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Rising Seas Trigger Carbon Sequestration in Tidal Marshes
Tidal marshes typically exhibit high rates of productivity. In the southern coastal region of North America, for example, the net primary production of these ecosystems averages approximately 8000 g m-2 yr-1 (Mitsch and Gosselink, 1993). Tidal marshes also exhibit low rates of organic matter decomposition, because the anaerobic decomposers of these oxygen-depleted environments operate at slower rates than do their aerobic counterparts of terrestrial environments (Humphrey and Pluth, 1996; Amador and Jones, 1997). Thus, it can readily be appreciated that as seas rise and encroach upon the land, rates of carbon sequestration in coastal marsh soils rise right along with them.

How significant is this phenomenon? In an earlier contribution to this series (6 March 2002), we highlighted the work of Choi et al. (2001), who studied coastal marshes of the St. Marks National Wildlife Refuge in Wakulla County, Florida, USA, where they discovered that high-marsh soils contain 73% more organic carbon than nearby upland forest soils, that older middle-marsh soils contain 100% more carbon, and that still-older low-marsh soils contain 287% more, indicative of the fact that as seawater encroaches upon the land, carbon sequestration at any submerged location grows ever larger. Now, additional evidence for this trend is provided by Hussein and Rabenhorst (2002) in a study of submerged upland tidal-marsh soils on the eastern seaboard of the United States in the lower eastern portion of the Chesapeake Bay area of Dorchester County, Maryland.

At each of two representative marshes in this area - Hell Hook and Cedar Creek - a transect was established that led from the upland area across the marsh to the main stream that feeds each marsh. Along these transects, soil cores were extracted and brought back to the laboratory for numerous analyses, including age dating and carbon (C) and nitrogen (N) content determinations. Among other things, these measurements demonstrated that the accumulation of organic C and total N were "significantly related (alpha = 0.01 with r2 of 0.94)," with an essentially constant C/N ratio of 20:1.

The authors also learned that over the 2000-year period they studied, the deposition of organic matter in the marshes generally kept pace with the sea-level rise, i.e., the top of the marsh-soil's organic matter horizon typically rose just as fast as sea level, which led them to conclude that "the sequestration of total N (g m-2 over the entire thickness of the organic horizon) will increase with time, and that sea-level rise is the primary driving force." In addition, we note that the tight coupling of the marsh-soil's organic C and total N contents implies essentially the same thing about soil carbon sequestration: it is driven by sea-level rise and will increase with time ... but only, of course, if sea level continues to rise.

With respect to the past, the authors determined the mean rate of total N sequestration at the Hell Hook and Cedar Creek tidal marshes was 1.47 0.3 g m-2 yr-1 over the past two millennia, while over the last century and a half it was 4.2 1.15 g m-2 yr-1, or nearly three times greater. Even more impressive were their projections for the coming century. Based on predicted rates of sea level rise, they calculated a mean total N sequestration rate of 20.0 7.9 g m-2 yr-1, which is nearly five times greater than the past century's rate and more than thirteen times greater than the mean rate of the prior two thousand years. And, again, the tight coupling the scientists observed between the organic C and total N contents of the marsh soils implies a similar amplification of marsh-soil carbon storage rate in the next century.

These observations illustrate one of the many important ways in which earth's biosphere tends to counter increases in the planet's temperature. The negative feedback effect is initiated by the photosynthetic removal of CO2 from the atmosphere by tidal-marsh vegetation and magnified by the enhanced sequestration of the biologically-captured carbon in submerged organic soils, which process is driven by rising sea levels that are sustained by the rising temperature. This interim effect either lowers the air's CO2 content, stabilizes it, or slows its rate of rise, which leads to a reduction in the atmospheric greenhouse effect that either reverses, stops or slows the rate of temperature rise.

Interestingly, when the air's CO2 content is in a rising mode, the power of this negative feedback phenomenon is even greater; for the meticulous and voluminous work of Dr. B.G. Drake and a host of collaborators on the very same Chesapeake Bay wetlands has abundantly demonstrated that elevated atmospheric CO2 concentrations significantly stimulate the productivity of the marsh plants that grow there (Drake et al., 1989; Curtis et al., 1990; Arp et al., 1991; Long and Drake, 1991; Drake, 1992; Jacob et al., 1995; Drake et al., 1996; Dakora and Drake, 2000). Hence, we can take comfort in the fact that earth's biosphere is functioning in such a way as to significantly limit the amount of warming that might possibly occur as a consequence of the ongoing rise in the air's CO2 content.

Dr. Sherwood B. Idso Dr. Keith E. Idso

References
Amador, J.A. and Jones, R.D. 1997. Response of carbon mineralization to combined changes in soil moisture and carbon-phosphorus ratio in a low phosphorus histosol. Soil Science 162: 275-282.

Arp, W.J. and Drake, B.G. 1991. Increased photosynthetic capacity of Scirpus olneyi after 4 years of exposure to elevated CO2. Plant, Cell and Environment 14: 1003-1006.

Choi, Y., Wang, Y., Hsieh, Y.-P. and Robinson, L. 2001. Vegetation succession and carbon sequestration in a coastal wetland in northwest Florida: Evidence from carbon isotopes. Global Biogeochemical Cycles 15: 311-319.

Curtis, P.S., Balduman, L.M., Drake, B.G. and Whigham, D.F. 1990. Elevated atmospheric CO2 effects on belowground processes in C3 and C4 estuarine marsh communities. Ecology 71: 2001-2006.

Dakora, F.D. and Drake, B.G. 2000. Elevated CO2 stimulates associative N2 fixation in a C3 plant of the Chesapeake Bay wetland. Plant, Cell and Environment 23: 943-953.

Drake, B.G. 1992. A field-study of the effects of elevated CO2 on ecosystem processes in a Chesapeake Bay wetland. Australian Journal of Botany 40: 579-595.

Drake, B.G., Leadley, P.W., Arp, W.J., Nassiry, D. and Curtis, P.S. 1989. An open top chamber for field studies of elevated atmospheric CO2 concentration on saltmarsh vegetation. Functional Ecology 3: 363-371.

Drake, B.G., Muehe, M.S., Peresta, G., GonzalezMeler, M.A. and Matamala, R. 1996. Acclimation of photosynthesis, respiration and ecosystem carbon flux of a wetland on Chesapeake Bay, Maryland to elevated atmospheric CO2 concentration. Plant and Soil 187: 111-118.

Humphrey, W.D. and Pluth, D.J. 1996. Net nitrogen mineralization in natural and drained fen peatlands in Alberta, Canada. Soil Science Society of America Journal 60: 932-940.

Hussein, A.H. and Rabenhorst, M.C. 2002. Modeling of nitrogen sequestration in coastal marsh soils. Soil Science Society of America Journal 66: 324-330.

Jacob, J., Greitner, C. and Drake, B.G. 1995. Acclimation of photosynthesis in relation to rubisco and nonstructural carbohydrate contents and in situ carboxylase activity in Scirpus olneyi grown at elevated CO2 in the field. Plant, Cell and Environment 18: 875-884.

Long, S.P. and Drake, B.G. 1991. Effect of the long-term elevation of CO2 concentration in the field on the quantum yield of photosynthesis of the C3 sedge, Scirpus olneyi. Plant Physiology 96: 221-226.

Mitsch, W.J. and Gosselink, J.G. 1993. Wetlands. Van Nostrand Reinhold, New York, NY.