Trace element toxicity in VA mycorrhizal cucumber grown on weathered coal fly ash

Mycorrhizal colonization is widely recognized as enhancing plant growth on severely disturbed sites. A greenhouse pot experiment was conducted to determine if inoculation with vesicular-arbuscular mycorrhizal (VAM) fungi will enhance vegetation establishment on abandoned coal fly ash basins. Spores of Glomus intraradices (Schenck and Smith) and Glomus etunicatum (Becker and Gerdemann) were added to weathered precipitator ash (EC = 0.91 dS m(-1); pH 5.0) and to a pasteurized soil of the same pH (Grossarenic Paleudult, 92% sand, 1% organic matter). Some soil and ash were left unamended as non-mycorrhizal controls. Cucumber (Cucumis sativus L. cv. Poinsette 76) seeds were sown, watered regularly, and fertilized periodically with macro-nutrient solution. By 8 weeks old, all ash-grown plants exhibited smaller leaves with leaf margin curl and necrosis, and plant biomass was significantly less (0.75 X) than soil-grown plants. Based on analysis of 18 elements in plant tissues, toxicity to B, Mn or Zn could have caused growth suppression, confirming trace element problems for plant growth on fly ash. For plants grown on fly ash, G. etunicatum was the only fungus that colonized roots (20% of root length reduced from 67% on soil) and it suppressed plant growth to 0.80 X that of uninoculated ash-grown plants. Correspondingly, shoot Zn concentration in G. etunicatum -inoculated plants was 3.5 X higher than in uninoculated plants and at generally toxic levels (273 mg kg(-1)). Glomus etunicatum had no other significant effects on elemental concentrations. These results indicate that VAM colonization in acid, weathered fly ash suppressed plant-growth by facilitating uptake of Zn to toxic levels, and implies a limitation to successful use of VAM for vegetation establishment on abandoned coal fly ash basins.