Nitrogen fixation in L. leucocephala L. as affected by rooting volume and competition with E. camaldulensis

Fast growing trees, in particular nitrogen fixing trees, are essential for the establishment of sustainable agroforestry practices by resource-poor and/or environmentally conscious aarmers. An important selection factor for trees is therefore high N2 fixing capability. This study was conducted to examine biological nitrogen fixation (BNF) in Leucaena 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 15N isotope dilution technique. Leucaena and Eucalyptus were grown either sole (SL, SE) or in two mixtures (1:1 - L1:E1 and 1:2 - L1:E2) for a period of 6 months. Planting pattern influenced (P < 0.05) dry matter and N content of shoot and roots of Eucalyptus, as the sole planted trees gave the highest yield. Dry matter, N content and total amount of N fixed by Leucaena significantly decreased with decreasing rooting volume. Also, the smaller the rooting volume (or soil weight), the higher was the proportion (%) of N derived from the atmosphere (%Ndfa) by leucaena. For each rooting volume, the sole or mixed planting pattern did not significantly influence either the proportion (%) or amount of N fixed by leucaena. For these trees, therefore, rooting volume rather than cropping pattern influenced the proportion and amount of nitrogen fixed.