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 (eCO(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 CO2 enrichment experiment with 88 C-3 or C-4 grassland plots, we found that biomass was markedly enhanced at eCO2 relative to ambient CO2 in C-3 but not C-4 plots, as expected. During the subsequent 8 years, the pattern reversed: Biomass was markedly enhanced at eCO(2) relative to ambient CO2 in C-4 but not C-3 plots. Soil net nitrogen mineralization rates, an index of soil nitrogen supply, exhibited a similar shift: eCO(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 eCO(2) biomass response. These findings challenge the current C-3-C-4 eCO(2) paradigm and show that even the best-supported short-term drivers of plant response to global change might not predict long-term results.