Quantification of the rate of carbon cycling in a coastal wetland in northwest Florida

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. The δ13C values of soil organic matter from the high marsh, however, 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 C4 plant to C3 plant, as a result of landward expansion of the wetland due to sea-level rise.; The soil organic carbon inventory was about 29 kg C/m2 in the low marsh, and 13 kg C/m2 in the high marsh. The much higher C storage in the low marsh than in the high marsh indicates that carbon sequestration increases significantly, as coastal wetland evolves from high marsh to low marsh. Much higher N and P inventories in the low marsh than the high marsh seem to correlate directly with above-ground productivity in the marshes.; C turnover time in the surface peat (0–10 cm) is 20 years in the low marsh, and 4 years in the high marsh. The carbon turnover time increases with depth in both low marsh and high marsh.; The current carbon accumulation rate in the surface peat (0–10 cm) is about 251 g C/m2/yr in the low marsh, and about 127 g C/m2/yr in the high marsh. This suggests that the low marsh is sequestrating C about twice as fast as the high marsh. The long-term C accumulation rate is 31 g C/m2/yr in the low marsh, and 25 g C/m2/yr in the high marsh. These data suggest that the marshes have been and continue to be a sink for atmospheric CO₂. The much higher current rates of C accumulation in the marshes, in comparison with the long-term rates, may be caused by increased productivity due to global warming, CO₂ fertilization effect and/or increased global N deposition.