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Physical Review E

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Roots at the percolation threshold

Eva Kroener^*, Mutez Ali Ahmed^†, and Andrea Carminati

Division of Soil Hydrology, Department of Crop Science, University of Göttingen, Göttingen, Germany

^*ekroene@gwdg.de
^†Permanent address: Department of Agricultural Engineering, Faculty of Agriculture, University of Khartoum, Khartoum, Sudan.

Phys. Rev. E 91, 042706 – Published 13 April, 2015

DOI: https://doi.org/10.1103/PhysRevE.91.042706

Abstract

The rhizosphere is the layer of soil around the roots where complex and dynamic interactions between plants and soil affect the capacity of plants to take up water. The physical properties of the rhizosphere are affected by mucilage, a gel exuded by roots. Mucilage can absorb large volumes of water, but it becomes hydrophobic after drying. We use a percolation model to describe the rewetting of dry rhizosphere. We find that at a critical mucilage concentration the rhizosphere becomes impermeable. The critical mucilage concentration depends on the radius of the soil particle size. Capillary rise experiments with neutron radiography prove that for concentrations below the critical mucilage concentration water could easily cross the rhizosphere, while above the critical concentration water could no longer percolate through it. Our studies, together with former observations of water dynamics in the rhizosphere, suggest that the rhizosphere is near the percolation threshold, where small variations in mucilage concentration sensitively alter the soil hydraulic conductivity. Is mucilage exudation a plant mechanism to efficiently control the rhizosphere conductivity and the access to water?

Article Text

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