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)).