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

We describe an approach for determining biological N₂ production in soils based on the proportions of naturally occurring ¹⁵N¹⁵N in N₂. Laboratory incubation experiments reveal that biological N₂ production, whether by denitrification or anaerobic ammonia oxidation, yields proportions of ¹⁵N¹⁵N in N₂ that are within 1‰ of that predicted for a random distribution of ¹⁵N and ¹⁴N atoms. This relatively invariant isotopic signature contrasts with that of the atmosphere, which has ¹⁵N¹⁵N 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 N₂ accumulation that accounts for up to 1.6% of the soil N₂. One-dimensional reaction-diffusion modeling of these soil profiles suggests that subsurface N₂ pulses leading to surface emission rates as low as 0.3 mmol N₂ m^-2 d^-1 can be detected with current analytical precision, decoupled from N₂O production.