ABSTRACT
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).