Carbon dynamics within agricultural and native sites in the loess region of western Iowa

In order to quantify the historical changes in carbon storage that result f rom agricultural conversion, this study compared the carbon dynamics of two sites in the loess region of Iowa: a native prairie and a cropland. Field data were obtained to determine present-day carbon storage and its variabil ity within a landscape (a stable ridgetop vs. eroding upper-midslope vs. de positional lower slope). Models were used to recreate the historical carbon budget of these sites and determine the cropland's potential to be a net C O2 source or sink, relative to the atmosphere. Regardless of slope position, the cropland site contains approximately half the amount of carbon as prairie. Variability in soil carbon storage within a site as a consequence of slope position is as large or larger (variation s of 200-300%) than temporal variation (similar to 200% at all slope positi ons). The most extreme difference in soil carbon storage between the cropla nd and prairie sites is found in the soil at the upper-midslope, which is t he area of greatest erosion. The models estimate that 93-172% of the carbon in the original topsoil has been lost from the cropland's eroding midslope . Much of this carbon is derived from deeper soil horizons. Either a small sink or strong source of carbon to the atmosphere is created, depending on the fate of the eroded sediment and its associated carbon.