DISTRIBUTION OF NITROGEN IN SOME HIGHLY ORGANIC TROPICAL VOLCANIC SOILS

The qualitative and quantitative distribution of N-compounds in 10 tropical soils, and in a number of humic materials extracted from representative samples thereof, was determined after 6 N HCl hydrolysis. Of the total N in the soils and humic material, 80-98% was hydrolyzable by 6 N HCl. Slightly less than 1/2 the hydrolyzable N in the soils and humic fractions consisted of amino acids. Well-drained soils and fulvic acids extracted form them contained unusually high concentrations of the acidic amino acids, aspartic and glutamic acids. Between 80-95% of the amino acids in the soils was accounted for in the humic materials + NaOH-insoluble organic residues. NH4+-N released by acid hydrolysis was generally higher for the soil samples than for the humic materials. Amino sugar-N constituted relatively small proportions of the total N in the soils and humic fractions. Large quantities of amorphous allophanic materials coupled with relatively high enzymic activity are responsible for the observed accumulation of acidic amino acids in the well-drained tropical volcanic soils.