Characterizing Soil Chemical Weathering Using Portable X-Ray Fluorescence Spectrometer.

Soil chemical weathering under natural conditions is one of essential processes for sustainability assessment of agriculture and land use, as well as for the identification of acid sensitive regions through the estimation of critical loads, which requires information on spatial differences in weathering rates. In the other hand, the intensity and status of the climate-driven chemical weathering were often well recorded in soil pedons in the course of soil formation; these pedologic records potentially contain substantial amount of paleoclimatic information, which are critical to interpretations and retrospective understanding of the change of climate in the history. In general, two distinct approaches can be applied to quantify weathering rates of soils; the first one is based on mass balance concept, requiring observations of inputs and outputs of the system (e.g., profiles and watersheds), and the second one is an approximation method based on the depletion of minerals with respect to an unweathered horizon under natural or laboratory conditions. The latter method demands geochemical characteristics of the soils in high resolution, vertically and/or horizontally. As portable X-ray fluorescence (PXRF) spectrometer provides an analytical approach to a broad range of elements in a time- and cost- efficient manner, the weathering rates of soils can therefore be efficiently characterized with PXRF, avoiding tedious sampling preparation and analysis. The major objectives of this study are to 1) characterize chemical weathering in terms of vertical variations of in different profiles and spatial differences in the fields based on PXRF readings, 2) compare the calculated chemical weathering rates based on PXRF readings and other methods, and 3) rationalize the cause of the variations.