The interactive effects of elevated carbon dioxide and water table draw-down on carbon cycling in a Welsh ombrotrophic bog

The effects of elevated atmospheric CO2 (eCO2) and water table draw-down on soil carbon sequestration in an ombrotrophic bog ecosystem were examined. Peat monoliths (11 cm diameter, 25 cm deep) with intact bog vegetation were exposed to ambient or elevated (ambient + 200 mg l−1) atmospheric CO2, combined with a natural water table (level with the peat surface) or a water table draw-down (−5 cm). Eight observations per treatment were included in the study, which was conducted over a 12 week period. Concentration of dissolved organic carbon (DOC), phenolic compounds and the fluxes of CO2 and CH4 were measured. The eCO2 treatment caused an increase in the CH4 and CO2 fluxes and a small decrease in both the DOC and phenolic concentrations. The water table draw-down invoked decreases in phenolic and DOC concentrations, a decrease in CH4 flux and a small increase in CO2 flux. The combined (eCO2 + water table draw-down) treatment caused a larger than expected CH4 flux decrease and CO2 flux increase and an increase in DOC concentration. Our results suggest very different effects on the system dependent on the treatment applied. The draw-down treatment principally increased oxidation of the rhizosphere resulting in increased decomposition and as such a removal of material from the dissolved carbon pool. The data also suggest labile carbon availability may be limiting the rate of decomposition and so slowing inorganic nutrient and carbon pool turn-over. The elevated CO2 addressed the labile-carbon limitation. Under the environment of the combined treatment, these limitations were effectively removed, culminating in a destabilisation of the carbon-sequestering environment to a weaker sink (or even a source) of atmospheric carbon.