The calcareous prairies of Louisiana are among the most endangered and the least understood ecosystems in North America. The major threat to this ecosystem is the invasion of woody plants, primarily eastern red cedar (Juniperus virginianaL.). We studied the vertical profile of soil organic C (SOC) and the stable carbon isotope (δ13C) from soil organic matter (SOM) to infer the vegetation history and to understand the C dynamics of these prairies. SOC decreased with depth (0–100 cm), but variation due to vegetation type (prairie, transition, forest) and landscape position were not statistically significant. The mean total SOC stock within the 0–30 cm depth from three sites, three vegetation types, and two landscape positions was 7.6 ± 0.4 kg/m2(mean ± SE,n= 18). The SOM δ13C values within the 0–10 cm depth showed that although the forest vegetation was exclusively C3, the transition and the prairie vegetation were composed of a mixture of C3 and C4 plant functional forms. Statistical comparison of SOM δ13C values from the forest, transition, and prairie with depth showed that the SOM δ13C from the prairie and transition are significantly different from that of the forest up to the 30–40 cm depth. SOM δ13C values of the prairie and transition were not significantly different at all depths, suggesting that the invasion of C3 trees and shrubs observed within the transition may be a recent phenomenon coinciding with fire suppression. SOM δ13C below 30–40 cm indicated that in the past, a C4 community might have dominated the entire site.