Role of nitrification and denitrification on the nitrous oxide cycle in the eastern tropical North Pacific and Gulf of California

Abstract

Nitrous oxide (N2O) is an important atmospheric greenhouse gas and is involved in stratospheric ozone depletion. Analysis of the isotopomer ratios of N2O (i.e., the intramolecular distribution of 15N within the linear NNO molecule and the conventional N and O isotope ratios) can elucidate the mechanisms of N2O production and destruction. We analyzed the isotopomer ratios of dissolved N2O at a site in the eastern tropical North Pacific (ETNP) and a site in the Gulf of California (GOC). At these sites, the flux of N2O to the atmosphere is extremely high but denitrification activity in the oxygen minimum zone (OMZ) also reduces N2O to N2. We estimated the isotopomeric enrichment factors for N2O reduction by denitrification. The factor was −11.6 ± 1.0‰ for the bulk (average) N, −19.8 ± 2.3‰ for the center N (α‐site nitrogen), −3.4 ± 0.3‰ for the end N (β‐site nitrogen), and −30.5 ± 3.2‰ for the 18O of N2O. Isotopomer analysis of N2O suggests that nitrifiers should contribute to N2O production more than denitrifiers at the oxycline above the OMZs in the ETNP (50–80 m) and in the GOC (80–300 m). In contrast, denitrifiers should largely contribute to the N2O production and consumption in the OMZs both in the ETNP (120–130 m) and in the GOC (600–800 m). The N2O isotopomer analysis will be a useful tool for resolving the distribution of water masses that carry a signal of N loss by denitrification.