The role of the external mycelial network in the effect of soil disturbance upon vesicular-arbuscular mycorrhizal colonization of maize.

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This study examined the mechanism by which vesicular-arbuscular (VA) mycorrhizal colonization of young maize (Zea mays L.) plants is inhibited by soil disturbance. Attention focussed upon the role of the external mycelial network. Plastic cylinders were divided into a central and two outer compartments by a fine nylon mesh screen which was impervious to roots but not to mycorrhizal hyphae. Soil was packed into each compartment and maize plants grown in the outer compartments over a series of 3-week cycles, leaving root-free central sections. Prior to a final growth cycle, the root-free soil in the central compartment was either disturbed and repacked or left undisturbed. In a third treatment the hyphal connections between the outer and central sections were severed, while leaving the soil in all sections undisturbed. Plants were then grown in the central compartments. Mycorrhizal colonization and shoot P and Zn concentrations of plants grown in the root-free zones were substantially reduced by disturbance of the soil. The reduction in colonization was similar to that found in control pots in which the root system and mycelial network were disturbed. This suggested that destruction of the hyphal network is an important component in the disturbance phenomenon. Removal of hyphal connections with roots in the outer compartments had no effect on colonization or nutrient absorption. Isolation of the mycelial network from the root system from which it had originated did not then seem to affect its infectivity. The mycelial network appears to be an important component of the inoculum potential of an undisturbed soil and it is likely that its destruction is directly responsible for much of the effect of soil disturbance upon mycorrhizal colonization. Evidence is presented indicating that the undisrupted mycelial network may also increase the nutrient absorption capacity independent of the degree of colonization.