As part of a long-term elevated CO2 experiment, we measured monthly porewater concentrations of sulfate and methane over a 5 year period. Samples were collected using porewater wells (i.e., sippers) in a Scirpus olneyi-dominated (C3 sedge) community and a Spartina patens-dominated (C4 grass) community in a brackish tidal marsh. Both plant communities were exposed to ambient and elevated (ambient + 340 ppm) CO2 levels for 15 year prior to porewater sampling, and the treatments continued over the course of our sampling.

Our results suggest that sulfate reduction was stimulated by elevated CO2 in the C3-dominated community, but not in the C4-dominated community. Overall, elevated CO2 also resulted in higher porewater concentrations of CH4 in the C3-dominated system; but, this effect was not as pronounced, likely due to the inhibition of CH4 production by sulfate reduction in this brackish system. These patterns mirror the plant response in these communities where elevated CO2 caused a sustained increase in total plant biomass in the C3-dominated community and no significant increase in plant biomass in the C4-dominate community. Our data suggest that the influence of elevated CO2 on anaerobic heterotrophic metabolism in this system is closely linked to plant productivity.