In Situ Quantification of Biological N Production Using Naturally Occurring NN.

We describe an approach for determining biological N production in soils based on the proportions of naturally occurring NN in N. Laboratory incubation experiments reveal that biological N production, whether by denitrification or anaerobic ammonia oxidation, yields proportions of NN in N that are within 1‰ of that predicted for a random distribution of N and N atoms. This relatively invariant isotopic signature contrasts with that of the atmosphere, which has NN 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 accumulation that accounts for up to 1.6% of the soil N. One-dimensional reaction-diffusion modeling of these soil profiles suggests that subsurface N pulses leading to surface emission rates as low as 0.3 mmol N m d can be detected with current analytical precision, decoupled from NO production.