Vegetation succession and carbon sequestration in a coastal wetland in northwest Florida: Evidence from carbon isotopes

Measurements of stable carbon isotopic ratios as well as carbon (C), nitrog en (N), and phosphorus (P) contents in soils and plants were made along a c hronovegetation sequence stretching from high marsh to low marsh in a coast al wetland in northwest Florida. The wetland is dominated by Juncus roemeri anus, which is a C3 plant and has an average delta C-13 of -27 parts per th ousand. Lesser amounts of other species, including C4 plants, are also pres ent in the area. The delta C-13 values of soil organic matter from low and middle marshes range from -24 to -27 parts per thousand, which are consiste nt with the current plant community. However, the delta C-13 values of soil organic matter from high marsh show significant variations, from -23 parts per thousand in the surface soil to -17 parts per thousand at depth. This large C isotopic variation within soil profiles indicates a shift in local vegetation, from a C4-dominated community to the current C3 plant-dominated marsh, as a result of landward expansion of the wetland due to sea level r ise. Radiocarbon dates on soil organic matter indicate that this ecological change occurred in the past hundred years or so as a result of sea level r ise presumably due to global warming. Soil organic carbon inventory was sim ilar to 29 +/- 3.6 kg m(-2) in low marsh (the oldest part of the wetland), 15 +/- 3.6 kg m(-2) in middle marsh, and 13 +/- 6.0 kg m(-2) in high marsh (the youngest and most inland part of the wetland). N and P inventories are also higher in low marsh than in high marsh and seem to correlate directly with aboveground productivity in the marshes. The much higher C storage in low marsh than in high marsh indicates that carbon sequestration increased significantly as coastal wetland evolves from high marsh (initial stage) t o low marsh (steady state). This has important implications to the global C cycle. As sea level rises owing to global warming, coastal wetlands are ex pected to expand landward in many areas where topography is gentle, which w ould provide a significant sink for atmospheric carbon dioxide.