A Heuristic Model for the Calculation of Dinitogen and Nitrous Oxide Flux from Nitrogen‐15‐Labeled Soil

Very sensitive measurements of N 2 and N 2 O flux from soil are possible when gas evolved from 15 N‐labeled soil is analyzed by isotope ratio mass spectrometry. This approach is useful for studying the fate of nitrogen fertilizer and for studying soil microbial processes contributing to the atmospheric increase of nitrous oxide, a radiatively active trace gas that can contribute to global warming and ozone depletion. Most systems of equations that relate isotopic analysis to gas flux are sufficiently complex that certain limitations and potentials of the 15 N approach may be overlooked. We describe a graphical representation of labeled N‐gas flux that illustrates the equations and encourages critical thinking regarding the implementation of related experiments. This model is used to interpret underestimation that occurs if flux derives from multiple pools of differing enrichment. A statistical derivation is presented for a previously published simulation of underestimation due to multiple pools. The same equations are applied to field data to explore whether temporal variation in soil nitrate enrichment is likely to cause significant underestimation. Two sampling strategies are proposed that may eliminate the assumption of pool uniformity, thereby eliminating a potential source of underestimation.