Moisture retention properties of a mycorrhizal soil

The water relations of arbuscular mycorrhizal plants have been compared oft en, but virtually nothing is known about the comparative water relations of mycorrhizal and nonmycorrhizal soils. Mycorrhizal symbiosis typically affe cts soil structure, and soil structure affects water retention properties; therefore, it seems likely that mycorrhizal symbiosis may affect soil water relations. We examined the water retention properties of a Sequatchie fine sandy loam subjected to three treatments: seven months of root growth by ( 1) nonmycorrhizal Vigna unguiculata given low phosphorus fertilization, (2) nonmycorrhizal Vigna unguiculata given high phosphorus fertilization, (3) Vigna unguiculata colonized by Glomus intraradices and given low phosphorus fertilization. Mycorrhization of soil had a slight but significant effect on the soil moisture characteristic curve. Once soil matric potential (Psi (m)) began to decline, changes in Psi (m) per unit change in soil water con tent were smaller in mycorrhizal than in the two nonmycorrhizal soils. With in the range of about -1 to -5 MPa, the mycorrhizal soil had to dry more th an the nonmycorrhizal soils to reach the same Psi m. Soil characteristic cu rves of nonmycorrhizal soils were similar, whether they contained roots of plants fed high or low phosphorus. The mycorrhizal soil had significantly m ore water stable aggregates and substantially higher extraradical hyphal de nsities than the nonmycorrhizal soils. Importantly, we were able to factor out the possibly confounding influence of differential root growth among my corrhizal and nonmycorrhizal soils. Mycorrhizal symbiosis affected the soil moisture characteristic and soil structure, even though root mass, root le ngth, root surface area and root volume densities were similar in mycorrhiz al and nonmycorrhizal soils.