N2 Efflux Research Articles

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

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.