ABSTRACT
Total soil N includes all forms of inorganic and organic soil N. Inorganic N includes soluble
forms (e.g., NO2 and NO3), exchangeable NH4þ, and clay-fixed nonexchangeable NH4þ.
Organic N content includes numerous identifiable and nonidentifiable forms (Stevenson
1986) and can be determined by the difference between total soil N and inorganic soil N
content. Total N analyses may be divided into two main types: (i) wet digestion (e.g.,
Kjeldahl method) or (ii) dry combustion (e.g., Dumas method). Wet digestion techniques
involve conversion of organic and inorganic N to NH4 þ in acid and its subsequent meas-
urement. Salts (e.g., K2SO4) and catalysts (e.g., Cu) are usually added to increase digestion
temperatures and accelerate oxidation of organic matter (Bremner 1996). The dry combus-
tion method normally involves an initial oxidation step followed by passage of the gases
through a reduction furnace to reduce NOx to N2. The quantity of N2 is usually determined
using a thermal conductivity detector. Near-infrared reflectance spectrometry has recently
been used for the determination of total soil N (Chang and Laird 2002), but the method will
not be described here.
