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Keywords:

  • fractionation;
  • hydroxylamine;
  • isotope effect;
  • N2O;
  • nitrification;
  • nitrifier-denitrification;
  • nitrous oxide;
  • oxygen exchange;
  • oxygen isotopes;
  • δ18O

Abstract

The ability to use δ18O values of nitrous oxide (N2O) to apportion environmental emissions is currently hindered by a poor understanding of the controls on δ18O–N2O from nitrification (hydroxylamine oxidation to N2O and nitrite reduction to N2O). 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 δ18O–N2O formed by nitrification. Modeling exercises used a set of defined input parameters to emulate the measured soil δ18O–N2O data (Monte Carlo approach). The Monte Carlo simulations implied that abiotic oxygen (O) exchange between nitrite (NO2) and H2O is important in all soils, but that biological, enzyme-controlled O-exchange does not occur during the reduction of NO2 to N2O (nitrifier-denitrification). Similarly, the results of the model simulations indicated that N2O consumption is not characteristic of aerobic N2O formation. The results of this study and a synthesis of the published literature data indicate that δ18O–N2O formed in aerobic environments is constrained between +13‰ and +35‰ relative to Vienna Standard Mean Ocean Water (VSMOW). N2O formed via hydroxylamine oxidation and nitrifier-denitrification cannot be separated using δ18O unless 18O tracers are employed. The natural range of nitrifier δ18O–N2O is discussed and explained in terms of our conceptual model, and the major and minor controls that define aerobically produced δ18O–N2O are identified. Despite the highly complex nature of δ18O–N2O produced by nitrification this δ18O range is narrow. As a result, in many situations δ18O values may be used in conjunction with δ15N–N2O data to apportion nitrifier- and denitrifier-derived N2O. However, when biological O-exchange during denitrification is high and N2O consumption is low, there may be too much overlap in δ18O values to distinguish N2O formed by these pathways.