Nature and distribution of organic nitrogen in tropical soils

Briones, Angelina Mariano
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Distribution of total nitrogen, organic nitrogen forms, and organic matter fractions was studied in -a number of tropical soils. The mineralizable fraction of organic nitrogen and nature of humic and fulvic acids in soils as well as the interaction of these organic matter fractions with soils were also examined. Amounts of total soil nitrogen and its organic forms (hydrolyzable ammonium-N, amino sugar-N, amino acid-N, unidentified hydrolyzable-N, and acid insoluble-N) were markedly influenced by climate. The most striking difference in the amounts of these fractions in various soils was obtained in the acid insoluble-N fraction. Volcanic ash soils developed under high moisture (150 to 250 inches mean annual rainfall) were found to have high nitrogen contents but contained small amounts of acid insoluble-N. In contrast, the acid insoluble-N content of soils occurring under lower rainfall (20 to 40 inches) was considerably higher. Soils derived from crystalline basalt exhibited similar trends. The Lualualei soil developed from alluvium however contained negligible amounts of acid insoluble-N. Changes In the amounts of the organic nitrogen forms after incubation (12 to 18 months) were determined to pinpoint the mineralizable nitrogen fraction. The amounts of hydrolyzable ammonium-N, amino sugar-N, unidentified hydrolyzable-N, and acid insoluble-N slightly increased or decreased with time. Amino acid-N consistently decreased with incubation time. The decrease in amino acid-N was significantly correlated to the increase in mineralized nitrogen content. Distribution of humic and fulvic nitrogen in volcanic ash soils varied markedly with climate. Humic acids predominated in soils developed under low rainfall while fulvic acids were high in soils developed under high rainfall. Basalt-derived soils behaved similarly except for Waialua and Lualualei soils. The Lualualei soil yielded no humic acid and the Waialua soil possessed a low proportion of this fraction. The trend in humic acid distribution followed the distribution pattern of acid insoluble-N. The chemical nature of acid insoluble-N is still unknown. In order to provide more information on the nature of nitrogen in particular fractions of organic matter, the proportions of acid insoluble and hydrolyzable nitrogen in humic acids were determined. It was found that the amount of acid insoluble-N is significantly correlated to the amount of humic acid in soils. Infrared data showed that humic acids of soils developed under high rainfall had lower amounts of aromatic groups than those from soils formed under lower rainfall. Fulvic acids were found to have a more intense absorption band for carboxylic groups than the humic acids. Adsorption of humic and fulvic acids on organic matter-free soils showed that adsorption was strongly concentration and pH dependent. More organic acids were adsorbed in the acid range, and for a given pH adsorption increased exponentially with concentration.
Bibliography: leaves 127-137.
xv, 137 l graphs, tables
Soils -- Nitrogen content
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Theses for the degree of Doctor of Philosophy (University of Hawaii at Manoa). Soil Science; no. 245
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