Effect of Elevated CO2on Carbon Pools and Fluxes in a Brackish Marsh

The effects of long-term exposure to elevated atmospheric CO2(ambient + 340 ppmv) on carbon cycling were investigated for two plant communities in a Chesapeake Bay brackish marsh, one dominated by the C3sedge Schoenoplectus americanus and the other by the C4grass Spartina patens. Elevated CO2resulted in a significant increase in porewater concentrations of DIC at 30 cm depth (p < 0.1). The CO2treatment also yielded increases in DOC (15 to 27%) and dissolved CH4(12-18%) in the C3marsh (means for several depths over the period of June 1998 and June 1999), but not at a significant level. Elevated CO2increased mean ecosystem emissions of CO2(34-393 g C m-2yr-1) and CH4(0.21-0.40 g C m-2yr-1) in the C3community, but the effects were only significant on certain dates. For example, CO2enrichment increased C export to the atmosphere in the C3community during one of two winter seasons measured (p = 0.09). In the C4community, gross photosynthesis responded relatively weakly to elevated CO2(18% increase, p > 0.1), and the concomitant effects on dissolved carbon concentrations, respiration, and CH4emissions were small or absent. We concluded that elevated CO2has the potential to increase dissolved inorganic carbon export to estuaries.