Soil Processes in the Southwestern United States.

The desert Southwest spans a broad range of soil forming factors overlying a complicated tectonic history, providing for a complex array of soil-landscapes and soil forming processes. The objective of this presentation is to provide an overview of the complex distribution of soils and soil forming processes in the desert Southwest, with particular emphasis on the Sonoran Desert. Over the Cenozoic, a series of volcanic and tectonic events shaped the modern landscape, forming the basin and range province that is typified by a regular spacing of uplifted mountain ranges and intervening basins filled with sedimentary deposits. The complex topography in this region generates strong elevation controlled climate gradients, extending from hot, dry desert scrub to cool, wet mixed conifer forests, with concomitant change in observed soil properties driven by a shift in dominant soil forming process from physical weathering to chemical weathering and carbon cycling. Underlying these local climate gradients, parent material exerts particularly strong control on soil properties, particularly in systems at and just beyond the transition from water to energy-limited climate conditions. Basin soils dominate the area geographically, and strongly reflect Pleistocene epoch paleoclimate variation in terms of alluvial fan deposition, erosion and soil development; older soils that have experienced several glacial-interglacial cycles typically exhibit substantial subsurface accumulation of clay, carbonates and/or other soluble salts, and Fe-oxyhydroxides. Examples will be provided highlighting these interactions of soil forming factors and landscape evolution processes and the exciting array of soil-landscapes present in the desert Southwest.