Denitrification in a clay soil under pasture and annual crop: Losses from super(15)N-labelled nitrate in the subsoil in the field using C sub(2)H sub(2) inhibition

Inhibition of the reduction of nitrous oxide (N sub(2)O) to dinitrogen (N sub(2)) by acetylene (C sub(2)H sub(2)) provides a method for measuring denitrification in the presence of atmospheric N sub(2). Our objective was to measure N sub(2)O losses, using C sub(2)H sub(2) inhibition, from super(15)N-labelled fertilizer added to the subsoil in the field. N sub(2)O concentrations in a clay soil were measured beneath a cultivated crop, black gram (Vigna mungo)), and a permanent pasture, green panic (Panicum maximum var trichoglume ). Three treatments 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 C sub(2)H sub(2); and (c) water plus C sub(2)H sub(2) plus glucose. At the same time, KNO sub(3) (98 atom % super(15)N excess) was applied with all treatments at a rate equal to 100 kg N/ha at each depth. The soil atmosphere at each depth was sampled at 0 and 6 h, and 1, 3, 7 and 14 days after the application of the treatments. At depths of 5-10 and 20-25 cm, none of the treatments resulted in N sub(2)O or super(15)N-N sub(2)O rising above ambient concentrations. At 50-55 cm, N sub(2)O production occurred beneath black gram, with rapid production occurring above a threshold CO sub(2) production of 26.5 mg CO sub(2)/kg soil. At 110-115 cm, significant N sub(2)O and super(15)N-N sub(2)O production occurred from under both systems between 7 and 14 days after application of the treatments, but this did not appear to be influenced by CO sub(2) concentration. In these experiments, conducted at close to field capacity, carbon substrate, in the form of glucose, was necessary for N sub(2)O production to occur.