Development of a helium atmosphere soil incubation technique for direct measurement of nitrous oxide and dinitrogen fluxes during denitrification

A technique is described in which the upper surfaces of intact soil cores are enveloped in a flowing atmosphere of He and O2 after first purging the soil and incubation vessel free from N2. This allows the independent measurement of N2O and N2 fluxes during denitrification of added or indigenous NO3−N by direct flushing to twin gas chromatographs and without recourse to acetylene blocking. Square section cores are extracted from random locations in the field and assembled without air gaps to make composite turves in the incubation vessel, thus preserving field aerobicity and orientation but allowing the spatial variability in denitrification to be accommodated. An N2-free irrigation assembly attached to each incubation vessel can be used to apply substrates during an experimental run, which is conducted in a temperature-controlled room. Use of the technique is demonstrated with measurements of N2O and N2 efflux from a wet, fine-textured soil under grassland management amended with nitrate and glucose. Peak concentrations were registered earlier than with previously-reported incubation techniques, with the flow rate of the incubation atmosphere having a substantial influence on the N2O to N2 ratio. Inclusion of acetylene as a component of the gas flow mixture stimulated denitrification and did not block N2 production completely. Application of the technique is limited by the extent to which atmospheric N2 contamination can be reduced and ultimately by the sensitivity of the gas chromatograph. The system in its present form has a detection limit for N2 from denitrification of about 50 g N ha−1 d−1 and is therefore most suitably applied to soils under productive agricultural management.