Organic Forms of Nitrogen

Determination of the chemical forms of soil organic nitrogen (N) has most commonly

involved a two-step procedure. First the soil is hydrolyzed by hot mineral acid (usually

hydrochloric acid [HCl]), and second the hydrolysate is either distilled to isolate operation-

ally defined N fractions or it is chromatographically separated for identification of common

amino acids (Bremner 1965; Stevenson 1994). Normally this approach identifies at most half

of the total soil N. This poor recovery may be explained by several factors: HCl hydrolysis

does not solubilize a considerable proportion (~20%–35%) of total soil N, a comparable

proportion is hydrolyzed from various N forms into NH4 þ, which obscures the original

forms, and ~10%–20% of total soil N is hydrolyzable but not identifiable as any of the

commonly measured amino acids (Stevenson 1994, 1996).