Stable Oxygen Isotope Ratios of Nitrate Produced from Nitrification: 18O-Labeled Water Incubations of Agricultural and Temperate Forest Soils

In many nitrate (NO3-) source partitioning studies, the delta O-18 value for NO3- produced from nitrification is often assumed to reflect the isotopic compositions of environmental water (H2O) and molecular oxygen (O-2) in a 2:1 ratio. Most studies that have measured or observed this microbial endmember have found that the delta O-18-NO3- was more positive (up to +15 parts per thousand higher) than the assumed value. Current understanding of the mechanism(s) responsible for this discrepancy is limited. Incubations of one temperate forest soil (organic) and two agricultural soils (mineral) were conducted with O-18-labeled H2O to apportion the sources of oxygen in NO3- generated from nitrification. The NO3- produced in all soils had delta O-18 values that could not be explained by a simple endmember mixing ratio of 2:1. A more comprehensive model describing the formation of microbial NO3- was developed, which accounts for oxygen exchange between H2O and NO2- and includes terms for kinetic and equilibrium isotope effects. Oxygen isotope exchange (i.e., the fraction of NO3--oxygen that originates from the abiotic exchange of H2O and NO2-) varied widely between the temperate forest soil (0.37) and the two agricultural soils (0.52 and 0.88). At present, the microbial endmember for nitrification cannot be successfully predicted.