Unexpected reversal of C3 versus C4 grass response to elevated CO2 during a 20-year field experiment

Theory predicts and evidence shows that plant species that use the C 4 photosynthetic pathway (C 4 species) are less responsive to elevated carbon dioxide ( e CO 2 ) than species that use only the C 3 pathway (C 3 species). We document a reversal from this expected C 3 -C 4 contrast. Over the first 12 years of a 20-year free-air CO 2 enrichment experiment with 88 C 3 or C 4 grassland plots, we found that biomass was markedly enhanced at e CO 2 relative to ambient CO 2 in C 3 but not C 4 plots, as expected. During the subsequent 8 years, the pattern reversed: Biomass was markedly enhanced at e CO 2 relative to ambient CO 2 in C 4 but not C 3 plots. Soil net nitrogen mineralization rates, an index of soil nitrogen supply, exhibited a similar shift: e CO 2 first enhanced but later depressed rates in C 3 plots, with the opposite true in C 4 plots, partially explaining the reversal of the e CO 2 biomass response. These findings challenge the current C 3 -C 4 e CO 2 paradigm and show that even the best-supported short-term drivers of plant response to global change might not predict long-term results.