Tuesday, August 3, 2010 - 8:40 AM

COS 16-3: Nitrogen-induced plant species shift constrains response of ecosystem productivity to elevated CO2

J. Adam Langley and J. Patrick Megonigal. Smithsonian Environmental Research Center

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.