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Nitrogen fixation in shallow-water sediments: Spatial distribution and controlling factors

Journal article
Authors Björn Andersson
Kristina Sundbäck
Hellman Maria
Hallin Sara
Christian Alsterberg
Published in Limnology and Oceanography
Volume 59
Issue 6
Pages 1932-1944
ISSN 0024-3590
Publication year 2014
Published at Department of Biological and Environmental Sciences
Pages 1932-1944
Language en
Keywords sediment, kväve, nifH, nitrogenas, marin
Subject categories Biological Sciences


Nitrogenase activity (NA) in shallow-water (< 1 m) sediments was investigated at 60 randomly selected sites along a 150 km stretch on the brackish-water Swedish west coast, without targeting any specific type of sediments, such as microbial mats. Benthic nitrogen (N) fixation and diazotrophs (nifH genes) were found at all sites, regardless of the presence of cyanobacterial or microbial mats. The majority of sites showed N fixation rates between 0.03 and 1 mmol N m−2 d−1. These rates were similar to those of benthic denitrification previously measured in the area. Maximum rates up to 3.4 mmol N m−2 d−1 were measured. A structural equation model was used to investigate direct and indirect effects of biogeochemical and physical factors on NA. Number of nifH genes had the largest direct positive influence on NA, whereas increasing wave exposure had an indirect negative effect on NA through its influence on the diazotrophic abundance. Increased salinity, previously been shown to suppress NA in coastal waters, was found to directly stimulate benthic N fixation, likely by generating favorable conditions for diazotrophic sulfate-reducing bacteria. Our field data confirmed previously observed negative effects of dissolved inorganic nitrogen on NA, which have so far mainly been experimentally studied. Both NA rates and the number of nifH genes correlated positively with pore-water dissolved inorganic phosphorus concentrations. These findings show that the potential for N fixation in illuminated sediments can be considerable, stretching beyond cyanobacterial mats, being controlled by complex interactions between biotic and abiotic factors.

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