Relationships among soil carbon and physiochemical properties of a typic Albaqualf as affected by years under cultivation

Soil organic carbon (SOC) is a dynamic soil property and a key component of the quality and sustainability of agricultural soils. Predictions of SOC change often rely on correlations between SOC and other soil physical and chemical properties that are assumed to be more stable over time. The objective of this study was to examine the long-term stability of relationships between SOC, physiochemical properties, and years under cultivation in a common rice (Oryza sativa L.) soil (i.e., a fine, montmorillonitic, thermic, Typic Albaqualf) of the Mississippi Delta region of the south-central United States. The field site consisted of an undisturbed native prairie and three agricultural fields that have been cultivated for 15, 26, and 44 years, respectively, in 2001. Neither SOC concentration nor mass was significantly correlated with sand, silt, or clay content, but both were significantly correlated with pH in 1987. Soil OC concentration and mass were significantly correlated with both silt and clay, but not sand content, and pH in 2001. However, the relationship between SOC concentration and sand content changed significantly, while the relationships between SOC and pH and silt and clay content remained stable and did not change significantly from 1987 to 2001. Soil OC decreased exponentially as years under cultivation increased, but the modeled relationship appeared to remain stable from 1987 to 2001. Results indicate that continuous cultivation of the Typic Albaqualf of this study will result in SOC stabilizing around 8-10 g kg(-1), which is less than 50% of the SOC in the adjacent undisturbed, native prairie soil. The results of this study can be used to strengthen predictions of how ecosystems, particularly flood-irrigated rice agroecosystems, will respond to future changes in management practices towards more sustainable use of the soil resource.