Denitrification in intact subsoil cores

Abstract

Intact, unamended subsoil cores taken from two contrasting field sites were incubated in the laboratory at 12 °C under aerobic (21% O2) and anaerobic (1.1–5.5% O2) conditions. Denitrification of 15N-NO3 – diffusing into the cores across a semi-permeable membrane was estimated by: (1) direct observation of 15N-labelled N2 and N2O, and (2) mass balance following sectioning at the end of the experiment. The in situ denitrification potential (rates where the supply of NO3 – is not limited by diffusion) was estimated using a finite-difference approximation to a diffusion reaction equation based on comparison of time and depth profiles of NO3 – and Br– in the incubated cores. Potentials between 0.2 and 2.5 mg N kg–1 day–1 were obtained under low O2 and natural C concentrations. These results indicate the potential for denitrification in glacial till subsoils to reduce NO3 – leaching to ground or surface waters to levels unlikely to result in a pollution hazard. The major product of NO3 – reduction in these subsoils was observed to be N2, rather than the greenhouse gas and catalyst of stratospheric O3 removal, N2O.