Direct measurement of dinitrogen evolution from soil using nitrogen-15 emission spectrometry

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 curvett and a sampling system was developed. The sampling system was evacuated to <10-1 Pa before gas from the curvett system was fed into a reservoir, through a liquid N2 trap and into five discharge tubes for ES. The final pressure in the tubes was 4 to 5 x 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 atoms % 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