Natural 15 N 15 N 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 15 N 15 N isotopologue of dinitrogen (N 2 ) provide a direct tracer of biological N 2 production across diverse aquatic environments. N 2 produced by denitrification and anammox has a near-stochastic 15 N 15 N distribution [0 per mil (‰)],
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 15 N 15 N isotopologue of dinitrogen (N 2 ) provide a direct tracer of biological N 2 production across diverse aquatic environments. N 2 produced by denitrification and anammox has a near-stochastic 15 N 15 N distribution [0 per mil (‰)], whereas atmospheric N 2 carries a distinct 15 N 15 N excess (19‰), allowing the two sources to be quantitatively distinguished. Across aquifers, stratified lakes, coastal basins, oxygen minimum zones, and marine sediments, 15 N 15 N measurements reveal widespread nitrogen loss previously obscured by physical gas accumulation and nitrogen fixation. Natural 15 N 15 N abundances therefore provide a general framework for directly constraining fixed nitrogen loss across the aquatic nitrogen cycle.
