Colloidal Clay Redistribution: Quantifying a Pedogenic Process and Its Influence On Ecosystems.

Few properties are as fundamental to the functioning of soil as clay content and distribution. The movement of clays as colloids suspended in soil water is also crucial to our understanding of pedogenesis. Until recently, assessment of this episodic process was generally limited to physical measurement of clay film accumulation, however, a new mass-balance model allows the redistribution of colloidal clays to be tracked over the course of pedogenesis. Quantification is based upon the different ratios of the low solubility, high field strength elements titanium and zirconium in colloidal clays, rock parent material, and soil. The model was applied to a granitic catena in South Africa where soil properties strongly influence ecosystem properties. Upslope, sandy, eluvial soils support broad-leaved trees and grasses with low nutrient content. Downslope, clay-rich, illuvial soils support fine-leaved trees and grasses with high nutrient content. The model shows losses of colloidal clay up to 110 kg m⁻² in eluviated soil. Downslope illuviated soil has gained up to 169 kg m⁻² colloidal clay. Colloidal mass fluxes were generally smaller than mass fluxes as dissolved elements. Mass loss by solution ranged from 1418 kg m⁻² to 195 kg m⁻². Fluxes of individual elements can be similarly distinguished. For example, up to 14% of aluminum loss can be attributed to colloids, and gains of colloidal aluminum in illuvial horizons have offset losses by dissolution. Because the new model relies heavily on the chemical composition of colloidal material, we have conducted experiments to compare the composition of colloids dispersed in the laboratory versus those sampled in situ. We have also mapped element distributions in granitic parent minerals to better understand the mobilization of titanium and zirconium, elements traditionally considered immobile. These efforts set the stage for wider application of the new model to understand this fundamental pedogenic process.