IMMOBILIZATION OF NITROGEN FROM UREA AND PLANT RESIDUES IN A FERRALLITIC SOIL - LABORATORY EXPERIMENTS AND STUDY BY SIZE-FRACTIONATION

The fate of N when incorporated in a ferrallitic soil was investigated during a 3-month incubation, using either N-15-labelled urea or N-15-labelled crop residues (sugarcane roots and leaves). The organic matter was characterized by particle-size fractionation. The urea-derived organic N-15 was mainly found in the clay-sized fractions and was ascribed to biological activity. The plant-derived N-15 was observed both in the sand-sized and in the clay-sized fractions; the former pool was ascribed to the persistence of crops residues, the latter to biological immobilization. The relative proportions of organic N-15 recovered in the various clay fractions (coarse, fine, and very fine) were similar, irrespective of the nature of the added N-15. The very fine clay fraction (F < 0.05-mu-m) showed the highest isotopic excess, and thus gave rise to the highest turnover rate.