Roots at the percolation threshold

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?

Figure 1

Water content distribution in the root zone of a young lupine after irrigation as imaged with neutron radiography. The gray values are proportional to the water content (dark denotes wet). The radiograph shows that the water content in the rhizosphere was markedly drier than in the adjacent bulk soil. The image is taken from the experiments described in [15].

Figure 2

Nonequilibrium relation (1) between rhizosphere water content and water potential in the rhizosphere during the rewetting phase [16]. The water content recovers more slowly than the water potential, resulting in a deviation from the equilibrium curve.

Figure 3

Conceptual model of rewetting of the dry rhizosphere. (a) Illustration of water content in the rhizosphere during drying and after rewetting, (b) rewetting of a pore covered with dry mucilage, and (c) pore network model of bonds covered with mucilage (disconnected) and bonds not covered with mucilage (connected).

Figure 4

Bond percolation in a cubic lattice with varying percentage of disconnected pores. (a) Water-filled pores averaged across the sample and for a horizontal cross section. (b) Water-filled nodes as a function of disconnected bonds. (c) Water-filled nodes at the outflow as a function of disconnected bonds. Close to the percolation threshold a small change in the number of disconnected nodes strongly affects the hydraulic behavior.

Figure 5

Capillary rise in soil columns with a small layer of soil mucilage mixture as an analog of the rhizosphere. Among the soil columns particle size as well as mucilage concentration of the soil mucilage mixture varied: (a) fine sand (diameter of 0.125–0.2 mm), (b) medium sand (0.2–0.355 mm), and (c) coarse sand (0.63–1.0 mm). The gravimetric concentration (mass of dry mucilage per unit mass of dry soil) in the rhizosphere layer is given for each soil column.

Figure 6

Relation between particle diameter and threshold concentration (7). The gray region is related to the error in the measurement of $c_{\mathrm{hy}}$.