Chemistry of Coastal Wetland Soil Organic Matter in the Atchafalaya and Barataria Basins

Recent studies have revealed that the deterioration of Louisiana coastal wetlands with the loss of soil organic matter could be one of the main reasons leading to the hypoxia in the northern Gulf of Mexico. In this study, Atchafalaya basin undergoing land building and Barataria basin experiencing land loss were evaluated for soil organic matter (SOM) chemistry and relation to carbon source and degradation dynamics. Both lignin biomarkers and SOM molecular compositions from wetland soil profiles up to 50 cm depth of the two contrasting basins were characterized and compared. Lignin monomers were determined using alkaline CuO oxidation followed by gas chromatography-mass spectrometry (GC/MS) detection and SOM compositions were measured directly by pyrolysis-GC/MS. Results showed that Barataria basin wetland soil profiles were dominated by nonwoody angiosperms, whereas Atchafalaya basin wetland soil profiles were dominated by both nonwoody gymnosperms and nonwoody angiosperms. The SOM degradation of the two basin wetland soils was influenced by different factors. Soil acidity had a strong negative effect on the SOM degradation in the Atchafalaya basin, whereas high N contents inhibited lignin degradation in the Barataria basin. Soil electrical conductivity (EC) negatively affected organic matter degradation in the profiles of low salinity wetlands but positively influenced SOM decomposition in higher salinity wetlands. Wetland ecosystems along an increasing salinity gradient of Louisiana coast also exhibited a large variation in SOM composition. Freshwater marsh conditions influenced the amount of N-containing compounds of SOM, whereas brackish and saline marshes exerted more influence on the presence of lignin and aliphatic compounds in SOM, respectively. Overall, coastal wetland SOM compositions were mainly influenced by vegetation sources of specific ecosystems with different salinity tolerance and degradation status that was facilitated by specific environmental factors