Quantifying the Rates of Soil Genesis by Geochemical Mass Balance.
Kyungsoo Yoo, Department of Plant and Soil Sciences, University of Delaware, 152 Townsend Hall, Newark, DE 19716
Existing soil chronosequence studies demonstrate that soils are dynamic and historical objects. Most importantly, the soil thickness and vertical profile of soil chemistry significantly change as the soil ages. Much progress has been made in identifying physical and chemical processes responsible for these temporal changes. However, we are still poorly prepared to quantify the rates of the individual processes, the functional relationships among the processes, and the feedbacks between the processes and the soil properties as soils form. This is a critical intellectual gap which needs to be bridged to legitimately place soil genesis in the context of ecosystem change in human to geological time scales. What is urgently needed, to address this challenge, is a process-based mathematical model which can guide us not only to determine what to measure and what relationships to seek but also to integrate the empirical data. To address this issue, we combined existing models of geochemical mass balance and geomorphic sediment transport. While the geochemical mass balance tracks the mass loss or gain by chemical dissolution and precipitation within and from a soil profile, the sediment budget describes physical soil mixing and soil production from underlying parent material in relation to soil thickness. Based on the new combined model, we propose the following data as key information for mechanically understanding soil formation; total chemistry, thickness, bulk density of entire soil profile, and the rates of soil production and mixing. This empirical data set, via our model, can be converted to the rates of net mass loss/gain by chemical dissolution and precipitation within and from a soil profile. The model quantitatively links the chemical weathering rates to physical processes of soil mixing and breakdown of parent material to finer soil materials. Lastly, by comparing those rates to the observed soil thickness and profile chemistry along a studied soil chronosequence, one can quantitatively study the feedback between earth surface processes and soil properties. There are increasing demands for integrating the knowledge of soil formation with process geomorphology, and we provide this model as a springboard for this potentially fruitful interdisciplinary research.