NEW FORMULAS FOR MASS-SPECTROMETRIC ANALYSIS OF NITROUS-OXIDE AND DINITROGEN EMISSIONS

Formulae currently used in mass spectrometric studies of N(2) and N2O emissions require the initial atmosphere to be in a state of isotopic equilibrium with a N-15 atom fraction equal to that of the global background (gamma). This is not always the case (e.g., in sequentially sampled incubations and those conducted under artificial atmospheres). Here I present new formulae that are valid whatever the initial conditions. These formulae are simpler than previous versions. They are expressed directly in terms of the N-15 atom fractions and N-30(2) molecular fractions of two gas samples (taken before and after emission). I have provided secondary expressions for converting the instrumental output of any (single- or dual-inlet, double- or triple-collector) mass spectrometer into the atomic and molecular fractions required. In common with previous treatments, my new calculations hypothesize a homogeneous soil N pool. I have proven that where this hypothesis is false the calculated value of the apparent pool enrichment is wrong (too high where the actual enrichment exceeds that of the atmosphere; too low in the exceptional case where the atmospheric enrichment exceeds that of the pool) and the flux is invariably underestimated. Computer simulations suggest that this underestimation may be relatively constant (about 25%) if the atmosphere is initially at isotopic equilibrium with N-15 fraction gamma, but that it may be larger (about 40%) and more variable where this is not the case.