Rhizosphere effects on soil solution composition and mineral stability

Abstract

Rhizosphere properties are known to differ from those of bulk soil, but differences are not always predictable and may be influenced by the kinds of plant roots and by soil mineral composition. Parent material primary minerals have a great influence on the soil secondary mineral suite, and the secondary phyllosilicates can influence soil solution composition. We hypothesized that 1) bulk soil solution and rhizosphere soil solution differ, 2) soil clay mineral Si content, hence parent material mineral composition, controls soil solution Al activity, 3) soil solution composition of soils acidified by agricultural fertilization practices differs from soil solution composition of soils acidified by long-term weathering, and 4) monocots and dicots differ in their rhizosphere effects. We compared the effects of a monocot (fescue, Vulpia myuros) and a dicot (tomato, Solanum lycopersicum) on rhizosphere solution composition in soils with a wide range of Si content. The greenhouse pot experiment was conducted using soils derived from parent materials of sialic, mafic, and mixed lithology. Differences between non-acidified, agriculturally-acidified, and naturally-acidic phases of soil within each parent material were compared so that a soil member of each parent material could be evaluated for its influence on rhizosphere chemistry. Mineral stability diagrams based on soil clay-fraction mineralogy for each parent material type served as guides to interpret rhizosphere effects on soil solution composition. These diagrams demonstrated that rhizosphere solution extracts are farther from equilibrium than are bulk soil solution extracts, that rhizosphere influences are greater in non-acidified than in agriculturally-acidified soils, and that the dicot changed the rhizosphere more than the monocot, relative to bulk soil. The sialic soil diagram suggests that conditions are not favorable for hydroxyl-Al interlayered 2:1 material (HIM) formation in this parent material, presumably because solution Si competes with 2:1 interlayer space for solution Al. The diagrams also suggest a montmorillonite→beidellite→kaolinite transformation in the mafic soil. We can draw no clear conclusions about which solid-phase sink controls solution Al in the non-acidified and agriculturally acidified mixed soils, but SRO aluminosilicate formation may attenuate the Al3+ activity in the naturally acidic soil.