Physical response of rigid and non-rigid soils to analogues of biological exudates

Exudates produced by plants and microorganisms can alter greatly the physical behaviour of soil. There is limited research that quantifies directly the underlying hydrological and mechanical mechanisms concerned, and so in this study we amended soils with a range of analogue biological exudate compounds with different physical and chemical properties: polygalacturonic acid (PGA), dextran, xanthan and lecithin. These were added to a structurally rigid soil (Plinthosol) and a non-rigid soil (Gleysol) that were formed as repacked cores and exposed to five cycles of wetting and drying (WD). Aggregate stability, tensile strength, water sorptivity and water repellency were measured initially and after the first, third and fifth WD cycle. Improved aggregate stability was only found for some exudates and differed between the soils. Xanthan had the greatest impact on aggregate stability, causing a 95% increase in the Plinthosol and 75% increase in the Gleysol. Xanthan also caused the greatest increase in tensile strength (50% in the Plinthosol and 148% in the Gleysol) but had minimal impact on water repellency in both soils, indicating mechanical stabilization. Lecithin reduced tensile strength but caused the greatest increase in water repellency, indicating hydrological stabilization. Both PGA and dextran had clear positive impacts on soil stability, but the underlying processes were not detected in the hydrological and mechanical tests. Increasing the number of WD cycles diminished aggregate stability, tensile strength and water repellency more rapidly in the non-rigid Gleysol than in the rigid Plinthosol. This study demonstrated that the effects of analogous biological exudates on aggregation and stabilization depend on the nature of exudate, the rigidity of soil structure and the number of WD cycles.