Rheological stabilization of wet soils by model root and fungal exudates depends on clay mineralogy

Changes in soil rheological behaviour by a root mucilage analogue (polygalacturonic acid, PGA) and a fungal polysaccharide (scleroglucan) were studied for a range of soils with different clay mineralogies. An oscillation recovery test was conducted with a parallel plate rheometer with stress steps of 10 Pa (linear viscoelastic), 500 Pa (resistance) and then 10 Pa (resilience). Five soils that were either dominated by 2:1 or 1:1 clay minerals were examined: a pure Ca montmorillonite; a 2:1 Cambisol; a 2:1 Luvisol; a 1:1 clay loam Ferralsol; and a 1:1 sandy loam Ferralsol. The viscosity of all 2:1-dominated samples increased with decreasing water content, ranging from 151 Pa s to 237 800 Pa s. The model biological exudates increased viscosity by up to 10-fold in some 2:1 clay-dominated soils at the wettest water contents, with the relative effect diminishing as soils dried. The viscosity of 1:1-dominated soils also increased with decreasing water content, ranging from 17 320 Pa s to 1 333 000 Pa s. In these soils, scleroglucan increased viscosity 22-fold for the 1:1 clay loam soil, but PGA had little impact. For the 1:1 sandy loam soil, neither exudate influenced viscosity. When a greater stress of 500 Pa was applied, viscosity decreased in all samples because of shear thinning. Scleroglucan maintained greater viscosity under this greater stress for the montmorillonite, 2:1 Luvisol and 1:1 clay loam, whereas PGA did not have a significant impact. After the stress was removed, only the scleroglucan-amended 1:1 clay loam recovered to viscosities greater than the other treatments. The increased viscosity and resistance caused by biological exudates will help to ‘drive’ soil structure formation, particularly in the rhizosphere.