Swell ways to measure how plant roots, biological exudates and temporal weathering impact soil structure and infiltration characteristics

Time results in large changes to soil infiltration characteristics due to weather, mechanical stability and the action of biology.  Even as the water status changes in a wetting soil, swelling may alter infiltration characteristics. Our laboratory has developed several novel approaches to measure how soil water infiltration characteristics vary over time and are influenced by biological processes or weathering stresses.  The measurements are often combined with an assessment of mechanical properties and pore structure so that underlying processes driving soil structure dynamics can be disentangled. An overview and a discussion of the benefits and challenges of the approaches will be provided.

A small-scale infiltrometer (sub-mm size) was adapted to allow for measurements of water infiltration and repellency at aggregate or rhizosphere scale.  It has been applied in numerous studies exploring the impacts of biological exudates, plant roots and weathering.  More recent research has compared results from this infiltrometer with X-Ray CT imaging to determine the impacts of soil pore structure on infiltration characteristics.  A challenge with a small-scale infiltrometer is experimental error caused by tip contact with the soil and the shape of the wetting front.  This has been demonstrated from repeated tests on repacked sands and sieved soils.

If soil aggregates, spatial variability or hot spots like the rhizosphere are not of interest, conventional infiltration measurements with flow across the entire surface of a soil core offer less laboratory experimental error.  We used this approach to explore the dynamics of soil wetting and swelling as affected by a range of biological exudates.  Repacked soil discs were wetted by a sintered disc attached to a weighed water reservoir, with swelling measured dynamically in horizontal and vertical directions using infra-red sensors.  Whereas polygalacturonic acid (PGA) had no affect on sorptivity, increasing concentrations of lecithin and actigum decreased sorptivity, likely due to different mechanisms of surface tension and viscosity respectively.  Total swelling was positively correlated with water sorptivity for both lecithin and actigum, suggesting that an expanding pore structure in the unconfined soil discs may enhance water uptake rates.  Biological exudates therefore have dual impacts on decreasing wetting and swelling rates, which will affect soil structural stability.

Current research is exploring soil structural stability impacts on soil hydrological properties over time.  This includes field studies exploring the impacts of soil amendments and management practices, and laboratory studies with controlled structural changes from wetting/drying and mechanical stresses.  In this work, changes in water infiltration due to stresses are explained from pore structure analysis with X-Ray CT imaging and mechanical stability tests.