PHYSICAL EROSION BENEATH THE SURFACE: QUANTIFYING COLLOIDAL GAINS AND LOSSES IN SOIL

Physical erosion is typically considered a process that influences soils and landscapes at the surface, but particulate matter also moves through soil macropores as colloids suspended in soil water. Such a process is highly size-selective and influences soil development differently than redistribution of bulk material. Subsurface transport of suspended solids is likely to be particularly important where parent materials with abundant sand-sized, resistant minerals (e.g. quartz) form soils with high infiltration capacities that reduce surface runoff. In such cases, the resistant grains can form a less-mobile skeleton through which plasma, the combined dissolved and suspended load in soil water, can move and exert influence on soil development. A new mass balance model has been developed to quantify such redistribution of colloids over the course of pedogenesis. Quantification is achieved by using ratios of the low solubility, high field strength elements titanium and zirconium in colloids, parent material, and soil. Due to the challenges of in situ sampling, laboratory protocols were developed to disperse colloids from soil for chemical characterization. The model was applied to a series of granitic soils on a hillslope in South Africa where slow physical erosion has yielded an exceptionally well-differentiated catena. The model shows losses of colloidal material up to 110 kg m⁻² from sandy, eluviated soil in upslope positions. Clay-rich, illuviated soil has gained up to 170 kg m⁻² of colloidal material in downslope positions. Mass losses via solution were generally greater and ranged from 1420 kg m⁻² to 200 kg m⁻². Dissolved versus suspended fluxes of individual elements can be similarly distinguished by the model, yielding insights into how weathering and soil development mobilize low solubility elements such as aluminum, titanium and zirconium.