Controlling organic interference in determination of soil mineral nitrogen

Inorganic nitrogen (N) in the form of exchangeable ammonium (NH₄⁺), nitrate (NO₃⁻), or nitrite (NO₂⁻) is normally extracted by shaking soil with a neutral salt solution and is subject to interference by soluble organic N (SON). After optimizing sequential diffusion methods to expedite recovery of NH₄⁺–N and (NO₃⁻ + NO₂⁻)-N, ¹⁵N-tracer studies were conducted to ascertain whether extraction is quantitative when performed on soils amended with 2 g ¹⁵N kg⁻¹ using 0.2, 1, and 2 M potassium chloride (KCl) and can be carried out by a simple leaching method instead of conventional shaking-filtration. The results verified a significant decrease in SON interference with the optimized diffusion procedures and showed that (a) interference is more serious for NH₄⁺–N than for (NO₃⁻ + NO₂⁻)-N, (b) 2 M KCl is required for quantitative recovery of ¹⁵NH₄⁺–N, and (c) leaching virtually eliminates organic interference during diffusion of (NO₃⁻ + NO₂⁻)-N. The leaching-diffusion approach minimizes the inflating effect of SON on soil inorganic N analyses and will be especially useful in N isotope studies.