Forms of nitrogen in cultivated soil profiles in Tanzania

In cultivated soils, total soil N, organic C and C-to-N ratios were in the range of 0.24–0.49%, 3.1–5.8% and 10.7–15.0, respectively in the surface horizons and decreased with depth. Native fixed NH⁺₄-N accounted for 2.3–3.0% of total soil N in surface horizons but while the quantities of fixed NH⁺₄-N decreased with depth, the proportion to total soil N increased. Exchangeable NH⁺₄-N ranged from 15 to 32 and NO^?₃-N from 26 to 73 μg g^?1 soil in surface horizons, and both decreased with depth. Exchangeable-N accounted for 1.1–2.4% of total soil N. Over 97% of total soil N was organically bound.Of the total soil N in the surface horizons, 29.0–79.0% was acid hydrolysable and 21.0–71.0% was nonhydrolysable. The range of proportions of each of hydrolysable NH⁺₄-N, hexosamine-N, serine plus threonine α-amino acid-N, identified-N, and unidentified-N to total soil N in the surface horizons were 14.5–22.4, 4.8–9.2, 0.2–5.8, 4.0–16.7, 23.3–48.8, and 0.3–41.5%, respectively. Hydrolysable NH⁺₄-N constituted the largest proportion of the identified-N fraction. Distribution patterns of the organic-N fractions in the profiles varied from soil to soil. Sixteen amino acids were identified which accounted for 82–100% of the α-amino acid-N fraction in the soils; glycine and alanine alone accounted for 35–40%. All the organic-N fractions were transformed to varying degree during aerobic incubation.