Ecosystem response to elevated CO(2) levels limited by nitrogen-induced plant species shift.

Terrestrial ecosystems gain carbon through photosynthesis and lose it mostly in the form of carbon dioxide (CO(2)). The extent to which the biosphere can act as a buffer against rising atmospheric CO(2) concentration in global climate change projections remains uncertain at the present stage. Biogeochemical theory predicts that soil nitrogen (N) scarcity may limit natural ecosystem response to elevated CO(2) concentration, diminishing the CO(2)-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 CO(2)-fertilization effect. However, conclusive demonstration that added N enhances plant productivity in response to CO(2)-fertilization in natural ecosystems remains elusive. Here we manipulated atmospheric CO(2) concentration and soil N availability in a herbaceous brackish wetland where plant community composition is dominated by a C(3) sedge and C(4) grasses, and is capable of responding rapidly to environmental change. We found that N addition enhanced the CO(2)-stimulation of plant productivity in the first year of a multi-year experiment, indicating N-limitation of the CO(2) response. But we also found that N addition strongly promotes the encroachment of C(4) plant species that respond less strongly to elevated CO(2) concentrations. Overall, we found that the observed shift in the plant community composition ultimately suppresses the CO(2)-stimulation of plant productivity by the third and fourth years. Although extensive research has shown that global change factors such as elevated CO(2) concentrations and N pollution affect plant species differently and that they may drive plant community changes, we demonstrate that plant community shifts can act as a feedback effect that alters the whole ecosystem response to elevated CO(2) concentrations. Moreover, we suggest that trade-offs between the abilities of plant taxa to respond positively to different perturbations may constrain natural ecosystem response to global change.