improving ecosystem...
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