Diffusion Methods to Determine Different Forms of Nitrogen in Soil Hydrolysates

Abstract

Conventional steam‐distillation techniques for fractionating the N in soil hydrolysates have generally indicated little variation in the chemical distribution of soil organic N, regardless of soil type, cropping, cultivation, or N‐fertilization history. Nitrogen‐15 tracer studies to evaluate these techniques showed that determinations of amino sugar–N are subject to serious underestimation, and that analyses for amino acid–N are vitiated by incomplete conversion of amino acid–N to NH4–N following incomplete removal of hydrolyzable NH4 and amino sugar–N. Diffusion methods were developed for fractionating the N in soil hydrolysates that are far more accurate and specific than steam distillation, while also being much simpler and more convenient. In these methods, total hydrolyzable N is measured by Kjeldahl digestion of the hydrolysate and diffusion of the digest with NaOH; diffusion is performed with MgO to determine hydrolyzable NH4–N; (NH4 + amino sugar)–N is recovered by diffusion with NaOH, after which amino acid–N is liberated by ninhydrin oxidation at pH <1.8 and recovered by diffusion with NaOH. Analytical accuracy and specificity were evaluated using a wide variety of purified organic‐N compounds and by checking the recovery of 15N added to soil hydrolysates as (NH4)2SO4, glucosamine, or glycine. Studies using a diverse set of soils showed that distillation and diffusion usually agreed to within 10% for quantitative analysis of total hydrolyzable N, NH4–N, and amino acid–N, whereas analyses of amino sugar–N were 74 to 317% greater by diffusion than by distillation.