The physics of the rhizosphere and its implications for soil and plant sciences

The increasing demand of food production and the consequent pressure on soil and water resources are the motivation to identify mechanisms improving the ability of plants to take up water and nutrients from soils, in particular when these resources are scarce. Plants can adapt to water shortage by growing deeper roots that are capable to extract water stored in the subsoil. An alternative adaptation strategy of roots consists in the modification of the soil in their vicinity, the rhizosphere.

Plant roots exude up to 10% of the carbon assimilated through photosynthesis into the soil, a process referred to as rhizodeposition. The carbon exuded into the soil helps roots to take up nutrients and promotes positive feedbacks between plants and microorganisms. I will discuss how the mucilaginous fraction of the rhizodeposits, referred to as mucilage, alters the soil physical properties playing a crucial role on soil-plant water relations.

Mucilage is a gel that can absorb large volumes of water, altering the physical properties of the rhizosphere and maintaining the rhizosphere wet and conductive when the soil dries. Acting as a hydraulic bridge between roots and the soil, mucilage facilitates root water uptake and maintains transpiration and photosynthesis in dry soils. Mucilage contains also a small fraction of lipids, which decrease the surface tension of mucilage and cause mucilage hydrophobicity upon drying. Mucilage hydrophobicity results in a temporarily water repellence of the rhizosphere and in a slow rewetting of the rhizosphere compared to the adjacent bulk soil.

The high water absorbing capacity of mucilage and its temporarily hydrophobicity after drying make of the rhizosphere a region with a dual behavior: mucilage increases the soil hydraulic conductivity during drying, but it limits water flow during rewetting. I suggest that such a dual behavior helps the plants to reversibly up- and down-regulate the water flow across the root-soil interface and it is a plant strategy to adapt to the heterogeneous and varying soil conditions. In conclusion, I propose that mucilage exudation is a plant trait that confers drought tolerance.

Additionally, I expect that the complex, hysteretic and unique physical properties of the rhizosphere have also an impact on the availability and diffusion of nutrients and on the spatial distribution and activity of soil microorganisms.

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Invited by Prof. Matzner, Soil Ecology

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