Organic carbon storage in floodplain soils of the US prairies

Two data sources, field-collected samples and values in the NRCS SSURGO database, were used to estimate organic carbon concentration (%) and stock (MgC/ha) in floodplain soils along rivers of the tallgrass and shortgrass prairie within the United States. Field sampling of 6 sites in the tallgrass prairie and 6 sites in the shortgrass prairie (total sample size of 370 vertical cores) indicates that percent organic carbon within a planar cross section through floodplain sediment at a site is spatially heterogeneous and does not decline systematically with depth, but statistical analyses indicate that soil organic carbon is randomly distributed. The median values of organic carbon concentration at both field-sampled and sites remotely sampled based on soil maps in the tallgrass prairie are significantly higher than those of the shortgrass prairie. Median values of organic carbon stock are not significantly different between those obtained from forested sites for comparison and either shortgrass or tallgrass prairie sites but are significantly higher at tallgrass than at shortgrass prairie sites. These results are surprising because upland net primary productivity in prairies is lower than in forested sites. We infer that the historical abundance of floodplain wetlands in river corridors of the tallgrass prairie results in high organic carbon stocks in tallgrass prairie river corridors. This implies that management designed to enhance carbon sequestration should focus on floodplain soils, especially in the tallgrass prairie region, as well as on upland forests.