The role of root mucilage and Extracellular Polymeric Substances in shaping soil structure and maintaining plant-soil contact

The hydraulic properties of the rhizosphere are essential for understanding root-soil interactions. Diverging from common assumptions in soil modelling, which often equate rhizosphere properties with those of bulk soil, research shows that the rhizosphere is distinct in its physical, biological, and chemical attributes.

There is a broad agreement on the role of mucilage and extracellular polymeric substances (EPS) in modifying soil water dynamics in the rhizosphere. However, the mechanisms of how these substances interact with the soil matrix and impact its hydraulic properties remain unclear. In this study, we assessed the forces exerted by Xanthan gum, used as a stand-in for EPS, maize root mucilage, and water - formed liquid bridges on particle pairs.

Forces were quantified for 1 microL liquid bridge between a pair of glass beads — one standing on a precision balance and the other fixed to a static stand. While the water bridges broke upon drying due to capillary forces, mucilage and Xanthan gum formed resilient filaments that maintain connectivity and tensile forces between the glass beads. The continuous recordings of weight changes by the balance provided crucial data for quantifying the force exerted on the beads during drying.

Our results show that both Xanthan gum and maize mucilage liquid bridges exert tensile strengths that are substantially greater than those of water bridges. The polymer solutions initially behave similarly to water, but the forces exerted on particle pairs deviate as the solutions dry, becoming progressively stronger. The tensile strength of water reaches around 10^-1 mN, while maize and Xanthan gum are respectively 1 and 2 order of magnitudes bigger. This increase is caused by the stretching of the polymer network and the development of elastic forces.

The significant aggregating force observed in our study suggests that EPS and mucilage play a crucial role on the mechanics of the root-soil interface. They contribute to soil structure formation in the rhizoshere and to maintain root and soil contact as roots shrink in drying soils.