Diffusion Method To Prepare Soil Extracts For Automated Nitrogen‐15 Analysis

TLDR

The procedure is ideal for preparing low‑N soil samples (50–200 µg N) without cross‑contamination. The study develops a labor‑saving diffusion method for preparing soil KCl extracts for 15N analyses of NH₄⁺ and NO₃⁻. The method uses disposable 140 mL containers with 7‑mm acidified GF/D filter paper disks on stainless‑steel wire, employing Devarda’s alloy and MgO as reductant and base, allowing 40–60 mL samples to diffuse over 6 days at room temperature without shaking, and enabling analysis by direct combustion or automated NaOBr after drying or mailing the disks. Paired t‑tests showed no significant difference between this diffusion method and steam distillation for soil extracts of varying ¹⁵N enrichment (p = 0.64).

Abstract

Abstract A diffusion method that requires less labor than distillation methods was developed for preparing soil KCl extracts for 15 NH + 4 and 15 NO ‐ 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 15 N enrichment by this method and steam distillation showed no significant difference ( p = 0.64). Use of KHSO 4 as the trapping acid allows the paper disks to be placed in a Sn capsule and analyzed by direct combustion mass spectrometry. Alternately, H 2 SO 4 can be used and the disks analyzed by an automated NaOBr method. The disks can be easily dried in a desiccator over H 2 SO 4 and mailed for 15 N analysis by service laboratories.