Carbon Pools in Floodplain Soils Undergoing Restoration: Potential for Deep Carbon Sequestration.

Soil is the largest terrestrial carbon (C) reservoir estimated at > 2x the quantity held in biomass and the atmosphere combined. Global storage estimates for the upper meter of C rich soils vary between 500 to 3000 Pg C (~ 1750 Pg average). Subsoil horizons (1-3+ m depth) generally have lower C concentrations and are thought to be stable over time. These horizons are generally not considered in global C pool estimates and may store considerable quantities of C. This research assessed deep soil C storage in alluvial floodplain soils transitioning from conventional agricultural management to restoration practices to support the return of a native vegetation community. Samples from 25 auger holes (to depths of 3-6 m) were collected across the 400 ha riparian restoration site along the Cosumnes River in the Central Valley of California. Soils were collected representing 30 cm increments and analyzed for total C. On average surface soil (0-1 m) accounted for only 33 % (10.6 kg/m2) of total C pool (33 kg/m2 - 0-6 m). The range of C is large and demonstrates how agriculture masked historic spatial variability of soil properties - C poor alluvial soils (13 kg/m2) were sampled adjacent to historic wetlands which contained the most C (50 kg/m2). Our results indicate that deep soil C is stable and appears generally resistant to land use change over a time scale of 50-100 y. The subsurface C at this site (up to 20 Mg/ha) may be considered a substantial offset of sequestered C and credit in emerging C markets.