Elevated CO2 affects porewater chemistry in a brackish marsh

As atmospheric CO2 concentrations continue to rise and impact plant communities, concomitant shifts in belowground microbial processes are likely, but poorly understood. We measured monthly porewater concentrations of sulfate, sulfide, methane (CH4), dissolved inorganic carbon and dissolved organic carbon over a 5-year period in a brackish marsh. Samples were collected using porewater wells (i.e., sippers) in a Schoenoplectus americanus-dominated (C-3 sedge) community, a Spartina patens-dominated (C-4 grass) community and a mixed (C-3 and C-4) community within the marsh. Plant communities were exposed to ambient and elevated (ambient + 340 ppm) CO2 levels for 15 years prior to porewater sampling, and the treatments continued over the course of our sampling. Sulfate reduction was stimulated by elevated CO2 in the C-3-dominated community, but not in the C-4-dominated community. Elevated CO2 also resulted in higher porewater concentrations of CH4 and dissolved organic carbon in the C-3-dominated system, though inhibition of CH4 production by sulfate reduction appears to temper the porewater CH4 response. These patterns mirror the typical divergent responses of C-3 and C-4 plants to elevated CO2 seen in this ecosystem. Porewater concentrations of nitrogen (as ammonium) and phosphorus did not decrease despite increased plant biomass in the C-3-dominated community, suggesting nutrients do not strongly limit the sustained vegetation response to elevated CO2. Overall, our data demonstrate that elevated CO2 drives changes in porewater chemistry and suggest that increased plant productivity likely stimulates microbial decomposition through increases in dissolved organic carbon availability.