The capacity of biology to shift the physical behaviour of soil underlies a myriad of processes related to crop productivity, biodiversity and environmental protection.  There has been considerable research in the area, but much of this reports shifts in soil structure only, without information on the hydrological and mechanical processes that underlie these changes. 

This talk will describe some physical processes in soil that are influenced by biology.  Novel testing approaches and analyses are essential for much of this research. It starts with the seed.  Myxospermous seeds are covered with mucilage that hydrates rapidly and sticks to soil particles.  Rheological tests described the mucilage as a weak gel, with viscosity at 10% concentration 400000 times greater than water. Root exudates also impact the mechanical behaviour of soil.  Soils initially amended with model root exudates were found to have a decrease viscosity, suggesting a dispersion mechanism that may help release nutrients from soil particles.  Over time, however, the production of secondary metabolites by microbes from root exudates causes increased soil viscosity, suggesting aggregation.  Organic matter incorporation and its decomposition also impacts visco-plastic behaviour, as shown by viscosity shifts following the decomposition of added composts to field soils or a range of organic compounds added to soil in laboratory studies.  In soils amended with humic acid (HA), we found minimal shifts in viso-elastic behaviour up to 160 mg HA g-1 soil, but increased pH by NaOH amendment decreased viscosity.  At larger scales, plant roots mechanically stabilise soils by particle enmesment as reinforcing rods.  This drives mechanical stabilisation from the scale of macroaggregates through to entire hillslopes.  The talk will be concluded by discussing how these data can start to be assembled to develop quantitative models describing the impact of soil biology on hydrological and mechanical behaviour.

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