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

Abstract

Nitrous oxide (N 2 O) is an important atmospheric greenhouse gas and is involved in stratospheric ozone depletion. Analysis of the isotopomer ratios of N 2 O (i.e., the intramolecular distribution of 15 N within the linear NNO molecule and the conventional N and O isotope ratios) can elucidate the mechanisms of N 2 O production and destruction. We analyzed the isotopomer ratios of dissolved N 2 O 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 N 2 O to the atmosphere is extremely high but denitrification activity in the oxygen minimum zone (OMZ) also reduces N 2 O to N 2 . We estimated the isotopomeric enrichment factors for N 2 O 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 18 O of N 2 O. Isotopomer analysis of N 2 O suggests that nitrifiers should contribute to N 2 O 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 N 2 O production and consumption in the OMZs both in the ETNP (120–130 m) and in the GOC (600–800 m). The N 2 O isotopomer analysis will be a useful tool for resolving the distribution of water masses that carry a signal of N loss by denitrification.