INFLUENCE OF MYCORRHIZA ON SELECTED TREE SPECIES: Gliricidia sepium, Acacia auriculiformis, Leucaena leucocephala
CHAPTER ONE
INTRODUCTION
Importance of Phosphorus in Agroforestry Systems in the Humid Tropics Agroforestry, the managed combination of tree production with that of crops or livestock, can be a viable land-use system on marginal soils in the humid tropics. Agroforestry systems fulfill various needs, including those for food, fuel-wood, or livestock feed, in areas with erodible soils and low soil fertility. Phosphorus has been identified as the nutrient of most concern to the success of agroforestry systems in tropical regions (Palm et al., 1991; Shepherd, 1991). This thesis addresses the problem of P constraints to agroforestry systems in the humid tropics by investigating strategies of low-P tolerance in nitrogen-fixing tree (NFT) species adapted to that environment. One reason for the concern with P is the prevalence of soils with high levels of P-fixation in the tropics. Soils with high P-fixing capacities are particularly widespread in the humid tropics, accounting for 38% of the land in this region (Sanchez and Logan, 1992). Focus on P limitation in agroforestry also results from the realization that P is necessarily exported out of agricultural systems with harvests, especially of P-rich components such as grain. Phosphorus inputs are required to sustain any system from which there are P losses. In regions where economic and infrastructural constraints forbid copious use of chemical fertilizers, employing species that are inherently well-adapted to low P fertility reduces the need for external inputs. A third reason for concern with P in agroforestry systems is the importance of this nutrient for biological nitrogen fixation (BNF) (Cassman et al., 1980 and 1981; Gates, 1974; Israel, 1987). Nitrogen, as the most limiting nutrient in agriculture (Singer and Munns, 1987), is often a major constraint to tree and crop growth. The use of NFTs in agroforestry systems can alleviate the problem of N deficiency in soil for both trees and companion crops or livestock (Dommergues, 1987; Siaw et al., 1991; Szott et al., 1991). To realize the benefits of BNF to the system, P supply should be sufficient to maintain the BNF symbiosis. In this thesis, low-P tolerance of NFTs is investigated in acid soil since high P-fixation is