Measuring the Role of Ecological Structure and Environmental Change on Organic Carbon Preservation in Coastal Wetlands from the Texas Gulf Coast

English Abstract:

The estimated magnitude of the carbon (C) stocks contained in the first meter of US coastal wetland soils represents ~10% of the entire C stock in US soils (4 vs. 52 Pg, respectively). Because this stock extends to several meters below the surface for many coastal wetlands, it becomes paramount to understand the fate of C under ecosystem shifts (e.g. woody encroachment), varying natural environmental constraints (shifts in salinity, hydrological regime, average and/or winter temperatures, nutrient inputs, etc) and changing land use. This study presents a synthesis of a NASA-funded project characterizing the quantity and quality of C sequestered in mangrove vs. saltmarsh ecosystems over the entire coast of Texas. The research outcomes of this multidisciplinary study are threefold: a) quantification of changes in vegetation cover (ecological shifts or wetland loss) was achieved and compared over two decades (1990 and 2010) using Landsat Thematic Mapper 5 images, b) changes in wetland areal coverage were converted into shifts in C stocks using sediment cores collected along the Texas coast, and c) control mechanisms of C preservation were elucidated using elemental, stable isotope, and biomarker characterization of organic matter in fresh plants, litter, and sediment cores. The most substantial loss of aboveground OC in Texas coastal salt marsh and mangrove ecosystems is due to the transformation of these wetlands into tidal flats and open water (-7.53.10⁸ and 1.57.10⁷ g OC, respectively). Similarly, loss of salt marsh and mangrove coverage resulted in significant decreases in sedimentary OC inventories (-3.69.10⁹ g OC and 5.71.10⁷ g OC, respectively). Mangrove expansion into mudflat and salt marsh environments offsets only 70% of the total calculated OC loss (-4.51.10⁹ g OC). Biomarker data show that neutral carbohydrates and lignin contribute 30-70% and 10-40% of total OC, respectively, in plant litter and surface sediments in these ecosystems. Sharp declines of carbohydrate yields with depth occur parallel to increases in lignin degradation ratios indicating substantial decomposition of both the polysaccharide and lignin components of litter detritus. Biomarker data suggest litter chemistry is the primary control of C preservation in these wetland ecosystems.