Immobilization of nitrogen from urea and plant residues in a ferrallitic soil: Laboratory experiments and study by size-fractionation

摘要

The fate of N when incorporated in a ferrallitic soil was investigated during a 3-month incubation, using either15N-labelled urea or15N-labelled crop residues (sugarcane roots and leaves). The organic matter was characterized by particle-size fractionation. The urea-derived organic15N was mainly found in the clay-sized fractions and was ascribed to biological activity. The plant-derived15N 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 organic15N recovered in the various clay fractions (coarse, fine, and very fine) were similar, irrespective of the nature of the added15N. The very fine clay fraction (F<0.05 μm) showed the highest isotopic excess, and thus gave rise to the highest turnover rate