Vast amounts of water flow through a thin layer of soil around the roots, the rhizosphere – an important hydrological hotspot. The rhizosphere was shown to turn water repellent upon drying, which has been interpreted as the effect of root and/or microorganism exudates, in particular root mucilage. Our understanding of the biophysical mechanisms controlling the rhizosphere water repellency remains largely speculative. Our hypothesis is that the key to describe the emergence of water repellency lies within the microscopic distribution of wettability on the pore-scale. At a critical mucilage concentration, a sufficient fraction of pores is blocked and the rhizosphere turns water repellent. A percolation approach is here introduced to predict the flow behaviour near this critical concentration.
Material and Methods
We performed WDPT (water drop penetration time) tests to quantify the wettability of glass beads (0.1-0.2 mm diameter) mixed with different amounts of mucilage. We simulated the process of water drop infiltration using a simplified pore-network model. Mucilage was distributed heterogeneously in the pores.
Results & Conclusions
The model matched well the measured WDPT data and captured the high variability in infiltration rates near the percolation threshold, when samples switched from wettable to water repellent. Our study highlights the importance of a non-uniform pore-scale distribution of contact angles in the emergence of water repellency in soils affected by root exudates.
Sand particles (0.5-0.63mm in diameter) mixed with wet mucilage at a dry mucilage content of 0.8 mg per g of sand particles. The sample was air dried and mucilage deposits were stained with blue ink. A stained filament of dry mucilage (blue) connects adjacent sand particles.