Determination of 30N2 and Application to Measurement of N2 Evolution During Denitrification
1982
Siegel, R. S. | Hauck, R. D. | Kurtz, L. T.
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 × 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.
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