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.