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

Abstract

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 in suboxic and anoxic sediments, however the magnitude of its importance appears to be linked to the presence of dissolved iron or sulfide in the pore waters.