Evaluation of Nitrogen‐15 Tracer Techniques for Direct Measurement of Denitrification in Soil: I. Theory

Mass spectrometric procedures for determination of dinitrogen (N 2 ) and nitrous oxide (N 2 O) evolved during denitrification in soil treated with 15 N‐labeled fertilizer are based on the hypothesis that the nitrate (NO − 3 ) undergoing denitrification can be assumed to exist in a single pool that is isotopically uniform. In the work reported, it is shown that 15 X N (the average mole fraction of 15 N in the NO − 3 undergoing denitrification) will be overestimated when this assumption is invalid. Moreover, it is proved mathematically that the amount of N 2 or N 2 O evolved is underestimated when 15 X N does not exceed 0.5. This proof does not hold for higher values of 15 X N , but it is concluded that underestimation also occurs in these cases, because no exceptions were found in over three million computer simulations involving evolution of N 2 from multiple pools having 15 N enrichments as high as 99.6 atom %.