Response of soil δ15N and nutrients to eastern red cedar (Juniperus virginiana) encroachment into a relict calcareous prairie

Abstract

The calcareous prairies of Louisiana have been threatened by the encroachment of woody 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. The knowledge of whether eastern red cedar encroachment has altered these soils is also of interest. We studied the depth distribution, at contrasting vegetation types (prairie, transition, forest) and landscape positions, of δ15N, total N, organic C, C/N ratio, Ca, Mg, K and pH of three relict prairie-forest associations in north central Louisiana, USA. The effect of vegetation type was significant for soil δ15N and Ca. Plant leaf samples from prairie, transition, and forest showed similar δ15N signals, and mean values ranged between −1.6‰ and −1.1‰. The order of soil 15N enrichment of the 0–10 cm depth relative to corresponding leaves was forest soil > transition soil > prairie soil. The forest soil was significantly enriched with 15N compared with the prairie soil and transition soil. Except for C/N ratio, all the soil properties significantly decreased with depth while δ15N increased with depth. Significant differences in C/N ratio, Ca and Mg were associated with landscape position. The change in soil pH due to woody encroachment was restricted to the 0–10 cm depth. The results suggest that the prairie soil was distinctly different from the forest soil and that the vegetation at transition (encroaching woody plants) was altering the surface soil pH towards ‘forest-like’ conditions.