DENITRIFICATION IN A CLAY SOIL UNDER PASTURE AND ANNUAL CROP - LOSSESFROM 15N-LABELED NITRATE IN THE SUBSOIL IN THE FIELD USING C2H2 INHIBITION

Inhibition of the reduction of nitrous oxide (N2O) to dinitrogen (N2) by acetylene (C2H2) provides a method for measuring denitrification in the presence of atmospheric N2. Our objective was to measure N2O loss es, using C2H2 inhibition, from N-15-labelled fertilizer added to the subsoil in the field. N2O concentrations in a clay soil were measured beneath a cultivated crop, black gram (Vigna mungo), and a permanent p asture, green panic (Panicum maximum var trichoglume), Three treatment s were applied by injecting through stainless steel tubes to depths of 7.5, 22.5, 52.5 and 112.5 cm: (a) water; (b) water PlUS C2H2; and (c) water PlUS C2H2 plus glucose. At the same time, KNO3 (98 atom % N-15 excess) was applied with all treatments at a rate equal to 100 kg N ha -1 at each depth. The soil atmosphere at each depth was sampled at 0 a nd 6 h, and 1, 3, 7 and 14 days after the application of the treatment s. At depths of 5-10 and 20-25 cm, none of the treatments resulted in N2O or N-15-N2O rising above ambient concentrations. At 50-55 cm, N2O production occurred beneath black gram, with rapid production occurrin g above a threshold CO2 production of 26.5 mg CO2 kg-1 soil. At 110-11 5 cm, significant N2O and N-15-N2O Production occurred from under both systems between 7 and 14 days after application of the treatments, bu t this did not appear to be influenced by CO2 concentration. In these experiments, conducted at close to field capacity. carbon substrate, i n the form of glucose, was necessary for N2O production to occur.