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Determination of ³⁰N₂ and Application to Measurement of N₂ Evolution During Denitrification¹

ABSTRACT

A method for the precise determination of the ³⁰N₂/(²⁸N₂ + ²⁹N₂) ratio with a dual-inlet ratio mass spectrometer is proposed. Precise measurements of this ratio (SE = 2.4 x 10^-7) performed on atmospheric samples taken from enclosed chambers above soils fertilized with highly enriched ¹⁵N-labeled KNO₃ (63.6 atom % ¹⁵N) allowed direct determination of rates of N₂ efflux in the field as low as 2.5 g N₂ ha^-1 day^-1 during denitrification. Rates as high as 250 g ha^-1 day^-1 were also observed. Measurement of the ²⁹N₂/²⁸N₂ ratio in conjunction with the ³⁰N₂/(²⁸N₂ + ²⁹N₂) ratio in atmospheric samples containing evolved N₂ gas allowed calculation of the average ¹⁵N enrichment of the NO^-₃ pool undergoing denitrification in the soil without destructive sampling. Isotopic enrichment of the NO^-₃ pool in one plot was reduced by one-third after 28 days. Laboratory studies verified the use of ²⁹N₂/²⁸N₂ and ³⁰N₂/(²⁸N₂ + ²⁹N₂) ratios for the calculation of the ¹⁵N content of the NO^-₃ undergoing denitrification. The laboratory studies showed little effect of mole fraction ¹⁵N in isotopically labeled NO^-₃ on the rate of N₂ efflux from flooded soils, indicating microbial activity was not influenced by high concentrations of ¹⁵N.

Key Words: ¹⁵N • N₂ purification • dinitrogen • oxygen removal

¹ Contribution from Dep. of Agronomy, Univ. of Illinois, Urbana, 61801 in cooperation with the Nat. Fertilizer Center, TVA, Muscle Shoals, AL 35660.

² Research Associate, Univ. of Illinois; Soil Scientist, TVA; and Professor, Univ. of Illinois, respectively.

Received for publication May 4, 1981. Accepted for publication August 27, 1981.

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