Elevated CO 2 promotes long‐term nitrogen accumulation only in combination with nitrogen addition

Biogeochemical models that incorporate nitrogen (N) limitation indicate that N availability will control the magnitude of ecosystem carbon uptake in response to rising CO 2 . Some models, however, suggest that elevated CO 2 may promote ecosystem N accumulation, a feedback that in the long term could circumvent N limitation of the CO 2 response while mitigating N pollution. We tested this prediction using a nine‐year CO 2 xN experiment in a tidal marsh. Although the effects of CO 2 are similar between uplands and wetlands in many respects, this experiment offers a greater likelihood of detecting CO 2 effects on N retention on a decadal timescale because tidal marshes have a relatively open N cycle and can accrue soil organic matter rapidly. To determine how elevated CO 2 affects N dynamics, we assessed the three primary fates of N in a tidal marsh: (1) retention in plants and soil, (2) denitrification to the atmosphere, and (3) tidal export. We assessed changes in N pools and tracked the fate of a 15 N tracer added to each plot in 2006 to quantify the fraction of added N retained in vegetation and soil, and to estimate lateral N movement. Elevated CO 2 alone did not increase plant N mass, soil N mass, or 15 N label retention. Unexpectedly, CO 2 and N interacted such that the combined N+ CO 2 treatment increased ecosystem N accumulation despite the stimulation in N losses indicated by reduced 15 N label retention. These findings suggest that in N‐limited ecosystems, elevated CO 2 is unlikely to increase long‐term N accumulation and circumvent progressive N limitation without additional N inputs, which may relieve plant–microbe competition and allow for increased plant N uptake.