A mass-balance model to separate and quantify colloidal and solute redistributions in soil

Studies of weathering and pedogenesis have long used calculations based upon low solubility index elements to determine mass gains and losses in open systems. One of the questions currently unanswered in these settings is the degree to which mass is transferred in solution (solutes) versus suspension (colloids). Here we show that differential mobility of the low solubility, high field strength (HFS) elements Ti and Zr can trace colloidal redistribution, and we present a model for distinguishing between mass transfer in suspension and solution. The model is tested on a well-differentiated granitic catena located in Kruger National Park, South Africa. Ti and Zr ratios from parent material, soil and colloidal material are substituted into a mixing equation to quantify colloidal movement. The results show zones of both colloid removal and augmentation along the catena. Colloidal losses of 110 kg m−2 (− 5% relative to parent material) are calculated for one eluviated soil profile. A downslope illuviated profile has gained 169 kg m−2 (10%) colloidal material. Elemental losses by mobilization in true solution are ubiquitous across the catena, even in zones of colloidal accumulation, and range from 1418 kg m−2 (− 46%) for an eluviated profile to 195 kg m−2 (− 23%) at the bottom of the catena. Quantification of simultaneous mass transfers in solution and suspension provide greater specificity on processes within soils and across hillslopes. Additionally, because colloids include both HFS and other elements, the ability to quantify their redistribution has implications for standard calculations of soil mass balances using such index elements.

Research highlights
► Ratios of Ti and Zr can trace colloidal redistribution in soils.
► Our model quantifies colloidal redistribution.
► Our model separates colloidal versus solution mass change.
► Solution losses are larger, but colloids redistribute significant mass.
► Ti and Zr redistribution have implication for other weathering calculations.