An Isotopic Method for High Precision, Independent Determination of Dinitrogen and Nitrous Oxide Fluxes from Soils

Bergsma, T.T., N. E. Ostrom, M. Emmons, and G. P. Robertson

Presented at the All Scientist Meeting (1998-07-21 to 1998-07-22 )

 The increasing concentration of nitrous oxide in Earth’s atmosphere contributes to radiative forcing of climate change and to the catalytic destruction of stratospheric ozone. Understanding the nitrous oxide budget requires understanding the relative proportions of nitrous oxide and dinitrogen produced during denitrification, which is the major source of nitrous oxide in terrestrial environments. Characterization of the N2O:N2 flux ratio requires highly precise simultaneous flux estimates for both gases. We have developed a method that estimates both fluxes independently from the shift in isotopic character of gases in confined atmospheres above labeled soil. This method has been developed to estimate N2 flux, which is difficult to measure by other means. We extend the method to estimate flux of N2O. Very small fluxes can be measured since isotopic analysis of nitrous oxide can be performed reliably on less than 0.2 micrograms of N as N2O. Using the same method for both N2 and N2O improves the reliability of the flux ratio estimate.Results from a field study in April and May of 1998 illustrate the application of the technique. Incubations were conducted before and after natural precipitation, using a soil cover over labeled soil. Headspace samples were collected at the beginning and end of each incubation. Figure 1 illustrates the isotopic character of N2O for each sample; soil N enrichment and N2O flux can be infered graphically or mathematically. N2 analysis is similar. Figure 2 summarizes gas fluxes during the period of study. Flux estimates of N2O by mass spectrometry agree well with results from gas chromatography. Estimates for N2 are near the detection limit for this experimental configuration.Figure 1. Isotopic character of initial and final N2O for 9 field incubations.Figure 2. Gas fluxes and precipitation over several days.Return to Contents

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