Differences in carbon storage between seasonal and permanent wetland in agricultural ecosystems

Background/Question/Methods

The amount of carbon storage in wetlands plays a crucial role in global biogeochemical cycles although wetlands only comprise about 4 % of Earth’s land area. Wetland soils have the high carbon density from high productivity and low decomposition rate. In the Central Valley of California, approximately 90 % of the wetland in this 58000 km²region had been lost since the 1900s. Currently, there are extensive efforts underway to restore wetlands for habitats and other ecosystem services. Many of these new wetlands will be managed as seasonal wetland. The question arises as to whether these restored wetlands will represent a significant storage of C in the region. In this study, we tested the hypothesis that seasonal wetlands will not store as much carbon as permanent wetlands due to aerobic decomposition processes occurring during dry periods. Sediment cores were collected from seasonally flooded and permanently flooded wetlands in the San Joaquin River National Wildlife Refuge which receives seasonal agricultural drainage. Flash 2000 elemental analyzer (CE Elantech) was used for measurement of total carbon and total nitrogen. Inorganic and organic fractions for soil carbon were determined by vapor acidification. Acid persulfate oxidation method was used for total phosphorus determination.

Results/Conclusions

Carbon in the surface sediments was similar within permanently and seasonally flooded wetlands. In the first 5-15 cm from the surface both of the wetlands show an initial rapid decline in carbon. However, carbon concentrations declined to approximately 30 % of surface concentrations in seasonally flooded areas, but remained at approximately 70 % of surface values in permanently flooded areas. Below 15 cm, carbon concentrations were largely consistent. The carbon concentration in the permanently flooded wetland is consistently higher than those of the seasonally flooded wetland. The combination of both anaerobic conditions and ecosystem productivity that makes permanently flooded wetland soils highly organic.  Carbon/Nitrogen ratio did not differ between seasonal and flooded. But, Nitrogen/Phosphorus and Carbon/Phosphorus ratio is different from surface to deep. Carbon concentration changed from surface (13 g C/kg) to deep (3 g C/kg) in the seasonally flooded area and carbon concentration changed surface (14 g C/kg) to deep (10 g C/kg) in the permanently flooded. Inorganic carbon was low every sample and suggesting low carbonate levels in the soils and that nearly all C was organic. The future work, using 210-Pb to determine soil accumulation rates, to link concentration by depth to rates of carbon storage.