A Model of Root Water Uptake Coupled with Rhizosphere Dynamics

Water flow from soil to roots is controlled by the hydraulic behavior of the soil near the roots, the rhizosphere. Recent experiments showed that during drying, the rhizosphere held more water than the bulk soil. After irrigation, the rhizosphere remained temporarily dry, and it slowly rewetted after a few days. The hypothesis is that the observed hydraulic behavior was caused by mucilage exuded by roots. Mucilage is a polymeric material that is capable of holding a large amount of water but that contains also lipids that makes it hydrophobic when it dries. A model of root water uptake coupled with swelling and shrinking of mucilage id proposed. Water flow is modeled solving the Richards equation in a domain composed of two concentric regions, bulk soil and rhizosphere, with the root in the center as a boundary condition. It is assumed that during drying, mucilage is in equilibrium with the bulk water. After irrigation, mucilage does not rehydrate immediately, and water content and water potential in the rhizosphere are decoupled. The rhizosphere rewetting rate is assumed to be proportional to the difference between the water potential in the rhizosphere and the potential that the rhizosphere would have if it was in equilibrium at the actual water content. The proportional coefficient depends on mucilage properties and pore characteristics. The model reproduced well the observed water dynamics in the rhizosphere. According to this model, the rhizosphere conductivity is not a unique function of soil water potential, but it is variable and depends on the drying and wetting history. The study illustrates the dynamic and interacting nature of water flow to roots.