Nitrous oxide cycling in the water column and sediments of the oxygen minimum zone, eastern subtropical North Pacific, Southern California, and Northern Mexico (23°N-34°N)

Abstract

Identifying sources and sinks of N2O can illuminate N cycling processes in marine systems, particularly where changes in dissolved O2 can lead to changes in N cycling pathways (i.e., nitrification versus denitrification). We measured N2O and concentration and their stable isotope ratios (δ15N and δ18O) in the water column and sediments of the oxygen minimum zone in the nearshore eastern subtropical North Pacific (23°N–34°N). Atmospheric efflux of N2O ranged from 2.2 to 17.9 μmol m−2 d−1 or about 2–20 times higher than in oxygenated regions of the North Pacific. Surface waters were a source of 15N-depleted and 18O-enriched N2O to the atmosphere, indicating a bacterial, not archaeal, nitrification N2O source. Stable isotopes indicated that nitrification in both surface and intermediate waters (∼0–200 m) was the major source of N2O in this study area, with denitrification acting as a small N2O sink in strongly O2-depleted waters. Denitrification had a larger impact on observed patterns of N2O and concentrations and isotope ratios in the southern oxygen minimum zone. Sediments were generally neutral or a weak sink for N2O, with only one site (Soledad basin) showing a positive efflux of +3.5 ± 1.0 μmol N2O-N m−2 d−1. Sediment fluxes of N2O at all sites were several orders of magnitude smaller than fluxes of dinitrogen, nitrate, and ammonium measured in previous studies and did not appear to impact water column N2O concentrations. N2O was less than 0.1% of the N2 efflux from sedimentary denitrification.