Plants alter biomass allocation to optimize resource capture. Plant strategy for resource capture may have important implications in intertidal marshes, where soil nitrogen (N) levels and atmospheric carbon dioxide (CO₂) are changing. We conducted a factorial manipulation of atmospheric CO₂ (ambient and ambient + 340 ppm) and soil N (ambient and ambient + 25 g m^-2 year^-1) in an intertidal marsh composed of common North Atlantic C₃ and C₄ species. Estimation of C₃ stem turnover was used to adjust aboveground C₃ productivity, and fine root productivity was partitioned into C₃-C₄ functional groups by isotopic analysis. The results suggest that the plants follow resource capture theory. The C₃ species increased aboveground productivity under the added N and elevated CO₂ treatment (P < 0.0001), but did not under either added N or elevated CO₂ alone. C₃ fine root production decreased with added N (P < 0.0001), but fine roots increased under elevated CO₂ (P = 0.0481). The C₄ species increased growth under high N availability both above- and belowground, but that stimulation was diminished under elevated CO₂. The results suggest that the marsh vegetation allocates biomass according to resource capture at the individual plant level rather than for optimal ecosystem viability in regards to biomass influence over the processes that maintain soil surface elevation in equilibrium with sea level.