30 years of free air carbon dioxide enrichment (FACE): What have we learned about future crop productivity and the potential for adaptation?

Authors: Ainsworth, Elizabeth - Lisa; LONG, STEPHEN - University Of Illinois

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/21/2020
Publication Date: 11/2/2020
Citation: Ainsworth, E.A., Long, S.P. 2020. 30 years of free air carbon dioxide enrichment (FACE): What have we learned about future crop productivity and the potential for adaptation? Global Change Biology. 27(1):27-49. https://doi.org/10.1111/gcb.15375.
DOI: https://doi.org/10.1111/gcb.15375

Interpretive Summary: The response of crops to rising atmospheric carbon dioxide concentrations has been studied in open-air field experiments using Free Air CO2 Enrichment (FACE) technology for 30 years. In this paper, we update a previous meta-analysis to synthesize the results from three decades of FACE experiments. Recent experiments have confirmed that C4 crops including maize do not show a direct stimulation of seed yield at elevated CO2 in the absence of drought stress. Additionally, the yield response of C3 crops to elevated CO2 is diminished by nitrogen deficiency. Another consistent response of crops to elevated CO2 is reduced mineral content in the seeds. We review potential biotechnological approaches to improve crop responses to rising CO2 along with studies that have identified significant genetic variation in response to rising CO2, both of which could be used to improve future yields.

Technical Abstract: FACE allows open-air elevation of CO2 without altering the microclimate. Its scale uniquely allows simultaneous study of crop physiology, growth and yield to soil moisture dynamics and microbial associations. Fifteen years ago we used meta-analysis to summarize results of the then 28 published observations. Here we add insights from many additional FACE studies of more crops, including multi-year studies. We also examine the subsequent combination of FACE with soil moisture manipulation and open-air elevation of ozone and temperature. Two of our original conclusions were that C4 crops would not be more productive in elevated [CO2], except under drought, and that yield responses of C3 crops would be diminished by nitrogen deficiency and wet conditions. Both have stood the test of time. Multi-year studies of maize showed no increase in yield, except in drought, while soybean yields were negatively affected by wet conditions in the early growing season. Decreased stomatal conductance of 18%-24% was seen across C3 and C4 in our previous analysis. Given feedback effects on evaporative demand by increased canopy temperature and leaf area in a less humid microenvironment, it was uncertain that crop evapotranspiration would decrease. Subsequent open-air micrometeorological measurements of canopy to atmosphere transfer confirmed significant reductions in C4 and C3 transpiration. Another consistent trend revealed by subsequent study is reduced levels of nutrients that depend on transpiration for movement, notably Zn and Fe, and lower nitrogen and protein in seed of non-leguminous crops. On a promising note we review studies that show sufficient genetic variation within crop germplasm to both maintain nutrient content and yield responsiveness to rising [CO2].