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The influence of previous application of organic matter for a period of 16 years on the amounts and distribution of organic N was investigated using profile samples from four rice field plots. This study was conducted by determining soil horizon samples for nonhydrolyzable Nand hydrolyzable (total, ammonium, amino sugar, and unidentified) N with the hot 6 N HCl hydrolysis procedure. Organic matter treatments, including the application of rice compost, rice straw, and Italian ryegrass residues, led to a marked increase in all forms of soil N whose content was highest in the surface soil and decreased down the profile. The proportion of hydrolyzable N (40-81%) to the total N decreased with depth and that of nonhydrolyzable N (19-60%) increased with depth. Furthermore, the proportion of amino acid N (4-28%), amino sugar N (2-5%), and unidentified N (12-32%) decreased with depth, whereas that of hydrolyzable ammonium N (16-24%) remained fairly constant throughout the profile. In general, the profile distribution pattern indicated no differences regarding the proportion of each organic N fraction, so that organic matter application appeared to have little effect on these parameters. In spite of the considerable differences observed in the amino acid content, the amino acid composition of all horizon samples in the profile was remarkably similar independently of the depth as well as the treatments. Neutral amino acids were present in very large amounts followed by acidic and basic amino acids with a rough ratio of 6.5 : 2.5 : 1.0. Moreover, the amino acid compositional pattern in the soil was closely consistent with that obtained from the organic residues used for soil application. In the linear correlation analysis, mineralizable N derived from anaerobic incubation method was found to be significantly correlated with all the N fractions including even nonhydrolyzable N. Stepwise regression analysis indicated, however, that there were no differences in terms of the relative contribution from each soil N fraction. Presumably, the degradation of the soil N compounds by the chemical treatment resulted in a pattern not closely related to the biological mineralization of soil N.

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