Tracking Short-Term Effects of Nitrogen-15 Addition on Nitrous Oxide Fluxes Using Fourier-Transform Infrared Spectroscopy

Synthetic fertilizer N additions to soils have significantly increased atmospheric N2O concentrations, and advanced methods are needed to track the amount of applied N that is transformed to N2O in the field. We have developed a method for continuous measurement of N2O isotopologues ((NNO)-N-14-N-14-O-16, (NNO)-N-14-N-15-O-16, (NNO)-N-15-N-14-O-16, and (NNO)-N-15-N-15-O-16) following 0.4 and 0.8 g N m(-2) of N-15-labeled substrate as KNO3 or urea [CO(NH2)(2)] using Fourier-transform infrared (FTIR) spectroscopy. We evaluated this method using two 4-wk experimental trials on a coastal floodplain site near Nowra, New South Wales, Australia, which is managed for silage production. We deployed an automated five-chamber system connected to a portable FTIR spectrometer with multipass cell to measure N2O isotopologue fluxes. Emissions of all isotopologues were evident immediately following N-15 addition. All isotopologues responded positively to rainfall events, but only for 7 to 10 d following N addition. Cumulative N-15-N2O fluxes (sum of the three N-15 isotopologues) per chamber for the 14 d following N-15 addition ranged from 1.5 to 10.3 mg N m(-2). Approximately 1% (range 0.7-1.9%) of the total amount of N-15 applied was emitted as N2O. Repeatability (1 sigma) for all isotopologue measurements was better than 0.5 nmol mol(-1) for 1-min average concentration measurements, and minimum detectable fluxes for each isotopologue were <0.1 ng N m(-2) s(-1). The results indicate that the portable FTIR spectroscopic technique can effectively trace transfer of N-15 to the atmosphere as N2O after N-15 addition, allowing powerful quantification of N2O emissions under field conditions.