Burras et al – Final report 2004 1 2004 FINAL REPORT Soil Carbon and Quality in Seymour and Clarinda Soil Map Units, Chariton Valley, Iowa Prepared for the Chariton Valley RC&D (ISU Project # 400-41-71-15-4216 CHARITON VAL/2-AGR-BUR Chariton Valley Biomass Project #DE-FC36-96GO10148) Submitted by C. Lee Burras, Julie M. McLaughlin, Skye A. Wills, Mike Barker & E. Charles Brummer March 24, 2005 Abstract Soil organic carbon (SOC) content and soil quality are increasingly important factors in many policy decisions yet both remain elusive to spatially quantify. This study sought to quantify each by comparing three soil map units (SMU) from each of two soil series, Clarinda and Seymour. These SMU’s represented different erosion classes. SMU’s under a variety of land uses were sampled, including row crop, switchgrass, pastures, and woods. The study area was the Chariton Valley of south central Iowa. Field methodologies including using centroid pedons, grid pedons, and grab samples. Laboratory analyses consisted of standard morphological descriptions followed by horizon-by-horizon analyses for bulk density, SOC content, stable aggregate content (SAC), and cation exchange capacity (CEC). Results include average SOC contents of about 5 kg m -2 *20 cm, 10 kg m -2 *50 cm and 12 kg m -2 *100 cm for all six SMU’s sampled, regardless of whether centroid or grid pedons were used. Depending on the method of sampling and statistical analyses, row cropped SMU’s generally have lower SOC content than the other land uses. Some data supports SOC content increasing proportional to stand age of perennial vegetation. No clear differences were observed between SOC content for either Clarinda or Seymour series with regards to uneroded and eroded SMU’s. Stable aggregate content, CEC and A horizon thicknesses do segregate by land use. Erosion classes of SMU’s do not appear to be important controls or predictors with regards to SAC, CEC and A horizon thickness. The relative rate of recovery for SOC contents is unclear for both the Clarinda and Seymour SMU’s. The recovery rate for soil quality is correlated with stand age of perennial vegetation. Recovery rates do not appear to be different between Clarinda and Seymour SMU’s. SAC increases by about 2 to 3 percent annually. The A horizon thickness increases by about 0.4 cm annually. Soil CEC is thought to be annually increasing by about 0.02 cmole c kg -1 . Finally, applying geostatistics and GIS to the data does result in clear, useable maps although we caution users to realize these maps are no better than the data used to make them. This means - given the overlap in properties across SMU’s and, to some degree, land uses – maps like the ones we present are probably best at the field or even regional scale and less accurate at the SMU scale.