SummaryWetland rice was cultivated in pots of puddled soil under continuous and intermittent flooding conditions. The soil was either fertilized with the surface application of prilled urea in three split doses or once with urea supergranules applied at different soil levels.The grain yield, fertilizer efficiency and percent nitrogen recovery by the grains were increased by deep placement of urea supergranules independent of the water regime. Grain yield was always lower with intermittent flooding, particularly when the plants were fertilized with the surface application of prilled urea.Nitrogen loss by ammonia volatilization, measured in a closed cuvette system, was reduced from 24% with the surface application of urea prills and 20% with surface application of urea supergranules to approximately 2% with deep placement of urea supergranules. Intermittent flooding created conditions which promoted additional nitrogen loss by nitrification and denitrification processes. The total nitrogen loss, measured in an open cuvette system, was about 38% with the surface application of urea supergranules, whereas this loss was reduced to 10% with deep placement of urea supergranules. Furthermore, deep placement of urea fertilizer reduced the nitrogen loss irrespective of water regime.
When a 15N‐enriched N source is added to the soil, denitrification results in a nonrandom distribution of N2 isotopes. As a result, it has been necessary to apply highly enriched 15N‐fertilizers to allow mass spectrometry (MS) determinations of 28N2, 29N2, and 30N2 to quantify the N2 evolution. An alternative method, based on the determination of N2 evolution by measurement of the 28N2/29N2 ratio with emission spectrometry (ES), is proposed. An apparatus consisting of a cuvette and a sampling system was developed. The sampling system was evacuated to <10−1 Pa before gas from the cuvette system was fed into a reservoir (10 mL), through a liquid N2 trap and into five discharge tubes for ES. The final pressure in the tubes was 4 to 5 × 102 Pa before they were sealed off, and the 28N2/29N2 ratios in the gases were determined with ES. It was verified that random distribution of 28N2, 29N2, and 30N2 was obtained in different N2‐gas mixtures made from natural atmosphere and 98 atom % 15N‐N2 by the high frequency discharge of the ES. The system was used to measure fertilizer N2 loss when 15N‐enriched KNO3 was applied at a depth of 2.5 cm in flooded soil. After 60 d, 18% of the applied KNO3 was lost as N2.
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