Belowgound carbon allocation is a major component of a plant's carbon budget, yet relatively little is known about the response of roots to elevated atmospheric CO{sub 2}. We have exposed three brackish marsh communities dominated by perennial macrophytes to twice ambient CO{sub 2} concentrations for two full growing seasons using open top chambers. One community was dominated by the C{sub 3} sedge Scirpus olneyi, one was dominated by the C{sub 4} grass Spartina patens, and one was a mixture of S. olneyi, S. patens, and Distichlis spicata, a C{sub 4} grass. Root and rhizome growth were studied in the 2nd yr of exposure by measuring growth into peat cores previously excavated and refilled with sphagnum peat devoid of roots. Growth under elevated CO{sub 2} resulted in an 83% increase in root dry mass per core in the Scirpus community. Those roots were also significantly lower in percentage of nitrogen than roots from ambient-grown plants. There was no effect of elevated CO{sub 2} on root growth or nitrogen content in the Spartina community or in the C{sub 4} component of the Mixed community.