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Separation of Vesicular-Arbuscular Mycorrhizal Fungus and Root Effects on Soil Aggregation

USDA-ARS Western Regional Research Center, Albany, CA 94710

R. L. Franson

Castle Mountain Venture, P.O. Box 68, Searchlight, NV 89046

G. J. Bethlenfalvay^*

USDA-ARS Horticultural Crops Research Lab., 3420 N.W. Orchard Ave., Corvallis, OR 97330

*Corresponding author.

ABSTRACT

Mycorrhizae influence soil stability, but relative contributions by plant or fungal endophyte to aggregation are little known. We studied the effects of both symbionts together and of each alone on waterstable soil aggregate (WSSA) formation. Split-root soybean [Glycine max (L.) Merr.] plants were grown in containers. One side of the split root was colonized by the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe; from here the VAM hyphase penetrated through a screen (44 – µ openings) to a root-free chamber. On the other side of a solid barrier, the non-VAM portion of the root was in contact through a screen with control soil free of roots and VAM hyphae. Thus, the four treatments, VAM roots (roots and hyphae), VAM hyphae, non-VAM roots, and control, were contained in the same experimental unit. Root length and mass were greater in the VAM-root than in the non-VAM-root chamber, whereas the density of VAM hyphae in the soil was lower in the VAM-root than in the VAM-hyphal chamber. The relative amount of WSSA was highest with VAM roots, lowest in the control chamber, and intermediate and similar with non-VAM roots and VAM hyphae. This incidence of WSSA was lower in all of the chambers at harvest (8 mo.) than in the starting material due to slaking. The composition of WSSA showed that the differences in WSSA between treatments was due to a variable slowing of the slaking process by the mycorrhizae. Non-VAM roots and VAM hyphae had similar effects on soil aggregation.

Received for publication June 8, 1992.

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