NITROGEN-FIXATION IN LEUCAENA-LEUCOCEPHALA L AS AFFECTED BY ROOTING VOLUME AND COMPETITION WITH EUCALYPTUS-CAMALDULENSIS

Fast growing trees, in particular nitrogen fixing trees,are essential for the establishment of sustainable agroforestry practices by resourc e-poor and/or environmentally conscious farmers. An important selectio n factor for trees is therefore high N-2 fixing capability. This study was conducted to examine biological nitrogen fixation (BNF) in Leucae na leucocephala L., as affected by growth in pots containing 4, 10, 15 and 30 kg soil and when grown together with Eucalyptus camaldulensis. BNF was measured by the N-15 isotope dilution technique. Leucaena and Eucalyptus were grown either sole (SL, SE) or in two mixtures (1:1 - L1:EI and 1:2 - L1:E2) for a period of 6 months. Planting pattern infl uenced (P < 0.05) dry matter and N content of shoot and roots of Eucal yptus, as the sole planted trees gave the highest yield. Dry matter, N content and total amount of N fixed by Leucaena significantly decreas ed with decreasing rooting volume. Also, the smaller the rooting volum e (or soil weight), the higher was the proportion (%) of N derived fro m the atmosphere (%Ndfa) by leucaena. For each rooting volume, the sol e or mixed planting pattern did not significantly influence either the proportion (%) or amount of N fixed by leucaena. For these trees, the refore, rooting volume rather than cropping pattern influenced the pro portion and amount of nitrogen fixed.