Land-use change effects on soil organic carbon, total soil nitrogen, and soil erosion in a temperate forest and grassland

There are several factors that influence the retention of soil organic carbon (SOC) and total soil nitrogen (TSN) including soil texture, climate, slope length and gradient, and current and past land-use. The purpose of this study was to compare by land use the retention of SOC and TSN on sloping landscapes. Two separate paired sites were used in the study, one a forest and cropland on a loess-derived Alfisol in Southern Illinois and the other a prairie and cropland on a Mollisol derived from loess over glacial till in Western Iowa. Tree ring data suggest that the forested site has been intact for about 70 years, although part of the site was used for grazing for a period. A monument marker at the prairie site suggests the location has not been farmed since at least 1946 and states that it has never been plowed; however it may have been used for grazing. The croplands at both sites utilized no-till management and a corn-soybean rotation. SOC and TSN concentrations of several layers and landscape positions were determined to a depth of 1m at the Iowa sites and 0.75 m at the Illinois sites due to the presence of a root-restricting fragipan layer. Fly ash was used as a tracer for physical erosion and levels were determined to a depth of 30 cm for all sites. In Iowa the prairie site had much higher SOC and TSN levels than the cropland at all landscape positions but the toeslope. This difference was seen throughout the full 1 m sampling depth. In Illinois the forest site showed less statistical difference, with the shoulder and lower backslope being the only positions with more SOC stock than the cropland. Again the lowest section of the landscape had more SOC in the cropland. Most of the difference in the forestland was seen in the upper 15 cm. TSN levels for the Illinois sites showed little difference except in depositional areas where the cropland had higher levels. Results suggested that the conversion of prairie to agriculture resulted in a 51% loss of the SOC, a 38% loss of the TSN, and a 37% loss of fly ash across all landscape positions and depths. The cropland of the Illinois site retained about 90% of the SOC, 100% of the TSN, and 54% of the fly ash of the forestland. It was also estimated that conversion of the forest or prairie to cropland would result in the release of 44.6 Mg CO2/ha and 168.7 Mg CO2/ha respectively. If climate change continues to push warmer temperatures north and forests and prairies are converted to cropland the results suggest that soil erosion will increase and SOC stock will decrease in the soil even with the use of no-till management