Unexpected reversal of C versus C grass response to elevated CO during a 20-year field experiment.

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