Climatic Controls on Soil and Deep Regolith Development in Southern Sierra CZO

Abstract:

The weathered bedrock zone, below the soil and above hard bedrock, may serves as a large reservoir of water and nutrients. Characterization of weathered bedrock, under different weathering environments, can lead to an improved understanding of the regulating factors for forest health and drought tolerance. Little is known about spatial patterns of weathered bedrock characteristics in the southern Sierra Nevada, because of the challenges and cost of sampling. The objective of this study is to evaluate morphological, physical and chemical properties of soil and weathered granitic bedrock, along an elevation gradient in the southern Sierra Nevada Critical Zone Observatory. Three catchments were selected at elevation of 400 m, 1100 m, and 2000 m. Cores were collected using a Geoprobe to the depth of refusal. Preliminary results show that weathered bedrock thickness increased with elevation, while the degree of soil development (as indicated by clay stocks in soil) was greatest at the mid-elevation (1100 m). Weathered bedrock at 2000 m is a large reservoir for water and nutrient (plant available P & K) due to its thickness. The storage capacity of water and nutrients in weathered bedrock decreases dramatically as elevation and regolith thickness decreases. While carbon content in weathered bedrock is low, the C stock at 1100 m and 2000 m sites was similar to that of soil at 400 m. In general, trends across the elevation gradient for C, P and K are similar when comparing soil and weathered bedrock, increasing with elevation with the exception that available P in soil was highest at 1100 m. If relationships between characteristics of soil and weathered bedrock can be established, soil properties could be used to predict conditions in weathered bedrock that regulate forest productivity, which are currently unobtainable at broad scales.