NITROGEN USED STRATEGIES OF NODULATED AMAZONIAN LEGUME: <em>INGA</em> <em>EDULIS</em>

Abstract

Symbiotic nitrogen fixation in legumes is strongly diminished by mineral nitrogen. Nevertheless, recent work with some tropical legumes revealed low sensitivity to mineral nitrogen or even enhancement of nodulation by ammonium. Thus, in this study, we investigated plant growth and nodulation of Inga edulis over a period of 128 days using different sources of nitrogen, i.e. ammonium (15 mM), nitrate (15 mM) and symbiotic nitrogen fixation. Plant growth was evaluated through dry mass, height, stem diameter and root/shoot ratio as well as nodulation by nodule number and dry mass. Nitrate, ureides and total amino acids were determined in roots, shoots and xylem sap. All three nitrogen sources were found to stimulate growth relative to the noninoculated N-free control, however ammonium was the most effective. Nodulation, both nodule number and mass, was strongly reduced by nitrate but not ammonium. The transport of total amino acids in the xylem sap was stimulated by ammonium without change in the composition with asparagine predominating in all treatments. Inga edulis can benefit from fertilisation with ammonium since even at high concentrations growth was improved without impairing nodulation, a condition appropriate for restoration of soil conditions.