Effects of nitrogen source and aluminum on growth of tropical tree seedlings adapted to low pH soils

The effects of N-source and Al on the growth of seedlings of Melastoma malabathricum, Acacia mangium, and Melaleuca cajuputi, which are tropical woody plants and are very tolerant to Al, and barley (Hordeum vulgare), which is a typical Al-sensitive plant, were investigated. The Al and N treatments consisted of the application of either 0 or 0.5 mM Al, and 2 mM NH4+ or NO3-, respectively. Growth of the tropical plants was enhanced by Al and NH4 application. In all the plant species, the pH of the culture solution decreased and the concentrations of soluble Al and P increased with the +NH4 treatment, which positively affected the growth of the tropical plant species. Excised roots of M. malabathricum dissolved insoluble Al with NH4 application and absorbed Al mainly from root tips. Al did not affect the leaf N concentration except in the case of barley. Roots of M. cajuputi exuded a large amount of citrate, which slightly increased by the +Al treatment. In A. mangium, the reactivity of soluble Al to PCV (pyrocatecholviolet) decreased in the culture solution of the +Al+NH4 treatment and Al concentration of roots in this treatment was very low. Roots of M. malabathricum released H+ along with Al uptake as well as NH4+ uptake. It is concluded that Al and NH4+ exert beneficial effects on the growth of tropical tree seedlings.