Measurement of gaseous emissions from denitrification of applied N-15 .3. Field-measurements

Abstract

Field experiments were conducted during autumn and winter (April-July) at four locations on Vertisol or Alfisol soils on the Darling Downs of Queensland in 1988 and 1989 to determine 15N losses when soil was saturated after applications of 15N labelled nitrate-N prior to sowing winter cereal crops. Losses of applied 15N were quantified by either gas emission or mass balance measurements on microplots (0.043 m2) confined laterally to a depth of 110 or 260 mm. At each field location, two experiments were established, one on a soil containing little visible crop residue where winter cereal had been harvested the previous November and another site containing residues of a recently harvested sorghum crop. Because shallow (110 mm) confinement was found to be unsatisfactory for both gas emission and mass balance measurement of 15N losses, comparison of the two methods was not applicable at one of the four field locations. Loss estimates for the six field sites by accumulating daily gas emissions averaged 80.7 � 33.4% (range 43-132%) of that estimated by mass balance. Loss estimates from peak emission measurements were generally closer to that estimated by mass balance 100.8� 39.9% (range 56-169%). Loss of applied 15N (40 kg N ha-1) when soils were saturated in April was several-fold more (19-29 kg N ha-1)) than that lost when soils were saturated in July (3.9-6.4 kg N ha-1)). Loss of 15N following saturation during July 1988 was similar in magnitude to the quantity of 15N apparently immobilized into soil organic forms (5.8-6.0 kg N ha-1)). Sorghum residues returned in March, or wheat straw added in December prior to a long period of dry weather, promoted loss of 15N applied prior to soil saturation in April or July. Alternatively, where residues of a previous winter cereal had decomposed considerably, loss of applied 15N was much lower than where sorghum residues had been added prior to saturations in April (15.3 cf. 28.6 kg N ha-1)) or July (3.9 cf. 6.4 kg N ha-1)).