Using rhizosphere-scale physical measurements we test the hypothesis that plant exudates gel together soil particles and on drying they enhance soil water repellency. Barley and maize root exudates were compared with chia seed exudate, a commonly used root exudate analogue. Sandy loam and clay loam soils were treated with root exudates at 0.46 and 4.6 mg exudate g-1 dry soil, and chia seed exudate at 0.046, 0.46, 0.92, 2.3 and 4.6 mg exudate g-1 dry soil. Soil hardness and modulus of elasticity were measured at -10 kPa matric potential using a 3 mm diameter spherical indenter. Water sorptivity and repellency index of air-dry soil were measured using a miniaturized infiltrometer device with a 1 mm tip radius. Soil hardness increased by 28% for barley root exudate, 62% for maize root exudate, and 86% for chia seed exudate at 4.6 mg g-1 concentration for sandy loam soil. For a clay loam soil, root exudates did not affect soil hardness, whereas chia seed exudate increased soil hardness by 48% at 4.6 mg g-1concentration. Soil water repellency increased by 48% for chia seed exudate and 23% for maize root exudate, but not for barley root exudate at 4.6 mg g-1 concentration for sandy loam soil. For clay loam soil, chia seed exudate increased water repellency by 45%, whereas root exudates did not affect water repellency at 4.6 mg g-1concentration. Water sorptivity and repellency were both correlated with hardness, presumably due to the combined influence of exudates on hydrological and mechanical properties of soils.
Naveed, M., Brown, L. K., Raffan, A. C., George, T. S., Bengough, A. G., Roose, T., Sinclair, I., Koebernick, N., Cooper, L., & Hallett, P. D. (2018). Rhizosphere-scale quantification of hydraulic and mechanical properties of soil impacted by root and seed exudates. Vadose Zone Journal, 17(1). https://doi.org/10.2136/vzj2017.04.0083