Nitrogen fixation in Acacia mangium plantations.

Abstract

A field experiment was set up in 1990 at Port-Bouët in lower Côte-d'Ivoire to assess the percentage of atmospheric nitrogen fixed by Acacia mangium in plantation conditions. The site was situated on sandy Quaternary coastal soil; previous cultivation on the site had been either the perennial leguminous cover crop Pueraria phaseoloides, or Imperata cylindrica grassland. Acacia mangium seedlings were planted within plantations of Eucalyptus urophylla. Three inoculation treatments with Bradyrhizobium sp., replicated in three blocks of a complete randomized block design, were tested: the first treatment involved trees inoculated with the Aust 13c strain (isolated from nodules collected in Queensland, Australia), the second, trees inoculated with the CB 756 strain (isolated from Macrotyloma africanum in Zimbabwe), and the third uninoculated (control) trees. Nineteen months after transplanting the trees to the field, tree height and diameter was measured. At the same time, foliage samples were collected to estimate the percentage of nitrogen derived from atmospheric N₂ (Ndfa%) fixed by Acacia mangium, which was measured using natural isotopic abundance in ¹⁵N. The Aust 13c strain had a positive growth effect on trees, with a significant increase in height increment (10%) and diameter (15%) compared with uninoculated trees or the trees inoculated with the CB 756 strain. The Ndfa% varied strongly from 20 to 90% depending on the block and inoculation treatments, and appeared to be negatively correlated with soil fertility.