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Aggregation of a Silty Clay Loam Soil by Mycorrhizal Onion Roots¹

ABSTRACT

Onion plants (Allium cepa L.) inoculated in the root zone with a vesicular-arbuscular mycorrhizal (VAM) fungus, or left uninoculated, were grown in potted soil for 230 d to determine the influence of the VAM fungus (Glomus macrocarpum Tul. and Tul.) on soil structure. The silty clay loam soil was maintained at a moisture content between 25 and 30%. Paired inoculated (+M) and uninoculated (–M) plants were harvested (20 pairs over 150 d) beginning 80 d after planting. Relationships between plant, fungal, and soil parameters and changes with time were evaluated by regression analysis. Root colonization by the VAM fungus in +M plants ranged from 49% to 60% over the sampling period, while no mycorrhizae were detected in the -M plants. Total dry mass of VAM plants was five to six times that of the non-VAM plants. Soil from the +M treatment was significantly better aggregated, more porous, and had greater water permeability than -M soil. Root dry mass and VAM hyphal density in the +M soil were both significantly correlated with the relative abundance of water-stable soil macroaggregates. Correlation of root mass with aggregate abundance was stronger, however, suggesting that soil changes were mainly mediated by direct root effects of a host plant whose growth was stimulated dramatically by its VAM fungal endophyte.

¹ Contribution from the Western Regional Research Center, USDA-ARS, Albany, CA 94710. Presented to Am. Assoc. Adv. Sci. at Los Angeles, CA, May 1985.

² Research physicist, soil scientist, soil microbiologist, research chemist and supervisory plant physiologist. Present address of S. Dakessian: Harding Lawson Associates, 7655 Redwood Blvd., P. O. Box 578, Novato, CA 94948. Requests for reprints should be sent to G. J. Bethlenfalvay.

Received for publication December 26, 1985.