Root- and microbial-derived mucilages affect soil structure and water transport

The production of exudates by plant roots and microbes in the rhizosphere, together with intense wetting and drying cycles due to evapotranspiration, stimulate changes in soil structure. We have attempted to separate these two processes using an experimental model with bacterial exopolysaccharides (dextran and xanthan) and root mucilage analogues (polygalacturonic acid, PGA), and up to 10 cycles of wetting and drying. To characterize the soil structure, tensile strength, water sorptivity and ethanol sorptivity of the amended soils were measured, and thin sections were made. Xanthan and PGA induced greater tensile strength of the amended soil, suggesting that they increased the bond energy between particles. Porosity increased with each cycle of wetting and drying, and this increase was less pronounced for the PGA 2 g l(-1) than for the xanthan and dextran. This suggests that PGA stabilized the soil against the disruptive effect caused by the wetting and drying. The PGA was the only polysaccharide that influenced water sorptivity and repellency, resulting in slower wetting of the treated soil. Wetting and drying led to an increase of the sorptivity and a decrease of the repellency for all treatments with the exception of the PGA-amended soils. The PGA may therefore stabilize the soil structure in the rhizosphere by increasing the strength of bonds between particles and decreasing the wetting rate.