Carbon Stocks in Soil and Deep Regolith Across the Southern Sierra Nevada, California.

Deep weathered bedrock has been shown to support forest productivity in California, yet carbon inventories have ignored this portion of the critical zone, likely due to difficulty of sampling. Thus, forest soil carbon inventories that are constrained to the upper 1-m of soil may underestimate carbon stocks. Due to its ecological significance in Sierran forests, we hypothesize that deep regolith is a significant reservoir of carbon. We sampled three catchments representing an elevation gradient (405 m, 1160 m, and 2015 m) across the Sierra Nevada. Regolith cores (soil + weathered bedrock) were collected to depth of refusal (up to 10.67 m) by a Geoprobe and subsampled over 30 to 50-cm intervals. Total carbon (TC) was measured using a Costech Carbon analyzer. In soil, TC density increased with elevation ranging from 3.34 kg C m-3 to 16.87 kg C m-3. TC density was much lower in weathered bedrock at all sites ranging from 0.39 kg C m-3 to 1.02 kg C m-3. Weathered bedrock thickness was greatest (2.45 m to 7.55 m) at higher elevation sites - 1160 m and 2015 m, and much less (0.80 m) at the low elevation site - 405 m. Small yet detectable carbon concentrations corresponded to substantial TC pools when integrated over weathered bedrock thickness. The TC stock in deep, weathered bedrock at the 2015-m elevation site was even larger (6.60 kg C m-2) than that of the soil at the 405-m elevation site (4.07 kg C m-2), representing a newly discovered and potentially stable C sink. Including weathered bedrock in this carbon inventory increased the carbon stock by 6 to 21% compared to a standard soil inventory at all catchments. Since TC in weathered bedrock is uniformly low, spatial variability in regolith thickness is essential for accurate carbon stock estimates at landscape scales.