In Situ Quantification of Biological N2 Production Using Naturally Occurring 15N15N

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.