Response of Acacia mangium to vesicular arbuscular mycorrhizal inoculation, soil pH, and soil P concentration in an oxisol

The vesicular-arbuscular mycorrhizal (VAM) fungus Glomus aggregatum and the legume species Acacia mangium were grown together in a manganese-rich oxisol at pH 4.3 to 6.0 and at soil P concentrations favorable for VAM-host growth (0.02 mg . L(-1)) and sufficient for non-VAM host growth (0.8 mg . L(-1)). At the lower P concentration, vesicular-arbuscular mycorrhizal fungal (VAMF) colonization of roots increased as soil pH increased from 4.3 to 5.0. However, colonization of roots was not significantly influenced by further increases in pH. Growth of A. mangium at 0.02 mg P/L in the presence of G. aggregatum was inferior to that observed at 0.8 mg P/L, suggesting that there was some degree of host-endophyte incompatibility. Increasing P concentration from 0.02 to 0.8 mg P/L increased target soil pH in the unlimed soil from 4.3 to 4.8 and reduced the concentration of available soil Mn from 13.2 to 2.1 mg . L(-1). Thus, the better plant growth observed at the higher P concentration at pH < 5 was mainly due to the alleviation of Mn toxicity due to the precipitation of the cation directly by excess phosphate and (or) phosphate-induced elevation of soil pH. The poor VAMF inoculation effect observed at the lower soil P level in the unlimed soil was thus largely due to the toxicity of high concentrations of Mn2+ and H+ ions.