Natural 15N15N abundances constrain fixed nitrogen loss.

Nitrogen regulates primary productivity across much of the biosphere, yet fixed nitrogen loss remains poorly constrained because existing methods rely on indirect proxies or ex situ experiments. In this study, we show that natural abundances of the rare 15N15N isotopologue of dinitrogen (N2) provide a direct tracer of biological N2 production across diverse aquatic environments. N2 produced by denitrification and anammox has a near-stochastic 15N15N distribution [0 per mil (‰)], whereas atmospheric N2 carries a distinct 15N15N excess (19‰), allowing the two sources to be quantitatively distinguished. Across aquifers, stratified lakes, coastal basins, oxygen minimum zones, and marine sediments, 15N15N measurements reveal widespread nitrogen loss previously obscured by physical gas accumulation and nitrogen fixation. Natural 15N15N abundances therefore provide a general framework for directly constraining fixed nitrogen loss across the aquatic nitrogen cycle.