Quantifying Physical Properties That Underlie Soil Structure Dynamics: Case Studies On the Impacts of Roots and Biota.

In materials science and engineering, structural development and breakdown can be quantified and modelled from fundamental properties such as bond energy, fracture resistance and rheology. In this talk, these concepts will be discussed in the context of soil structure dynamics. The derived data are necessary to begin developing deterministic models of soil structure formation. Most of the studies described will examine the influence of biological exudates and tissues, as biology is recognised to have considerable influence on soil structure. One of the driving factors in soil structural breakdown is the resistance to fracture. This is described by fracture mechanics, which has shown that organic compounds typical of root exudates can double the fracture toughness of dry clays. Although the stability of dry soils represents an extreme condition where greatest susceptibility to breakdown by water is often observed, only the top few millimetres ever reaches this condition in nature. Elastoplastic fracture mechanics can describe crack resistance in wet soils, with a water potential flux from -0.5 kPa to -50 kPa doubling bond energy due to capillary cohesion and shrinkage. Capillary cohesion was measured directly in a controlled system using parallel rough surfaces that were pulled apart in tension. These data showed that reduced surface tension improved spreading on increasingly rough surfaces, so the expected relationship between surface tension and cohesion was not observed. Microbes will often reduce surface tension, but in similar tests with microbial exudates, adhesion by biological ‘glues' increased the stress required to pull apart the plates. Further testing, using rheological approaches, found biological exudates to increase viscosity over a wide range of water contents for numerous soils with different mineralogy. In wet soils, a localised viscosity increase of soil by root and microbial exudates could nucleate the formation of aggregates. This is particularly relevant to rhizosphere formation.