A Synthesis of Corn Belt Region Soil Carbon Field Studies in Relation to Carbon Cycling Model Testing Needs.

We conducted a regional analysis for the extended Corn Belt region to determine important long-term relationships between soil carbon and cropping systems, tillage practices, fertilizer application rates, and other management, in support of testing carbon cycling models with datasets from the region. The ten states considered here (Indiana, Iowa, Illinois, Michigan, Minnesota, Missouri, Nebraska, Ohio, Pennsylvania, and Wisconsin) were determined as outlined by the Cropping Systems Coordinated Agricultural Project: Climate Change, Mitigation, and Adaptation in Corn based Cropping Systems (CSCAP; see http://www.sustainablecorn.org/) and relevant research literature. Soil can act as a source or sink of carbon; however short-term C dynamics are often misleading and inconclusive. Thus we focused on assessing important soil carbon sequestration trends discernible in long-term Corn Belt studies (> 10 year) including sites near Hoytville and Wooster in Ohio, the Morrow research plots in east central Illinois, and Sanborn Field in Columbia, Missouri. However, we also draw on research reported in some shorter studies (at least 5 years in length) that provide additional key insights regarding soil carbon dynamics in the region. Several aspects of soil carbon research were investigated including: (1) carbon cycling trends in active and passive pools, (2) the relationship of soil carbon with soil depth, (3) tillage effects on soil carbon sequestration and soil carbon transport on eroded soil, and (4) residue return effects on soil carbon levels from different cropping/management systems. We relate these factors to initialization and testing requirements of the Environmental Policy Impact Climate (EPIC) and DAYCENT field-scale models. We further discuss limitations and inconsistencies in reported field study data, and issues encountered with using these data to perform calibration and validation of both models across a range of environmental and climatic conditions in the Corn Belt.