Decoupling of Dissolved and Colloidal Export Across the Hydrograph of an Early Pedogenic Stage Critical Zone Observatory

ABSTRACT This study investigates colloid transport under early weathering conditions using the Landscape Evolution Observatory (LEO), a set of three replicated mesoscale basaltic hillslopes located in Tucson, Arizona. The hillslopes were subjected to a controlled irrigation regime designed to mimic natural wetting–drying cycles, potentially enhancing the in situ formation of colloids. After only 12 years of weathering, the hillslopes remain in an early pedogenic stage and have not yet developed vertical horizon differentiation. This structural homogeneity and textural isotropy make LEO an ideal experimental platform for investigating the mechanisms governing the mobilisation of weathering‐derived colloids. During a one‐month infiltration experiment, dissolved elements (Na, Mg, Al, Si, K, Ca, Fe) and their colloidal counterparts (Al, Fe) were monitored daily, the latter being analysed using high‐resolution inductively coupled plasma mass spectrometry in single particle mode (spHR‐ICP‐MS). This analytical approach allowed us to examine the concentration variations and mass distribution of colloids in response to changes in hydrologic discharge rate across the hydrograph. Results show that export is dominated by the dissolved fraction even for nominally insoluble phases (e.g., Al, Fe) and that the maximum instantaneous colloidal flux occurs during the recession of infiltration events. No significant relationship is observed between discharge and concentration, neither for dissolved nor colloidal forms of Fe and Al. However, for any given element, the dissolved and colloidal fractions do not follow the same dynamics in response to the discharge surge, suggesting distinct geochemical and hydrological controls. This study provides evidence of the role played by the development of the upper soil horizon in the export of colloids from soils and hillslopes by rivers during high‐flow periods, using an experimental hillslope with weak vertical contrasts (mineralogy, hydraulic conductivity) as an end‐member system.