Nitrogen fixation in actinorhizal plants

The actinorhizal plants (AP) that exhibit the highest N-2-fixing potential belong to two genera : Alnus and Casuarina. Thus the N-2-fixing potential 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. Therefore, it is necessary to develop strategies aiming not only at optimising the potentialities of ''host plants x Frankia'' symbioses but also at alleviating the impact of limiting factors. Most relevant strategies are briefly described. Provided that their N-2-fixing potential is fully exploited, AP are ideal candidates in many systems of land use :production forestry (especially mixed-tree plantations), agroforestry (such as enriched fallows, alley cropping, intercropping, living fences) protective forestry (especially sand dune stabilisation and windbreaks), reclamation forestry (especially reclamation of eroded, salinized or wasted lands and phytoremediation), urban forestry and amenity plantations.