In recent years, conversion of crop and marginal land to reconstructed prairie has been gaining momentum, due to the prairies’ aesthetic and environmental values. However, little attention has been given to the belowground biogeochemical processes, which have potential to reduce soil erosion, and sequester significant amounts of atmospheric CO₂ in the soil. The general objectives of this study were to determine the effects of years since establishment and slope position on soil C dynamics of reconstructed prairies by (1) quantifying changes in soil organic carbon (SOC) and soil total nitrogen (TN) stocks, (2) establishing whether a site is a sink or source for CO ₂ by measuring potential C input and C loss via microbial respiration, (3) potential erosion effects by measuring water infiltration, and (4) examining the interrelationships between soil aggregation and SOC sequestration.

The study was conducted in Jasper and Warren counties in Iowa. Soils in both Jasper and Warren counties formed in loess under native vegetation of tallgrass prairie. All of the reconstructed prairie sites were located in the Neal Smith National Wildlife Refuge. The remnant prairie was located approximately 95 kilometers southwest in Rolling Thunder Prairie in Warren County. All sampling plots were located on summit, mid-slope, and toe-slope positions and were chosen by year of establishment and the presence of relatively similar soils. There were three reconstructed prairie sites varying in establishment year: 1993, 1998, and 2003. These sites were then compared to an adjacent row crop production site, categorized as a no-till site, established in 2003. A prairie remnant site was included to identify the upper limits for selected soil properties, prior to conversion to cultivated row crop production. The experiment was designed so that site was the main treatment, replicated three times along different slope positions in plots of approximately 4 m ² and 30 m apart. Changes in soil organic carbon (SOC), total nitrogen (TN), water stable aggregates (WSA), bulk density (&rgr;_b), above- and belowground biomass, and soil surface CO₂ emissions were measured to examine soil C dynamics.

Results from this study show that slope position and time since establishment did have a significant impact on SOC sequestration rates in the top 15 cm of soil depth. In the summit positions, the youngest established prairie (2003) had the greatest SOC sequestration rate at 2.15 Mg ha^-1 yr ^-1, although rates sharply decreased to near zero or below zero as time since establishment increased after five years. Much smaller increases were observed at the toe-slope positions, where an average of 0.59 Mg ha ^-1 yr^-1 was determined over the first 14 years of prairie establishment compared to the summit position sequestration rate of 0.73 Mg ha^-1 yr^-1. At the mid-slope position, there were no detectable changes in SOC, due to the high variability and potential for soil erosion. In addition, increases in SOC sequestration rates were only observed when WSA macro-aggregate distributions were also increased. This suggests that the mechanism of formation of soil aggregates that aids in stabilizing soil organic matter is also critical in determining a site’s potential for SOC sequestration. Also, the two younger established reconstructed prairies sites (1998 and 2003) were superior in sequestering C (mainly through root biomass and surface litter) compared to the older prairies, and sinks for atmospheric CO₂. Findings from this study suggest that slope position and year since establishment may be the leading factors in determining the effectiveness of reconstructed prairies for soil C sequestration potential.