The effects of arbuscular mycorrhizas on soil aggregation depend on the interaction between plant and fungal species

• Arbuscular mycorrhizal fungi (AMF) and roots mediate soil stabilization, although the mechanisms and how their interactions affect soil stabilization are not known. We tested the effects of specific plant-fungus combinations on aggregate stabilization, and whether hyphal length and root biomass determine stabilization, predicting that fungi producing more hyphae, and plants with higher root biomasses, would better stabilize soils. • The percentage of water-stable aggregates (%WSA_{1-2 mm}), hyphal lengths, and root biomass were measured from a five AMF x nine plant factorial experiment. Arbuscular mycorrhizal fungi with greater extradical mycelium production were represented by the Gigasporaceae and plants of high root biomass by grasses. Other taxa represented lower hyphal lengths and root biomass. • An interaction between symbionts with respect to %WSA_{1-2 mm} was observed. Root biomass and total hyphal lengths were not positively correlated with %WSA. Combinations of grasses with Gigasporaceae fungi had the lowest %WSA. • Mechanisms underlying aggregation were not elucidated by measuring root biomass and total hyphal lengths alone, suggesting other physiological or architectural mechanisms may be responsible.