FACILITATION OF PLANT PHOSPHATE ACQUISITION BY ARBUSCULAR MYCORRHIZASFROM ENRICHED SOIL PATCHES .2. HYPHAE EXPLOITING ROOT-FREE SOIL

Effects of arbuscular mycorrhizas (AM) on plant exploitation of soil n utrient heterogeneity were studied with nonmycorrhizal and mycorrhizal Agropyron desertorum (Fisch. ex Link) Schult. in two-compartment cont ainers. A central cylindrical plant compartment was separated from an outer hyphal compartment by two layers of stainless-steel screen with a 2 mm air gap between the screen layers. Patchy or uniform nitrate (N O3-) and phosphate (P) distribution patterns were created in the outer compartment. Only AM hyphae could cross the double-screen barrier to access those nutrients. Mycorrhizal plants acquired significantly more labelled P in both the patchy- and the uniform-nutrient treatments th an did non-mycorrhizal plants. Mycelia in root-free soil delivered sim ilar amounts of P from the more distant rich patches to mycorrhizal pl ants as from the uniform and more proximate labelling. The uptake of a more mobile and abundant element, nitrate, was not affected significa ntly by either mycorrhizal infection or by nutrient distribution patte rns in the root-free soil. Despite a lower root:shoot mass ratio, myco rrhizal plants had significantly greater shoot phosphorus concentratio n than did nonmycorrhizal plants. There was no significant difference in P uptake or phosphorus concentration between the two nutrient distr ibution patterns for mycorrhizal plants, indicating that AM hyphae can explore the root-free soil for available P and transport it to host p lants equally well when P was distributed in either patchy or uniform patterns in the root-free soil.