Deep Carbon Stocks in a California Delta Floodplain: Evidence for Long Term Sequestration Potential of Seasonally Inundated Soils.

Abstract:

Soils account for the largest terrestrial pool of carbon (C) and have the potential for even greater amounts of carbon sequestration. California soil carbon pool estimates taken to 1 m typically vary between 95 and 260 Mg-C/ha depending on land use with a mean of 133 Mg-C/ha (Soil Survey Staff, 2013). Previously unaccounted for deep C pools (1-3m) are generally considered to have low carbon contents and be stable C pools. This study assessed these deep carbon pools associated with alluvial floodplain soils that are transitioning from agricultural production to restoration of native vegetation. The study site covers approximately 400 ha of riparian and floodplain systems along the Cosumnes River in the Central Valley of California. Surface soils (0-0.15m) and subsurface soils (0.15-3m) were collected and analyzed for C and N. Subsurface soils were found to have significantly more C than most California soil carbon stocks with nearly half of total C residing from 1-3m. Burial of sediments resulting from flooding is most likely the key process for the observed storage of large quantities of C in these soils as is seen in apparent buried horizons found at approximately 1 m in 14 of 25 subsurface boreholes (> 0.15 m). In addition, past marsh and riparian habitat would have promoted high C inputs prior to burial. Floodplain soils are highly complex and therefore are not considered homogenous. In spite of this, considerable amounts of C were found to be sequestered at depths greater than 1 m even in soils without a C concentration indicative of a buried O-horizon which suggests that these soils have significant amounts of unaccounted for C as well as the potential for future long term C storage when seasonal inundation is restored.