EXPERIMENTS ON THE ECOLOGICAL BASIS OF SUSTAINABILITY - EARLY FINDINGS ON NITROGEN, PHOSPHORUS, AND ROOT SYSTEMS

Experiments were established on fertile soil in the humid lowlands of Costa Rica to examine the effects of complementary use of resources be tween species and stand turnover time on sustainability. The experimen tal plantations are dominated by perennial plants of the two most succ essful life forms in this biome, dicotyledonous trees and self-support ing monocots, and are harvested and replanted at intervals of 1, 4, an d 16 years. Three main categories of responses are under study, includ ing soil fertility, plant-pest interactions, and productivity. Nitroge n is abundant in the study-site soil; nitrogen mineralization rates ar e about 0.85 kg ha-1 day-1 and nitrification rates are about 1.0 kg ha -1 day-1. Phosphorus is also abundant (30 to 70 mug/g, Olsen extractab le), and preliminary data indicate that much of the organically bound phosphorus is under microbial control. Root systems of the three main tree species under study differ greatly, ranging from a dense, compact mass of roots (in Hyeronima alchorneoides) that thoroughly exploit a modest soil volume, to sparse roots that extend very far from the base of the tree (in Cordia alliodora), to massive starch-rich roots that enable the tree to respond to herbivore attack (in Cedrela odorata).