The response across all levels of the ecosystem to elevated CO2 (i.e. photosynthesis, respiration, growth of shoots and roots, increased biogeochemical activity) suggests there is a substantial amount of carbon sequestered or cycled through this ecosystem. A negligible fraction of the additional soil carbon is exported from the site in shallow ground water.
The picture emerging from these long-term studies is that rising atmospheric CO2 will cause increased carbon cycling in the native plant communities through a variety of mechanisms all dependent on the responses of photosynthesis and transpiration. Although acclimation of photosynthesis has been repeatedly observed, this has not eliminated the impact of rising CO2 at the plant and ecosystem levels. The greatest effects of rising atmospheric CO2 on carbon assimilation and plant growth, and on microbial processes, result from interactions with environmental stress, primarily caused by interannual variation in rainfall. Although we have shown that elevated CO2 favors increased soil respiration (perhaps partly through a priming effect), we have been unable to show that the products of decomposition (dissolved organic and inorganic carbon) are flushed out of the marsh through movement of soil water. Based on these findings, we hypothesize that an accumulation of the products of CO2 stimulation of photosynthesis and growth in the litter and soil raises the surface of the marsh. This accumulation of carbon in the form of undecomposed litter is the most likely sink for the additional carbon.