Denitrification in intact subsoil cores

Intact, unamended subsoil cores taken from two contrasting field sites were incubated in the laboratory at 12 degrees 0 under aerobic (21% O-2) and anaerobic (1.1-5.5% O-2) conditions. Denitrification of N-15-NO3- diffusing into the cores across a semi-permeable membrane was estimated by: (1) direct observation of N-15- labelled N-2 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 O-2 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 N-2, rather than the greenhouse gas and catalyst of stratospheric O-3 removal, N2O.