Measurements of stable carbon isotopic ratios as well as carbon (C), nitrogen (N), and phosphorus (P) contents in soils and plants were made along a chronovegetation sequence stretching from high marsh to low marsh in a coastal wetland in northwest Florida. The wetland is dominated by Juncus roemerianus , which is a C3 plant and has an average δ13C of −27‰. Lesser amounts of other species, including C4 plants, are also present in the area. The δ13C values of soil organic matter from low and middle marshes range from −24 to −27 ‰, which are consistent with the current plant community. However, the δ13C values of soil organic matter from high marsh show significant variations, from −23‰ in the surface soil to −17‰ 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 rise. 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 rise presumably due to global warming. Soil organic carbon inventory was ∼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) to low marsh (steady state). This has important implications to the global C cycle. As sea level rises owing to global warming, coastal wetlands are expected to expand landward in many areas where topography is gentle, which would provide a significant sink for atmospheric carbon dioxide.
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