The ability to use delta 18O values of nitrous oxide (N 2O) to apportion environmental emissions is currently hindered by a poor understanding of the controls on delta 18O-N 2O from nitrification (hydroxylamine oxidation to N 2O and nitrite reduction to N 2O). In this study fertilized agricultural soils and unfertilized temperate forest soils were aerobically incubated with different 18O/ 16O waters, and conceptual and mathematical models were developed to systematically explain the delta 18O-N 2O formed by nitrification. Modeling exercises used a set of defined input parameters to emulate the measured soil delta 18O-N 2O data (Monte Carlo approach). The Monte Carlo simulations implied that abiotic oxygen (O) exchange between nitrite (NO 2-) and H 2O is important in all soils, but that biological, enzyme-controlled O-exchange does not occur during the reduction of NO2a to N 2O (nitrifier-denitrification). Similarly, the results of the model simulations indicated that N 2O consumption is not characteristic of aerobic N 2O formation. The results of this study and a synthesis of the published literature data indicate that delta 18O-N 2O formed in aerobic environments is constrained between +13 per mil and +35 per mil relative to Vienna Standard Mean Ocean Water (VSMOW). N 2O formed via hydroxylamine oxidation and nitrifier-denitrification cannot be separated using delta 18O unless 18O tracers are employed. The natural range of nitrifier delta 18O-N 2O is discussed and explained in terms of our conceptual model, and the major and minor controls that define aerobically produced delta 18O-N 2O are identified. Despite the highly complex nature of delta 18O-N 2O produced by nitrification this delta 18O range is narrow. As a result, in many situations delta 18O values may be used in conjunction with delta 15N-N 2O data to apportion nitrifier- and denitrifier-derived N 2O. However, when biological O-exchange during denitrification is high and N 2O consumption is low, there may be too much overlap in delta 18O values to distinguish N 2O formed by these pathways.
