Elevated CO2 affects porewater chemistry in a brackish marsh

Abstract

As atmospheric CO 2 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 (CH 4 ), 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 patensdominated (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) CO 2 levels for 15 years prior to porewater sampling, and the treatments continued over the course of our sampling. Sulfate reduction was stimulated by elevated CO 2 in the C 3 -dominated community, but not in the C 4 -dominated community. Elevated CO 2 also resulted in higher porewater concentrations of CH 4 and dissolved organic carbon in the C 3 -dominated system, though inhibition of CH 4 production by sulfate reduction appears to temper the porewater CH 4 response. These patterns mirror the typical divergent responses of C 3 and C 4 plants to elevated CO 2 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 CO 2 . Overall, our data demonstrate that elevated CO 2 drives changes in porewater chemistry and suggest that increased plant productivity likely stimulates microbial decomposition through increases in dissolved organic carbon availability.

Ecosystem response to elevated CO2 levels limited by nitrogen-induced plant species shift

“…Allocation to nutrient-acquiring roots instead of light-acquiring shoots represents a trade-off that should confer an advantage to S. americanus in N acquisition under conditions of low N availability. This view is consistent with the observation that porewater nitrogen concentration (that is, [N]) was over eight times higher in pure S. patens stands than in S. americanus stands in a previous study at the site 27 . Greater allocation to roots affords S. americanus enhanced ability to acquire N when porewater [N] is low.…”

Salinity Influence on Methane Emissions from Tidal Marshes

“…This dataset was limited to three studies because most studies reported just one of the three variables, usually [CH 4 ]. Data were extracted from two of the studies (Bartlett et al 1987, Tong et al 2010) by visual interpretation of graphs, while 3108 observations from a third study were obtained from the authors (Keller et al 2009). The methane flux and porewater data were compiled into three subsets: methane emissions versus salinity (31 observations), porewater methane versus salinity (3136 observations), and porewater methane versus sulfate concentration (3139 observations).…”

“…Measurements included salinity and porewater methane, taken at three locations in each of five marsh sites (Table 1). Porewater was sampled using a sipper (Online Appendix 2) and the analytical methods described by Keller et al (2009). Dissolved methane concentration was quantified with flame ionization gas chromatography after equilibration with an equal volume of ambient air.…”

“…Because two of the distributions were not normally distributed, methane emissions was log transformed before being tested with ANOVA and a least significant difference (LSD) means comparison test in SAS. Quantile curves of the relationships among the [CH 4 ], [SO 4 2− ] and salinity data from Keller et al (2009) were calculated by dividing the x-axis (i.e., [SO 4 2− ] or salinity) into 13 (approximately 1 mM) bins, calculating quantiles separately for each bin using SAS Proc Univariate, then fitting a curve to the quantiles versus the mid-point of each bin with a secondorder polynomial in Microsoft Excel. Quantiles of the Chmura et al (2003) marsh (not mangrove) data were calculated with S-Plus, which uses linear interpolation between ordered data.…”

“…The data available for evaluating the relationship between salinity and porewater methane concentration included a series of sites on the Nanticoke River that ranged in salinity from 0 to 5.5, a large dataset collected from a single marsh with an average salinity of 6.8 (Keller et al 2009), and a third dataset collected from a single marsh with average salinity of 2.3 (Tong et al 2010) (Fig. 2a).…”