HUMUS CONTENTS AND TRANSFORMATIONS IN NATIVE AND CULTIVATED SOILS

In spite of obvious differences in total humus content, and in the relative proportions of humus fractions from one soil to another, the relationships between the ecology and chemical nature of humus are still incompletely elucidated. Most studies centre upon the polymeric nature of humic acids, the mechanisms involved in their formation, and their interactions with non-humic biomolecules. Anthropic alterations of the ecosystem for agricultural purposes modify soil humus content and composition, sometimes with restricted renewal through crop root exudates and residues. In the tropics after forest clearing, even soil fertility can be affected; the amount of humic acids strongly decreases and they are renewed at a much slower rate than any other fraction. This decrease was studied in actual field situations, using natural 13C isotope measurements. © 1990.