Effectiveness of mycorrhizae on phosphorus uptake and nitrogen fixation of humid tropical leguminous tree seedlings in a P-deficient, acid soil

Information on the extent to which P nutrition and biological N fixation by acid-tolerant leguminous tree species benefit from vesicular-arbuscular mycorrhizae (VAM) colonization remains limited. Two greenhouse studies were undertaken to evaluate the influence of soil fertility (pH, Ca supply and P availability), and mycorrhizae and rhizobia inoculation on growth, nutrition, nodulation and mycorrhizal colonization of leguminous tree seedlings in an acid P-deficient B horizon of a Typic Kanhapludults. In the first experiment treatment variables included two soil pH levels (4.2 and 5.5), Ca supply, two Mehlich-1 soil P levels (3 and 6 μg cm−3), and presence or absence of mycorrhizae inoculum with a uniform rhitzobia inoculum. The effect of Ca supply associated with liming to increase soil pH was isolated by comparing gypsum against two lime sources, CaCO₃ and MgCO ₃. Two leguminous tree species were tested in the first experiment: Erythrina berteroana Urb. and Retalhuleu provenance of Gliricidia sepium (Jacquin) Steudel. Mycorrhizal infection and nodulation in roots of E. berteroana was not detected in any of the treatments. Mycorrhizae colonization of G. sepium were only detected when mycorrhizae inoculum was added to the soil. Nodule number and activity for G. sepium increased with either source of lime. Increases in shoot dry weight of both species were greater with applications of gypsum or lime than by increasing soil P availability from 3 to 6 μg cm −3. Lime increased shoot dry weight of G. sepium more than gypsum, and the response to mycorrhizae inoculation was less than that for P. Maximum Ca uptake for both species and P uptake for G. sepium occurred in gypsum treatments with 6 μg cm−3 of Mehlich-1 soil P. In the second experiment the soil was limed with CaCO₃ to pH 5.5 and G. sepium growth response to five levels of applied P (0 to 93 μg cm−3 was evaluated both in the presence and absence of mycorrhizae and rhizobium inoculation. Mycorrhizal root colonization was only detected among treatments receiving fungi inoculum, and was not affected by soil P supply or rhizobia inoculation. Phosphorus fertilization increased nodule number and activity when rhizobium inoculum was applied. Shoot dry weight and length of tap and lateral roots increased with P fertilization. Fertilizer P required to achieve the Mehlich-1 critical soil P level (1.5 μg cm−3 for optimum shoot dry weight corresponds to 54 μg cm−3. Mycorrhizae inoculum had a negative effect on lateral root length. Nitrogen and P uptake increased with P fertilization, but neither variable was significantly affected by mycorrhizae or rhizobium inoculation. Potassium uptake was increased by both P fertilization and mycorrhizae inoculum. The marginal response in the growth and nutrient uptake by G. sepium to mycorrhizae inoculation, relative to lime and P fertilization, indicates that this species is moderately dependent on VAM associations. The VAM dependency value for G. sepium , with the mycorrhizae species tested, never exceeded 35% among all treatments.