Effects of soil pH on the ectomycorrhizal response of Eucalyptus urophylla seedlings

To examine the effects of soil pH on ectomycorrhizal formation and function on Eucalyptus urophylla S. T. Blake, seedlings inoculated with nine ectomycorrhizal fungi (seven isolates of Pisolithus spp., Scleroderma cepa and Laccaria laccata collected under eucalypt stands in Australia and the Philippines) were transplanted into pots containing a non-sterile acid (pH 4.6) sandy loam amended with four levels of CaCO3 that raised the soil pH from 4.6 to 6.6 (5 mM CaCl2). Pots were placed in temperature-controlled water baths (28 +/- 2 degrees C) inside an evaporatively cooled glasshouse for 9 wk. Increase in soil pH from 4.6 to 6.6 significantly decreased plant d. wt and shoot nutrient content of uninoculated and inoculated seedlings. Inoculation with four Pisolithus spp. (H445, H2144, M56 and H4003) significantly increased the growth of E. urophylla seedlings at pH 4.6. At pH 6.6, eight ectomycorrhizal isolates significantly improved total d. wt compared with those of the uninoculated seedlings. Pisolithus isolates stimulated seedling growth more than L. laccata whereas S. cepa was ineffective at all pH levels. Total d. wt of H445 inoculated plants grown in P-deficient (8 mg P kg(-1) soil) soil was 147 % more than that of uninoculated plants given the same P rate and was 70 % that of plants fertilized with 64 mg P kg(-1) soil (P64) at pH 4.6. At soil pH 5.8 and 6.6, M56 was the best growth-promoter for E. urophylla. These results indicate that soil pH can significantly alter the development and function of ectomycorrhizal fungi. Soil pH did not significantly affect mycorrhizal formation by the different ectomycorrhizal fungi. However, the percentages of mycorrhizal root tips formed by the different ectomycorrhizal fungi differed significantly. Pisolithus isolate H445 formed the highest percentage of colonized roots and highest total d. wt at pH 4.6 and 5.2, implying its potential for commercial use in acidic conditions.