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

Abstract

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 (14N14N16O, 14N15N16O, 15N14N16O, and 15N15N16O) following 0.4 and 0.8 g N m-2 of 15N-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 15N addition. All isotopologues responded positively to rainfall events, but only for 7 to 10 d following N addition. Cumulative 15N-N2O fluxes (sum of the three 15N isotopologues) per chamber for the 14 d following 15N addition ranged from 1.5 to 10.3 mg N m-2. Approximately 1% (range 0.7-1.9%) of the total amount of 15N applied was emitted as N2O. Repeatability (1ω) 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 15N to the atmosphere as N2O after 15N addition, allowing powerful quantification of N2O emissions under field conditions. © American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America.