NITROGEN-FIXATION IN ACTINORHIZAL PLANTS

The actinorhizal plants (AP) that exhibit the highest N-2-fixing poten tial belong to two genera : Alnus and Casuarina. Thus the N-2-fixing p otential of 2-year old Casuarina equisetifolia is 116 kg N-2 ha(-1) an (-1), which is fairly high. In fact, the actual amount of N-2 fixed is often lower or much lower than the N-2-fixing potential because it is limited by various physical, chemical and biological constraints. The refore, it is necessary to develop strategies aiming not only at optim ising the potentialities of ''host plants x Frankia'' symbioses but al so at alleviating the impact of limiting factors. Most relevant strate gies are briefly described. Provided that their N-2-fixing potential i s fully exploited, AP are ideal candidates in many systems of land use :production forestry (especially mixed-tree plantations), agroforestr y (such as enriched fallows, alley cropping, intercropping, living fen ces) protective forestry (especially sand dune stabilisation and windb reaks), reclamation forestry (especially reclamation of eroded, salini zed or wasted lands and phytoremediation), urban forestry and amenity plantations.