Influence of organic carbon and nitrate loading on partitioning between dissimilatory nitrate reduction to ammonium (DNRA) and N2 production. (1st September 2015)

• Record Type: Journal Article
• Authors: Hardison, Amber K.; Algar, Christopher K.; Giblin, Anne E.; Rich, Jeremy J.
• Is Part Of: Geochimica et cosmochimica acta. Volume 164(2015:Sep. 01)
• Page Start: 146
• Page End: 160
• Publication Date: 2015-09-01
• Subjects: Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2015.04.049
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Biologically available nitrogen is removed from ecosystems through the microbial processes of anaerobic ammonium oxidation (anammox) or denitrification, while dissimilatory nitrate reduction to ammonium (DNRA) retains it. A mechanistic understanding of controls on partitioning among these pathways is currently lacking. The objective of this study was to conduct a manipulative experiment to determine the influence of organic C and NO3 − loading on partitioning. Sediment was collected from a location on the southern New England shelf (78 m water depth) and sieved. Half of the sediment was mixed with freeze-dried phytoplankton and the other half was not. Sediment was then spread into 1.5 mm, "thin discs" closed at the bottom and placed in large aquarium tanks with filtered, N2 /CO2 sparged seawater to maintain O2 limited conditions. Half of the discs received high NO3 − loading, while the other half received low NO3 − loading, resulting in a multifactorial design with four treatments: no C addition, low NO3 − (−C−N); C addition, low NO3 − (+C−N); no C addition, high NO3 − (−C+N); and C addition, high NO3 − (+C+N). Sediment discs were incubated in the tanks for 7 weeks, during which time inorganic N (NH4 +, NO3 −, and NO2 − ) was monitored, and sediment discs were periodically removed from the tanks to conduct 15 N isotope labeling experiments in vials to measure potential rates of anammox, denitrification, and DNRA. Temporal dynamics of inorganic N concentrations in the tanks were indicative of anoxic N metabolism, with strong response of the build up or consumption of the intermediate NO2 −, depending on treatments. Vial incubation experiments with added 15 NO2 − + 14 NH4 + indicated significant denitrification and DNRA activity in sediment thin discs, but incubations with added 15 NH4 + + 14 NO2 − indicated anammox was not at all significant. Inorganic N concentrations in the tanks were fit to a reactive transport model assuming different N transformations. Organic C decomposition rates were inferred based on modeled rates as well as stoichiometric conversions of NH4 + production in pre-incubated vials. Based on model results, partitioning between DNRA and N2 production was positively linearly related to the ratio of C decomposition to NO3 − reduction rates (C/NO3 − ) but not C decomposition alone. Based on vial results, partitioning was significantly related to C decomposition. Overall, this study supports the hypothesis that high organic C loading is a prerequisite for DNRA to be favored over denitrification but that N2 production may still be significant when organic C is high depending on NO3 − availability.