Carbon induced subsoil denitrification of N-15 labelled nitrate in 1 m deep soil columns

Nitrate in soils is potentially a contaminant of groundwater and can also be denitrified to form nitrous oxide (N2O), a global warming gas that is also involved with stratospheric ozone depletion. The objective of this work was to examine the fate of 15N-labelled NO3− when it was injected into the subsoil (80 cm) in the presence of carbon. Gas fluxes from the soil surface and gas concentrations in the soil profile were monitored for 38 d. On average, after this time only 13% of the 15N-labelled N was present as NO3− with immobilisation (54%), entrapment in soil pore space (7%), dissolution of N in soil water (2%), gas fluxes from the soil column surface (N2O<1%, N2 1.8%) and unaccounted for 15N (20%) making up the remainder of the 15N balance. As 15N-labelled N gases diffused from the zone of denitrification towards the soil surface the ratio of N2O–N:(N2O–N+N2) decreased (range of 0.90 to 0.17).