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Direct Measurement of Dinitrogen Evolution from Soil Using Nitrogen-15 Emission Spectrometry¹

ABSTRACT

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 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 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.

¹ Contribution from the Phytotron, Univ. of Oslo, P.O. Box 1066, Blindern, 0316 Oslo 3, Norway.

² Plant Physiologists (all), the Phytotron, Univ. of Oslo, P.O. Box 1066, Blindern, 0316 Oslo 3, Norway.

Received for publication December 5, 1986.