Agroforestry: N 2 -fixing trees in integrated agriculture.

Nodulated legumes and actinorhizal trees are the two main groups of N2-fixing trees. Details of some of the most important legume trees for agroforestry are given. These trees were: Acacia, Calliandra, Erythrina, Faidherbia, Flemingia, Gliricidia, Leucaena, Prosopis, Sesbania and Tephrosia. Species of Casuarina are the most important non-leguminous N2-fixing trees in the lowland tropics. Specific rhizobial requirements for inoculation are briefly discussed. Different methods used in measuring the amounts of N2-fixed by trees are reviewed as well as the various uses of N2-fixing trees in agroforestry such as fuelwood and timber, food and forage producers and promotion of soil fertility. The distribution of trees in fields of the arid zones and the problems with alley cropping are discussed. Agroforestry systems such as barrier fences, improved fallows, and cut-and-carry systems are reviewed. Finally, the role of N2-fixing trees for reclamation of degraded land is also discussed.