Measurement of fixed ammonium and nitrogen isotope ratios using dry combustion

Clay-fixed NH4-N ran represent up to 18 percent of the total soil N for Can adian agricultural soils. A modification of the Silva and Bremner's method for determining clay-fixed NH4 was developed. The Silva and Bremner procedu re incorporates dissolution of mineral residues by a HF-HCl digestion in or der to release bound NH4, following removal of exchangeable NH4 and organic N, Steam distillation is used to recover NH4 from the soil digest. The pro cedure presented in this paper substitutes combustion in an automated eleme ntal analyzer for the HF-HCl digestion step. Combustion with an elemental a nalyzer follows removal of exchangeable NH4 with 0.5 M KCl and organic N wi th wet digestion using KOBr. The elimination of the HF-HCl digestion and st eam distillation steps shortens the time required for analysis and avoids t he use of strong reagents. The procedure is also suitable for N-15 studies when combined with a mass spectrometer, The two techniques gave virtually t he same results for 17 soils with the slope of the regression of 1.03 and a coefficient of determination of 0.97. Simultaneous measurements of fixed N H, and N isotope ratios using continuous flow mass spectrometry can be tarr ied out.