THE ROLE OF NITROGEN-FIXATION AND NUTRIENT SUPPLY IN SOME AGROFORESTRY SYSTEMS OF THE AMERICA

There are a large number of agroforestry systems practiced in the Amer icas in which nitrogen-fixing trees are used to supply nitrogen and or ganic matter as well as to make other contributions to improve growth conditions for non-nitrogen-fixing components, including annual crops, perennial crops or animals. These systems can be divided into traditi onal systems, whose origins were before the European conquest of the A mericas, natural systems which may have existed previous to human occu pation of the Americas about 20 000 years ago, and systems devised by agricultural scientists in the last 100 years. An attempt is made to e valuate nitrogen fixation and the source of nutrients in over 10 such traditional and synthetic systems. Controlled experiments over a suffi cient time to establish an accurate picture of nitrogen fixation and d ecomposition of organic residues generally are lacking. Systems with l ow nutrient removal, in which trees are allowed to grow for long perio ds without being pruned, would be expected to make greater contributio ns to the improvement of soil conditions. Measurements from systems su ch as shaded plantations and alley farming, in which trees are frequen tly pruned and rates of removal of nutrients is high, generally show o nly a slow buildup of nitrogen, soil organic matter, levels of availab le nutrients and improvements in soil physical conditions. Perhaps non equilibrium conditions from pruning, burning and nutrient removal in c rops lead to faster accumulation of nutrients through nitrogen fixatio n and other processes characteristic of ''stressed'' systems. The succ ess of agroforestry systems may be favored by the maintenance of such nonequilibrium conditions. (C) 1997 Elsevier Science Ltd.