Comparing Simulated and Measured Soil Organic Carbon Content of Clay Soils for Time Periods Up to 60 Years

JIMMY R. WILLIAMS Texas A&M Experiment Station 808 East Blackland Road Temple, Texas 76502, USA ABSTRACT / The Erosion Productivity Impact Calculator model (EPIC) was recently altered to include algorithms based upon concepts found in the Century model with the exception of a daily time step. We compared measured soil organic carbon (SOC) content values with simulated values to validate the new EPIC simulation model carbon sequestration routine. The simulations were based upon detailed soils data for three clay soils (Udic Haplusterts) and actual weather data recorded near the sample sites. Historical cropping systems for central Texas were simulated for a period of 120 years, the period from the original breaking of the native prairie sod to modern times. In addition, the effect of tilling the soil for 60 or more years and then returning the site to grass was simulated. Periods of restored grass were 6, 26, and 60 years. It was necessary to adjust a parameter, the fraction of humus in the passive pool, to make realistic simulations. Once adjusted, EPIC simulated the decline in SOC with agriculture well at all three sites in central Texas. EPIC also simulated the relative difference in SOC content occurring between continuously tilled sites and sites with restored grass well.