| Focus 2/3 | Does rising atmospheric CO₂ threaten the nutritional quality of crops?

The FACE technique

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Figure 1. FACE facility in Phoenix (Arizona, USA) used to study the response of wheat to atmospheric CO₂ enrichment (photo: US Department of Agriculture).

FACE (Free-Air CO₂ Enrichment) devices are heavy equipment installed outdoors (Figure 1). They enable a “bubble” of CO₂-enriched atmosphere to be maintained outdoors for a long period of time above a cultivated plot or natural area. This is achieved by continuously injecting pure CO₂ into the air through a network of pipes and nozzles covering the plot. CO₂ sensors mounted on stakes measure the concentration of this gas in the atmosphere in real time and control the injection so that the concentration remains close to the set value (e.g. 550 ppm instead of the 415 ppm in the ambient air).

Compared to the closed chambers used in laboratories, these devices have the advantage of studying the response of plants to high CO₂ under natural conditions, with crops grown in soil and under real weather conditions. In addition to its very high cost, the disadvantage of this system is that it is often difficult to precisely control the CO₂ concentration because the injected gas diffuses freely outside the enrichment bubble and can even be quickly swept away if the wind is too strong.

Dozens of long-term experiments with FACE devices have been conducted since the early 1990s in many regions of the world and on very different types of vegetation (see, for example (see, for example [1]).

A recent synthesis of some fifty FACE studies provides a very comprehensive overview of the effects of elevated CO₂ on the main crops that form the basis of our diet [2].

Notes and references

Thumbnail. [Source part of US Department of Energy report – Free-Air CO₂ Enrichment experiments, DOE/SC-0202, 2020]

[1] Report from US Department of Energy – Free-Air CO2 Enrichment experiments, DOE/SC-0202, 2020_https://ess.science.energy.gov/wp-content/uploads/2020/12/facereport2020.pdf

[2] Ainsworth EA, Long SP. 30 years of free-air carbon dioxide enrichment (FACE): What have we learned about future crop productivity and its potential for adaptation? Glob Chang Biol. 2021 Jan;27(1):27-49. doi: 10.1111/gcb.15375. Epub 2020 Nov 2. PMID: 33135850