Soil Development in a Coastal Louisiana Wetland during a Climate-Induced Vegetation Shift from Salt Marsh to Mangrove

Mild winter temperatures are facilitating the expansion of black mangrove (Avicennia germinans) into smooth cordgrass (Spartina alterniflora) marshes along the northern Gulf of Mexico. These expansions have the potential to alter soil development because of differences in productivity and tissue chemistry between Spartina and Avicennia. Here, we examined changes in soil-nutrient chemistry at 2-cm intervals in 30-cm soil cores collected from Spartina and Avicennia habitats at two different sites in Fourchon, Louisiana. Beginning in 1959 and continuing through 2009, our chronology shows that the species shift had no effect on bulk density (mean +/- standard error [SE]: 0.68 +/- 0.02 g cm(-3)), organic matter (mean +/- SE: 8.36 +/- 0.29 %), or nitrogen content (mean +/- SE: 1.15 +/- 0.03 mg cm(-3)). Phosphorus densities were significantly greater in Avicennia habitats (mean +/- SE: 0.32 +/- 0.01 mg cm(-3)) than they were in Spartina habitats (mean +/- SE: 0.28 +/- 0.01 mg cm(-3)), which we attributed to Avicennia occurring at higher elevations in more oxidizing soils. We observed significant variability with depth (proxy for time) and between sites in all soil properties measured. This variability can be attributed to the dominance of allochthonous sediment deposition from natural and anthropogenic disturbances compared with the lesser influence of vegetation on autochthonous soil development. In the highly disturbed region of Fourchon, Louisiana, the shift from Spartina marshes to scrub Avicennia stands does not appear to affect the chemistry of soil development.