Ecosystem response to elevated CO2 levels limited by nitrogen-induced plant species shift

Terrestrial ecosystems gain carbon through photosynthesis and lose it mostly in the form of carbon dioxide (CO2). The extent to which the biosphere can act as a buffer against rising atmospheric CO2 concentration in global climate change projections remains uncertain at the present stage(1-4). Biogeochemical theory predicts that soil nitrogen (N) scarcity may limit natural ecosystem response to elevated CO2 concentration, diminishing the CO2-fertilization effect on terrestrial plant productivity in unmanaged ecosystems(3-7). Recent models have incorporated such carbon-nitrogen interactions and suggest that anthropogenic N sources could help sustain the future CO2-fertilization effect(8,9). However, conclusive demonstration that added N enhances plant productivity in response to CO2-fertilization in natural ecosystems remains elusive. Here we manipulated atmospheric CO2 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(10). We found that N addition enhanced the CO2-stimulation of plant productivity in the first year of a multi-year experiment, indicating N-limitation of the CO2 response. But we also found that N addition strongly promotes the encroachment of C-4 plant species that respond less strongly to elevated CO2 concentrations. Overall, we found that the observed shift in the plant community composition ultimately suppresses the CO2-stimulation of plant productivity by the third and fourth years. Although extensive research has shown that global change factors such as elevated CO2 concentrations and N pollution affect plant species differently(11-13), and that they may drive plant community changes(14-17), we demonstrate that plant community shifts can act as a feedback effect that alters the whole ecosystem response to elevated CO2 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.