Nitrogen cycling within suboxic and anoxic sediments from the continental margin of Western North America

Here we report rates of benthic nitrogen (N) cycling and assess controls on biological NO 3 − sequestration and transport in sediments underlying oxygen deficient regions of the ocean ranging from anoxic/suboxic ([O 2] of 0–2 μM) to more oxic (57 μM [O 2]) conditions. N mass balances were constructed based on benthic fluxes (N 2, NH 4 + and NO 3 −) and pore water profiles (N 2, NO 3 −, NO 2, N 2O, Fe and HS −) at sites in the Southern California Borderland and the Mexican Margin. Fluxes across the sediment-water interface for N 2, NO 3 − and NH 4 + were determined directly by whole core incubations, and fluxes of N 2 were also determined by modeling mm-scale pore water profiles. Estimates of the N 2 flux by these two methods agree to ± 50%, thereby establishing a range of net N 2 production in these settings. The average N 2 efflux was four times larger at a site with high pore water H 2S concentrations (Soledad Basin 3.14 ± 1.10 mmol N m − 2 day − 1 ) compared to an iron-rich site (Santa Monica Basin 0.74 ± 0.21 mmol N m − 2 day − 1 ) despite both sites having comparable NO 3 − uptake fluxes (− 0.93 ± 0.14 vs. − 0.82 ± 0.08 mmol N m − 2 day − 1 respectively). Pore water profiles from both sulfidic and iron-rich sites reveal subsurface maxima in NO 3 −, NO 2 −, and N 2O that are likely caused by the presence of NO 3 − sequestered by infaunal microbiota. In Soledad Basin, the sequestered pool of microbial NO 3 − contributes to NH 4 + production via DNRA resulting in an NH 4 + efflux (2.66 ± 0.52 mmol N m − 2 day − 1 ) to the overlying water. This flux exceeds the rate of NH 4 + production by C org degradation by 10 times. At the suboxic sites, a total N balance can only be achieved if the flux of NO 3 − into sediments is composed of two components: diffusive and biologically mediated transport. The more oxic site shows no evidence of a sequestered microbial NO 3 − pool and diffusive fluxes can account for all N transformations. Core incubations do not capture the total amount of NO 3 − uptake where biological transport is important as they do not account for NO 3 − sequestered prior to the start of the incubation. Pore water N 2O concentrations of up to 500 nM in sub-surface sediments greatly exceed the background concentration (7 nM) and are likely generated by the metabolic reduction of the intracellular nitrate pool, however, there was no measurable efflux of N 2O from sediments to the overlying water. Biological NO 3 − transport is a ubiquitous process