Controlling organic interference in determination of soil mineral nitrogen

Abstract

AbstractInorganic 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.