Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant, Enviromental and Soil Sciences

First Advisor

Wayne H. Hudnall


The calcareous prairies of Louisiana are among the most endangered ecosystems in North America. The major threat to this ecosystem is the invasion by woody and herbaceous plants, primarily eastern red cedar (Juniperus virginiana ). The restoration and management of these rare plant communities require a thorough understanding of the soils supporting them. Geostatistical, fractal, and stable isotopic procedures were integrated to study the soil and vegetation spatial variability. The prairie soil was alkaline, the forest soil was acidic, and the transition soil was neutral. The spherical model described the spatial variability of virtually all the soil properties considered with a spatial dependence expressed over a range of 20 to 30 m. The estimated fractal dimension for each soil property indicated short-range variation domination. Calcite was the major constituent of the soil carbonate. The delta13C values of the carbonate showed that it is of secondary origin. The delta13 C values of soil organic matter (SOM) and pedogenic carbonate covary, indicating an isotopic equilibrium between SOM and the pedogenic carbonate. The carbon isotope data from SOM within the 0--10 cm depth suggested that the prairies are composed of a mixture of approximately equal proportions of C3 and C4 plants. The relative proportion of C3 and C4 plants varied with season. The C4 plants dominated in the summer and the C3 plants in the spring. Comparison of delta13C values from the forest, transition, and prairie with depth suggested that the invasion of C3 trees and shrubs observed within the transition might not be a recent phenomenon. SOM delta 13C below 40--50 cm indicated that in the past a C4 community might have dominated the site. Plant leaves from prairie, transition, and forest showed similar delta 15N signals. Compared with the leaves, the soil was enriched with 15N. The order of enrichment of the 0--10 cm relative to corresponding leaves was forest soil > transition soil > prairie soil. The delta 15N and Ca were significantly affected by vegetation type. The differences in the soil delta15N is a clear indication that the vegetation within the transition zone (invading woody and herbaceous plants) is altering the N cycling towards more "forest-like" conditions.