Ecosystem response to elevated CO_2 levels limited by nitrogen-induced plant species shift

摘要

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 change10. 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.