Much had previously been hypothesised about the fate of the CO2 fertilization effect. Current plants take up every year about a quarter of all CO2 emissions. But will this ecosystem service persist? During decades, CO2 experiments have been fumigating plants with elevated levels of CO2 to reproduce the atmospheric CO2 concentration of the future and answer this question, but the results have been equivocal: plant growth did not respond to CO2 in some experiments because nitrogen availability limited growth, whereas CO2 did increase growth in others. Much has been speculated about the drivers of the CO2 fertilization effect: temperature, age, soil moisture, plant type, etc. However, none of these factors seemed to consistently explain the variety of plant responses to CO2.
Synthesising data from 83 CO2 experiments, we found that indeed none of those potential predictors have enough explanatory power to predict the observations from CO2 experiments. As previously anticipated, and discussed in the literature, nitrogen (N) availability is the most important factor driving the CO2 effect (low N = low CO2 effect). But N does not always limit the CO2 effect: plants that associate with ectomycorrhizal fungi can overcome N limitations and take advantage of elevated CO2 to grow as much as non N limited plants.
We concluded that, indeed, N availability is the most important element determining whether or not plants can grow bigger under elevated CO2; but some plants can partner with fungi that help them get it. The identification of the drivers of the CO2 fertilization effect has important consequences for the prediction and quantification of the amount of CO2 that future plants might be able to remove from the atmosphere.
Terrer C, Vicca S, Hungate BA, Phillips RP, Prentice IC. 2016. Mycorrhizal association as a primary control of the CO₂ fertilization effect. Science 353: 72–74.