a limiting element for biological productivity, nitrogen plays a central role in marine microbial ecosystems. The microbial cycling of fixed nitrogen in the ocean has a profound impact on the broader global nitrogen budget. The mechanisms are well known – nitrogen fixation converts inert N2 into ammonia, while denitrification and anaerobic ammonia oxidation (anammox) return fixed nitrogen to N2 – but the oceanic nitrogen fluxes due to these processes are still uncertain. As a result, the balance of fixed nitrogen sources and sinks is debated. Recent advances in mass spectrometry have enabled access to measure proportions of naturally occurring 15N15N in N2. Atmospheric N2 has a substantial 15N15N disequilibrium enrichment of 19 ‰ relative to equilibrium, while microbial N2 production yields 15N15N abundance in isotope equilibrium. We hypothesize that naturally occurring 15N15N in N2 can be used to unambiguously quantify in-situ microbial N2 production in oceanic waters. As a proof-of-principle application, we will extract and analyze N2 from waters in two typical oxygen minimum zones located offshore Southern California (USA) and offshore eastern Canada. We will apply an advection-diffusion-reaction model to quantify in-situ microbial N2 production within pelagic and benthic ecosystems. These results will be compared to those from conventional methods, such as the 15N isotope pairing technique and stoichiometric relationships of nutrients. The results of this project will provide new insights into oceanic and global nitrogen budgets.