Conservation Reserve Program (CRP) prairie restorations can sequester soil C and N, but the varied effects of soil order and ecosystem age are uncertain. Soil bulk density (Db) (0-20 cm) and soil organic C (SOC) and total N at 0 to 5, 5 to 10, and 10 to 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 (4-16 yr) and soil order (Alfisols and Mollisols). Several important outcomes were found regarding land conversion to CRP: (i) soil Db decreased on Alfisols (-0.12 {+/-} 0.11 g cm-3, P < 0.0001) but not Mollisols; (ii) 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 upper 5 cm; (iii) whole SOC and N to a depth of 25 cm did not change significantly; (iv) the annual average SOC sequestration rate declined (P < 0.05) as prairie age increased (from 72 {+/-} 105 to 13 {+/-} 25 g C m-2 yr-1 for youngest to oldest age groupings); and (v) short-term SOC and N increases could be lost with 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.
