HUMID TROPICAL LEGUMINOUS TREE AND PASTURE GRASS RESPONSIVENESS TO VESICULAR-ARBUSCULAR MYCORRHIZAL INFECTION

Phosphorus is the major nutrient limiting plant growth in a Costa Rica n silvopastoral system located on an acid, high P-retaining, volcanic soil. We investigated plant responsiveness to vesicular-arbuscular myc orrhizal (VAM) inoculation using the leguminous tree species Erythrina berteroana Urban, and the two dominant grass species Paspalum conjuga tum Berg and Homolepsis aturensis Chase of this silvopastoral system. We grew grass seedlings in the greenhouse for 15 weeks in a methyl bro mide-sterilized study soil to which either mixed-species VAM inoculum (Theobroma cacao feeder roots) or autoclave-sterilized cacao roots (no n-inoculated control) were added. E. berteroana was grown from both se edlings and vegetative stakes (40 cm long) for 30 and 19 weeks, respec tively. Upon harvest, we measured above and below ground biomass, N an d P content, root: shoot ratio, legume nodulation, and VAM infection l evels. The total aboveground and root biomass of mycorrhizae-inoculate d P. conjugatum seedlings were 2.5 and 2.8 times greater than those of noninoculated seedlings. In contrast, VAM-inoculated seedlings of H. aturensis produced 8.4 and 5.9 times more total above-ground and root mass than noninoculated seedlings. Mycorrhizae-inoculated E. berteroan a seedlings produced 10.6 times greater shoot biomass for inoculated v ersus noninoculated seedlings, while E. berteroana vegetative stakes e xhibited a negative growth response to VAM inoculation (an approximate ly 16% decrease in shoot biomass for VAM-inoculated cuttings). The dif ference in responsiveness between Erythrina growth forms is hypothesiz ed to reflect the cost-benefit relationship between plant host and fun gal symbiont for energy and nutrient reserves.