A Diffusion Method to Prepare Soil Extracts for Automated Nitrogen-15 Analysis

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Soil Science Society of America



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A diffusion method that requires less labor than distillation methods was developed for preparing soil KCl extracts for 15NH+4 and 15NO-3 analyses. The procedure is ideal for preparing samples having low N mass (50–200 µg N) with no cross contamination. The method uses 140 mL disposable specimen containers to hold the sample, and 7-mm diam, disks of acidified GF/D glass fiber filter paper on stainless steel wire as the acid trap. Devarda's alloy and MgO are used as the reductant and base respectively. Complete diffusion for 40 to 60 mL of sample takes 6 d at room temperature with no shaking. Paired t-tests comparing analyses of soil extracts of varying 15N enrichment by this method and steam distillation showed no significant difference (p = 0.64). Use of KHSO4 as the trapping acid allows the paper disks to be placed in a Sn capsule and analyzed by direct combustion mass spectrometry. Alternately, H2SO4 can be used and the disks analyzed by an automated NaOBr method. The disks can be easily dried in a desiccator over H2SO4 and mailed for 15N analysis by service laboratories.

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