Wet prairie soils (Aquolls) in the Midwest hold significant stores of soil organic carbon (SOC) and total nitrogen (TN). We intensively sampled crop fields, restored prairies of varying ages, and remnant prairies on a floodplain in southern Wisconsin, to estimate changes in SOC and TN due to the conversion of native prairie to row crops and the restoration of prairie on cropped land. In the top 10 cm of soil, remnant prairies contained an additional 24 Mg SOC ha-1 and 1.7 Mg TN ha-1, compared to currently cropped fields (P < 0.05), a difference of 35%. In the top 25 cm, remnant prairies contained an additional 2.4 Mg TN ha-1 compared to currently cropped fields (P < 0.05), but we could not detect a difference in SOC between unplowed remnants and cropped fields. Interestingly, ratios of soil inorganic carbon (SIC) to total carbon (TC) were significantly greater in cropped fields, likely due to mixing of the topsoil with carbonate rich subsoil through tillage. We detected no significant differences in SOC and TN mass between annually tilled and biennially tilled fields, nor any significant differences between currently cropped fields and prairie restorations less than seven years old. Because these soils are often saturated and were originally high in SOC and TN, changes in these properties due to land use have likely been tempered compared to upland sites. Additionally, the high variability in SOC content of crop sites under similar management shows the need to interpret spatial observational studies with care.