The PIs will conduct a study on the cycling of nitrous oxide (N2O) in the surface ocean. The results of this study would have broad impact beyond oceanic nitrogen cycling, as N2O plays a significant role in the reduction of ozone in the stratosphere and is itself an important greenhouse gas. The major goal of this study is to identify, using a combination of laboratory and field experiments, the biogeochemical conditions under which N2O is produced. More specifically, the PIs propose to 1) measure N and O2 isotopic compositions of N2O at natural abundance levels, 2) perform 15N tracer experiments to monitor N2O production via nitrification and denitrification pathways, and 3) measure the isotopic composition of the potential N and O sources of N2O. Field experiments at the Hawaii Ocean Time-series site (HOT) and on a cruise in the North Pacific are proposed.