Influence of organic carbon and nitrate loading on partitioning between dissimilatory nitrate reduction to ammonium (DNRA) and N2 production

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 15N 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 15NO2− + 14NH4+ indicated significant denitrification and DNRA activity in sediment thin discs, but incubations with added 15NH4+ + 14NO2− 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.