Effects of VAM on the growth and nutrition of sugarcane

A series of pots using a moderate P status Tully-Coom soil from Tully was established at The University of Queensland to determine the response of sugarcane cv. Q124 to three rates of fertiliser P (0, 50, 500 kg P ha-1 equivalent) and 4 V AM isolates extracted from a sugarcane soil and applied at 4 spores g-l soil. A set of nonmycorrhizal control pots was also established. After 13 weeks, mycorrhizal plants grown in the absence of fertiliser P had 14-25% greater shoot biomass and 77% greater mean P concentration in leaf tissue than non-mycorrhizal controls. When 50 or 500 kg P ha-1 were added, differences in shoot biomass between mycorrhizal and non-mycorrhizal plants were either negligible (1%) or negative (-1% to -12%). Tissue P concentrations of mycorrhizal plants improved by 8% in comparison to nonmycorrhizal plants in all except one isolate when P was added at 50 kg P ha-1, but were less than those of non-mycorrhizal plants at the highest P rate. Mean infection of fine roots remained at 50% with 0 or 50 kg P ha-1 applied, but dropped below 4% with 500 kg P ha-1. These results indicate V AM can playa beneficial role where sugarcane plants are unable to access sufficient P for maximum growth, and that excessive applications of fertiliser P can lead to yield reductions in the short term.