Nonequilibrium water dynamics in the rhizosphere: How mucilage affects water flow in soils

Abstract

The flow of water from soil to plant roots is controlled by the properties of the narrow region of soil close to the roots, the rhizosphere. In particular, the hydraulic properties of the rhizosphere are altered by mucilage, a polymeric gel exuded by the roots. In this paper we present experimental results and a conceptual model of water flow in unsaturated soils mixed with mucilage. A central hypothesis of the model is that the different drying/wetting rate of mucilage compared to the bulk soil results in nonequilibrium relations between water content and water potential in the rhizosphere. We coupled this nonequilibrium relation with the Richards equation and obtained a constitutive equation for water flow in soil and mucilage. To test the model assumptions, we measured the water retention curve and the saturated hydraulic conductivity of sandy soil mixed with mucilage from chia seeds. Additionally, we used neutron radiography to image water content in a layer of soil mixed with mucilage during drying and wetting cycles. The radiographs demonstrated the occurrence of nonequilibrium water dynamics in the soil-mucilage mixture. The experiments were simulated by numerically solving the nonequilibrium model. Our study provides conceptual and experimental evidences that mucilage has a strong impact on soil water dynamics. During drying, mucilage maintains a greater soil water content for an extended time, while during irrigation it delays the soil rewetting. We postulate that mucilage exudation by roots attenuates plant water stress by modulating water content dynamics in the rhizosphere.

Plant exudates improve the mechanical conditions for root penetration through compacted soils

“…Chia seed exudate has been widely used in other studies as a model root exudate (Ahmed et al 2014;Kroener et al 2014). It was extracted based on Naveed et al (2017) and Ahmed et al (2014) by mixing 100 g distilled water with 10 g chia seeds using a magnetic stirrer for 2 min at 50°C, followed by cooling to room temperature (20°C) and 4 h standing.…”

Effects of grass roots on soil-water retention curve and permeability function

“…The fourth would be related to organic carbon released by plant roots in the form of polymeric gel (the so-called root exudation, mucilage, or rhizodeposition) which were reported to decrease the mobility of clay colloids and enhance water retention around the roots (e.g. Grayston et al 1997;Traoré et al 2000;Kroener et al 2014;Tian et al 2015;Ghezzehei and Albalasmeh 2015). Recent work by Bengough (2012) shows that some of these root exudations exhibit hydrophobic or hydrophilic behavior, depending on the soil water content, causing the hysteretic-like behavior.…”

Water percolation through the root-soil interface

“…Young [1995] came to a similar conclusion after measuring a higher water content in the rhizosphere of wheat compared to the adjacent bulk soil. Recently, Ahmed et al [2014] and Kroener et al [2014] showed that mucilage exuded by chia seeds increases the water content of a sandy soil at any water potential if the system is in hydraulic equilibrium.…”