In Situ Quantification of Biological N(2) Production Using Naturally Occurring (15)N(15)N

[Image: see text] We describe an approach for determining biological N(2) production in soils based on the proportions of naturally occurring (15)N(15)N in N(2). Laboratory incubation experiments reveal that biological N(2) production, whether by denitrification or anaerobic ammonia oxidation, yields proportions of (15)N(15)N in N(2) that are within 1‰ of that predicted for a random distribution of (15)N and (14)N atoms. This relatively invariant isotopic signature contrasts with that of the atmosphere, which has (15)N(15)N proportions in excess of the random distribution by 19.1 ± 0.1‰. Depth profiles of gases in agricultural soils from the Kellogg Biological Station Long-Term Ecological Research site show biological N(2) accumulation that accounts for up to 1.6% of the soil N(2). One-dimensional reaction-diffusion modeling of these soil profiles suggests that subsurface N(2) pulses leading to surface emission rates as low as 0.3 mmol N(2) m(–2) d(–1) can be detected with current analytical precision, decoupled from N(2)O production.