Landscape position and age of reconstructed prairies effect on soil organic carbon sequestration rate and aggregate associated carbon

Abstract

Changes in agricultural land use such as the establishment of prairies on previously cultivated cropland provide an opportunity for greater soil organic carbon (SOC) sequestration rates. In this study, a topo- and chrono-sequence approach was used to investigate relationships between SOC and soil aggregate formation. Our hypothesis was that the greatest increases in SOC sequestration rates are associated with most recently established prairies on cultivated land, where SOC was the most depleted, due to the destruction of soil aggregate. The study was conducted in Jasper and Warren counties in Iowa, from 2005 to 2008. Soils in both counties were formed in loess under native vegetation of tallgrass prairie. There were three reconstructed prairie sites varying in establishment year—1993, 1998, 2003; a row crop production site under no-tillage; and a prairie remnant site. All soil and plant sampling plots were located on summit, midslope, and toe-slope positions. Results show that time since establishment and slope position had a significant impact on SOC sequestration rates in the top 15 cm (6 in) soil depth only. In the summit position, the greatest SOC sequestration rates were observed in the youngest prairie at 2.15 Mg ha⁻¹ y⁻¹ (0.96 tn ac⁻¹ yr⁻¹), and decreased to near equilibrium after approximately eight years. Additionally, SOC was shown to increase linearly at a rate of 0.73 Mg ha⁻¹ y⁻¹ (0.33 tn ac⁻¹ yr⁻¹) since prairie establishment during the 14-year period. In the toe-slope position, SOC sequestration rates were substantially lower at 0.59 Mg ha⁻¹ y⁻¹ (0.26 tn ac⁻¹ yr⁻¹). Furthermore, increases in SOC sequestration rates coincided with increases in water stable aggregate—associated carbon (C) in 0.25 to 2 mm (0.01 to 0.08 in) size fractions. This occurred in the youngest reconstructed prairie (2003) in the summit and in the midslope positions on prairies reconstructed in 1998 and 1993. This suggests that the mechanisms of soil aggregate formation aid in stabilizing SOC, which is significantly affected by slope position and years since prairie establishment.