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

Siegel, R. S., Hauck, R. D., Kurtz, L. T.
Soil Science Society of America journal 1982 v.46 no.1 pp. 68-74
denitrification, isotope labeling, isotopic enrichment, microbial activity, nitrogen, potassium nitrate, soil, spectrometers, stable isotopes
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.