The fate of fixed nitrogen in marine sediments with low organic loading: An in situ study

Research output: Contribution to journalArticle

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: 53-360gμNg-2g dayg-1) were comparable to those from Arctic and subarctic sediments worldwide (range: 34-344gμNg mg-2g dayg-1). Anammox accounted for 18-26g % 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 45g % of total nitrate reduction. At the offshore stations, the proportion of DNRA in relation to denitrification was lower (0-16g % 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 (237g ktg yrg-1, which corresponds to 184g% 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.

Details

Authors
  • Stefano Bonaglia
  • Astrid Hylén
  • Jayne E. Rattray
  • Mikhail Y. Kononets
  • Nils Ekeroth
  • Per Roos
  • Bo Thamdrup
  • Volker Brüchert
  • Per O. J. Hall
Organisations
External organisations
  • Stockholm University
  • University of Gothenburg
  • Technical University of Denmark
  • University of Southern Denmark
  • Calluna AB
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Geology
Original languageEnglish
Pages (from-to)285-300
Number of pages16
JournalBiogeosciences
Volume14
Issue number2
Publication statusPublished - 2017 Jan 19
Publication categoryResearch
Peer-reviewedYes