Soil and Weathered Bedrock Evolution along an Elevation Gradient in the Southern Sierra Nevada, California

摘要

Soil studies have traditionally been limited to the upper 2-m, while deep regolith -- semi-consolidated materials from soil to bedrock -- remains relatively unexplored, leaving many questions about the weathering processes of the near earth surface. Regolith (soil plus weathered bedrock) was studied across an elevation transect (405 m to 2700 m) of the west slope of southern Sierra Nevada mountains as a bioclimosequence.;In chapter one, effects of subtle differences in lithology and temperature were evaluated to measure degree of pedogenesis in regolith at rain- and snow- dominated sites. Biotite content in fine sand fractions was positively correlated with regolith thickness at rain-dominated and snow-dominated sites. Mineral grain size was negatively correlated with regolith thickness at the rain-dominated site. Meanwhile, the degree of regolith transformation, evaluated based on clay and secondary Fe oxide concentrations, was related to annual heat energy load from 0.5 to 10-m hard bedrock, where the proxy of energy input was mediated by temperature with depth.;In chapter two, a significant reservoir of carbon (C) within weathered bedrock was discovered. Small measurable amounts of C detected in deep regolith summed to large pools at locations where the regolith was thick. Although temperature and precipitation not effected on C density in deep regolith, the influence of climate had an indirect effect on C stock because it governed weathered bedrock thickness. Weathered bedrock had been reported to store deep C by 9% to 30% of the total regolith C inventory due to thickness of 0.7 to 7.6 m. Furthermore, a hypothetical scenario of global C stocks in weathered bedrock of 4-m thickness is 201 Pg, which help to explain a "hidden" C sink in the global C budget. In the third chapter, physical, chemical and mineralogical properties were measured at three elevations to understand climatic controls on weathering in soil and weathered bedrock. Evidence of chemical weathering was most pronounced in soil at the mid-elevation, while evidence of physical weathering was greatest in weathered bedrock at the high elevation where deep water percolation was highest. The concentration of nutrients was lower in weathered bedrock than that of soil, but due its thickness weathered bedrock was reservoir of plant essential nutrients. Moreover, available water content was large when summed over weathered bedrock thickness.;New spatial trends in ecosystem services were revealed by expanding our concept of soil to include deep regolith. This work was first to document factors that influence spatial trends of regolith (soil to weathered bedrock) characteristics in the Sierra Nevada. Characterization of deep regolith under different weathering environments improves our understanding of regulating factors for ecosystems health. The differentiation between weathering trends of soil and weathered bedrock could be used in the future to predict conditions in weathered bedrock at large scales.