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

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Science  20 Apr 2018:
Vol. 360, Issue 6386, pp. 317-320
DOI: 10.1126/science.aas9313

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A short-term trend reversed

Theory and empirical data both support the paradigm that C4 plant species (in which the first product of carbon fixation is a four-carbon molecule) benefit less from rising carbon dioxide (CO2) concentrations than C3 species (in which the first product is a three-carbon molecule). This is because their different photosynthetic physiologies respond differently to atmospheric CO2 concentrations. Reich et al. document a reversal of this pattern in a 20-year CO2 enrichment experiment using grassland plots with each type of plant (see the Perspective by Hovenden and Newton). Over the first 12 years, biomass increased with elevated CO2 in C3 plots but not C4 plots, as expected. But over the next 8 years, the pattern reversed: Biomass increased in C4 plots but not C3 plots. Thus, even the best-supported short-term drivers of plant response to global change might not predict long-term results.

Science, this issue p. 317; see also p. 263

Abstract

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-C4 eCO2 paradigm and show that even the best-supported short-term drivers of plant response to global change might not predict long-term results.

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