Digital Mapping of Topsoil Carbon Content and Changes in the Driftless Area of Wisconsin, USA

We mapped topsoil (0-20 cm) soil organic C (SOC) content in a 5200-ha site and identified key variables affecting SOC distribution in the Driftless area where Alfisols and Mollisols are common. About 47% of the area was under arable crops, 22% under pasture, 21% under forest, and the remaining 10% under roads, constructions, and others. Condition-based regression rules were used to predict SOC using 13 environmental variables. The average C content of the soils was 23 g kg(-1) and topsoils under forest had the highest levels of SOC (28 g kg(-1)). There was little difference in SOC contents of the different soil orders, but Inceptisols contained more SOC than Alfisols and Mollisols. Land use, soil order, and terrain parameters, such as elevation, insolation, wetness index were the key variables affecting SOC distribution. The highest prediction error was observed in the soils from the forest followed by pasture and crop lands. We also estimated SOC changes using the Projected Natural Vegetation Soil Carbon (PNVSC) approach as a reference to which the present day SOC predicted map was compared. The PNVSC represents a SOC map that would be observed in the landscape today if the area had remained under natural vegetation. Based on this approach, soils under forest had accumulated on average >2 g kg(-1), whereas crop land and pasture had lost 1.8 to 2.0 g kg(-1) SOC in about 10 yr. There was a relation between soil orders, land use, and SOC levels; and we have spatially quantified this relationship including an estimate of the uncertainty.