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 C₄ photosynthetic pathway (C₄ species) are less responsive to elevated carbon dioxide (eCO₂) than species that use only the C₃ pathway (C₃ species). We document a reversal from this expected C₃-C₄ contrast. Over the first 12 years of a 20-year free-air CO₂ enrichment experiment with 88 C₃ or C₄ grassland plots, we found that biomass was markedly enhanced at eCO₂ relative to ambient CO₂ in C₃ but not C₄ plots, as expected. During the subsequent 8 years, the pattern reversed: Biomass was markedly enhanced at eCO₂ relative to ambient CO₂ in C₄ but not C₃ plots. Soil net nitrogen mineralization rates, an index of soil nitrogen supply, exhibited a similar shift: eCO₂ first enhanced but later depressed rates in C₃ plots, with the opposite true in C₄ plots, partially explaining the reversal of the eCO₂ biomass response. These findings challenge the current C₃-C₄eCO₂ paradigm and show that even the best-supported short-term drivers of plant response to global change might not predict long-term results.