Background/Question/Methods The extent to which the biosphere will buffer against rising atmospheric CO₂ remains a vast uncertainty in global climate change projections. Biogeochemical theory predicts that soil nitrogen (N) scarcity may limit natural ecosystem response to elevated CO₂, diminishing the CO₂-fertilization effect on terrestrial plant productivity in unmanaged ecosystems. Recent models have incorporated such carbon-nitrogen interactions and suggest that anthropogenic N sources could help sustain the future CO₂-fertilization effect. Although some evidence indicates that N may become more limiting under elevated CO₂, conclusive demonstrations that added N actually enhances plant CO₂ response in natural ecosystems remains extremely scarce. How plant community shifts may feed back to alter the whole ecosystem response has not been explicitly considered in field studies. We hypothesized that when plant communities are allowed to shift freely, the ecosystem response to multiple global change factors may be quite different than that predicted by biogeochemistry. To address this, we manipulated atmospheric CO₂ and soil N availability in a dynamic tidal marsh community for four years.

Results/Conclusions   To address this hypothesis, we manipulated atmospheric CO₂ and soil N availability in a dynamic tidal marsh community for four years. N addition enhanced the CO₂-stimulation of plant productivity in the first year (causing a 54% greater CO₂ effect with added N), indicating N-limitation of the CO₂ response. However, N addition also stimulated C₄ grass productivity by a factor of 10. Because C₄ grasses respond weakly to elevated CO₂, this shift ultimately suppressed the CO₂-stimulation of total plant productivity by the third and fourth years. Although extensive research has shown that global change factors such as elevated CO₂ and N pollution affect plant species differently, and may drive community changes, we demonstrate that plant community shifts can feed back to dramatically alter the whole ecosystem response. Moreover, tradeoffs between the abilities of plants to respond positively to multiple perturbations simulataneously may constrain natural ecosystem response to global change.