The role of nitrogen fixation and nutrient supply in some agroforestry systems of the Americas

There are a large number of agroforestry systems practiced in the Americas in which nitrogen-fixing trees are used to supply nitrogen and organic 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 traditional systems, whose origins were before the European conquest of the Americas, natural systems which may have existed previous to human occupation of the Americas about 20 000 years ago, and systems devised by agricultural scientists in the last 100 years. An attempt is made to evaluate nitrogen fixation and the source of nutrients in over 10 such traditional and synthetic systems. Controlled experiments over a sufficient time to establish an accurate picture of nitrogen fixation and decomposition of organic residues generally are lacking. Systems with low nutrient removal, in which trees are allowed to grow for long periods without being pruned, would be expected to make greater contributions to the improvement of soil conditions. Measurements from systems such as shaded plantations and alley farming, in which trees are frequently pruned and rates of removal of nutrients is high, generally show only a slow buildup of nitrogen, soil organic matter, levels of available nutrients and improvements in soil physical conditions. Perhaps nonequilibrium conditions from pruning, burning and nutrient removal in crops lead to faster accumulation of nutrients through nitrogen fixation and other processes characteristic of ''stressed'' systems. The success of agroforestry systems may be favored by the maintenance of such nonequilibrium conditions. (C) 1997 Elsevier Science Ltd.