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

Abstract

Abstract A method for the precise determination of the 30 N 2 /( 28 N 2 + 29 N 2 ) 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 15 N‐labeled KNO 3 (63.6 atom % 15 N) allowed direct determination of rates of N 2 efflux in the field as low as 2.5 g N 2 ha ‐1 day ‐1 during denitrification. Rates as high as 250 g ha ‐1 day ‐1 were also observed. Measurement of the 29 N 2 / 28 N 2 ratio in conjunction with the 30 N 2 /( 28 N 2 + 29 N 2 ) ratio in atmospheric samples containing evolved N 2 gas allowed calculation of the average 15 N 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 29 N 2 / 28 N 2 and 30 N 2 /( 28 N 2 + 29 N 2 ) ratios for the calculation of the 15 N content of the NO ‐ 3 undergoing denitrification. The laboratory studies showed little effect of mole fraction 15 N in isotopically labeled NO ‐ 3 on the rate of N 2 efflux from flooded soils, indicating microbial activity was not influenced by high concentrations of 15 N.