Sulfate Reduction and Methanogenesis

The small size and long-term nature of the elevated CO2 plots limited our ability to take additional cores for measuring SO 4 reduction directly, so we relied primarily on pore-water concentrations of the primary substrate (SO 4 ) and primary product (H 2 S). Hydrogen sulfide concentrations are useful for interpreting SO 4 reduction activity at this site because the soils are organic and lack iron minerals.
     There was a pattern of higher H 2 S and lower SO 4 concentrations in the elevated CO2 plots than the ambient CO2 plots, which suggests that elevated CO2 stimulated SO 4 reduction. Given that SO 4 reducing bacteria are heterotrophic, the data suggest the process is at least partly C-limited and was stimulated by an increase in the supply of photosynthates at elevated CO2 (Figure 1).
     More evidence the process is limited by organic carbon is found in a comparison of the two consecutive years of porewater data when the system was subjected to either extreme drought (2002) or rainfall (2003). In the wet year at 40 cm depth, SO 4 concentrations declined and H 2 S concentrations increased compared to the dry year. This was presumably due to an increase in SO 4 reduction rates, and it coincided with an increase in primary productivity from 2002 to 2003 (Figure 2,3).
    Dacey et al. (1994) reported that elevated CO2 stimulated CH 4 emissions at our study site. The present study indicates that this effect was caused in part by the influence of elevated CO2 on SO 4 reduction. By increasing the organic carbon supply, elevated CO2 may have depleted the porewater SO 4 pool in one part of the soil profile, relieving methanogenic bacteria from carbon competition elsewhere in the profile (Figure 4).
    A greenhouse study was performed to test the hypothesis that elevated CO2 stimulated SO 4 reduction. Sciprus olneyi was grown in soils from the site for three months at ambient and elevated CO2 ; there were also no-plant control soils. Sulfate reduction was faster in the soils with plants than in the absence of plants due to the higher labile organic C supply (Figure 5). Elevated CO2 raised SO 4 reduction, even more, corroborating our field observations. Thus, it is reasonable to interpret the in situ changes in pore-water SO 4 and H 2 S levels as an elevated CO2 stimulation of sulfate reduction.
    Because of limitations on the number of soil samples that can be removed from our study plots, a limited number of small (2-cm diameter x 5-cm deep) soil cores were removed to measure SO 4 reduction rates in situ (Figure 6). There were either no significant effects of elevated CO2 on SO 4 reduction rate (2003), or lower SO 4 reduction in the elevated CO2 plots. Interestingly, SO 4 reduction was higher in 2003 than in 2002, which is consistent with higher plant production and a drawdown of the SO 4 pool. If elevated CO2 drew down the porewater SO 4 pool further deeper in the profile, it would explain low rates at the soil surface compared to ambient CO2 . A more intensive sampling effort that yields depth profiles of SO 4 reduction rates will be required to test this hypothesis.

Figure 1

Porewater concentrations of methane, sulfate and hydrogen sulfide at 40 cm depth in a C3-dominated brackish marsh exposed to elevated atmospheric CO2 .


Figure 2
A. Shoot biomass at the peak of the growing season and total annual rainfall for the 17 year period, 1988-2003. B. (right panel) Correlation between shoot biomass and annual rainfall, 1992-2000 (elevated, r 2 = 0.92; ambient, r 2 = 0.80) .


Figure 3
Shoot density in the C3 Scirpus -dominated community from 1987 to present (left panel) and the elevated CO 2 stimulation of shoot density over the same period (right panel). Stimulation calculated as [(A-E)/A]*100.
Figure 4
Relationship between SO 4 and CH 4 in C3-dominated plots treated with ambient or elevated CO2. Low [SO 4 ] in the wet year favored methanogenesis.
Figure 5

A greenhouse study on the effects of plants and elevated CO2 on sulfate reduction.

Figure 6

In situ measurements of SO 4 reduction in a C 3 -dominated tidal marsh exposed to ambient and elevated CO2.

Research Questions