Contribution of actinorhizal plants to tropical soil productivity and rehabilitation

The contribution of actinorhizal plants to soil productivity and rehabilitation depends not only on properties encountered in a number of non-N2-fixing trees but also on the input of fixed N2 that is subsequently transferred to soil and ultimately to associated crops. The nitrogen-fixing potential of a number of actinorhizal plants (e.g. Casuarina sp. and Alnus sp.) is high but the amount of N2 actually fixed in the field is often low because the expression of this potential is limited by unfavorable environmental conditions or improper management practices. Assessing the amount of fixed N2 transferred to soil is difficult mainly because of the recycling of fixed N2 except in open ecosystems. Many examples of successful introductions of actinorhizal plants into various systems of land management are given. To increase the input of fixed N2 into ecosystems two strategies can be adopted: the first one is to use proper management practices; the second one is to improve the performances of the N2-fixing system. Practically, in addition to optimizing actinorhizal fixation, it is recommended to develop the introduction of actinorhizal plants as soil improvers in a number of countries where they are not yet used, to domesticate hitherto neglected or overlooked actinorhizal plants, and to exploit their ability to contribute to the rehabilitation of wasted lands and possibly to the phytoremediation of polluted sites.