Pore‐Scale Distribution of Mucilage Affecting Water Repellency in the Rhizosphere

CORE IDEAS: During drying, mucilage is preferentially deposited in small pores. This microscopic heterogeneity critically affects macroscopic wettability. Infiltration is impeded when a critical fraction of pores is blocked by dry mucilage. Dry mucilage bridges are shaped like hollow cylinders connecting particles. The physical properties of the rhizosphere are strongly influenced by root‐exuded mucilage, and there is increasing evidence that mucilage affects the wettability of soils on drying. We introduce a conceptual model of mucilage deposition during soil drying and its impact on soil wettability. We hypothesized that as soil dries, water menisci recede and draw mucilage toward the contact region between particles. At low mucilage contents (milligrams per gram of soil), mucilage deposits have the shape of thin filaments that are bypassed by infiltrating water. At higher contents, mucilage deposits occupy a large fraction of the pore space and make the rhizosphere hydrophobic. This hypothesis was confirmed by microscope images and contact angle measurements. We measured the initial contact angle of quartz sand (0.5–0.63‐ and 0.125–0.2‐mm diameter), silt (36–63‐μm diameter), and glass beads (0.1–0.2‐mm diameter) mixed with varying amounts of chia (Salvia hispanica L.) seed mucilage (dry content range 0.2–19 mg g⁻¹) using the sessile drop method. We observed a threshold‐like occurrence of water repellency. At low mucilage contents, the water drop infiltrated within 300 ms. Above a critical mucilage content, the soil particle–mucilage mixture turned water repellent. The critical mucilage content decreased with increasing soil particle size. Above this critical content, mucilage deposits have the shape of hollow cylinders that occupy a large fraction of the pore space. Below the critical mucilage content, mucilage deposits have the shape of thin filaments. This study shows how the microscopic heterogeneity of mucilage distribution impacts the macroscopic wettability of mucilage‐embedded soil particles.