Conservation Reserve Program (CRP) prairie restorations can sequester soil carbon (C) and nitrogen (N), but the varied effects of soil order and ecosystem age are uncertain. Soil bulk density (BD) (0-20 cm) and soil organic C (SOC) and total N at 0-5, 5-10, and 10-25 cm were measured at 39 paired CRP-crop sites in Wisconsin to quantify SOC and N stock changes as a function of prairie age (4 to 16 years) and soil order (Alfisols and Mollisols). Several important outcomes were found regarding land conversion to CRP: (1) soil BD decreased on Alfisols (-0.12 ± 0.11 g cm-3, P < 0.0001) but not Mollisols; (2) SOC sequestration rates were not significantly different between Mollisols (49.7 ± 64 g C m-2 yr-1) and Alfisols (43.9 ± 86 g C m-2 yr-1), but were only detectable (P < 0.05) in the top five cm; (3) whole SOC and N to a depth of 25 cm did not change significantly; (4) the annual average SOC sequestration rate declined (P < 0.05) as prairie age increased (from 72 ± 105 g C m-2 yr-1 to 13 ± 25 g C m-2 yr-1 for youngest to oldest age groupings); and (5) short-term SOC and N increases could be lost over time. These data suggest that there may be a discontinuity between the intensity of continuing management that is needed for sustained, long-term SOC increases in planted prairies and the resources that the CRP has available to achieve this level of ecosystem functioning.
