The rheology of rhizosphere formation by root exudates and soil microbes

One of the major inputs driving the formation of soil structure is exudation from plant roots. We used approaches from rheology to test the hypothesis that root exudates initially disperse soil, thus easing root penetration and releasing nutrients, followed by gelling (i.e. aggregation) if the compounds are transformed by soil microbes. Soils were amended with 0, 1.5 and 15 mg/g soil of root-exudate compounds consisting of a mix of sugars, amino acids and organic acids and incubated for 13 days at either 2oC, to reduce microbial action, or 16oC. Incubation at 2oC significantly suppressed respiration rates compared to 16oC, suggesting microbial processes were impaired. A parallel plate rheometer was used to quantify the rheological behaviour of the incubated soils amended with root-exudate compounds. The drop in flow point (stress where breakdown occurs) at 2oC incubation and rise in flow point at 16oC incubation suggests dispersion by the root exudates followed by gelling if root exudates are transformed by microbes.