Why is oxygen released from plants

Knowledge : Plants are busier than expected

Carbon dioxide plays a key role in the global climate. Researchers therefore want to know exactly what amounts of gas are exchanged between the atmosphere, the oceans, plants and the ground. There could be a big mistake in this balance, known as the “carbon cycle”, reports a team led by Lisa Welp in “Nature” (vol. 477, p. 579). Not 120 billion tons of carbon would be absorbed by all plants on earth every year, but 150 to 175 billion tons. That would be 45 percent more.

The scientist from the University of California in San Diego and her colleagues have evaluated the isotopic composition of carbon dioxide (CO2) in the air over the past 30 years. Atoms of different weights of one and the same element are called "isotopes". In the specific case, it was about the light isotope oxygen-16 and the heavier oxygen-18. The researchers found that the content of these oxygen isotopes in carbon dioxide shows regular fluctuations that can be linked to El Niño events. The explanation: In El Niño years it is warmer and more rainy than usual in the tropics. The plants thrive, there is more photosynthesis. At the same time, a lot of water evaporates from the leaves. During this process, primarily light oxygen isotopes are released, the heavy ones accumulate in the plant water. This “water-isotope ratio” is also found in the CO2 that is given off by the vegetation. As a result, the isotope ratio of all carbon dioxide in the atmosphere also changes.

After every El Niño event, the “disturbed” isotope ratio in the CO2 in the atmosphere recovered unexpectedly quickly, noted Welp. She concludes from this that the vegetation supplies a lot of fresh CO2, i.e. converts the carbon much faster and therefore in greater quantities than previously assumed. That would have serious consequences, because the researchers' “carbon climate models” would have to be revised.

Matthias Cuntz from the Helmholtz Center for Environmental Research (UFZ) in Leipzig is skeptical. "Such estimates depend on many assumptions," he writes in an accompanying comment. Welp and colleagues assume, for example, that 43 percent of all CO2 molecules that reach the plant are actually converted into photosynthesis. But this number is only an estimate. Certain plants such as maize or millet convert 60 percent of the CO2 molecules due to a different metabolism. If more maize is planted, the quota increases. According to Cuntz’s view, the new findings do not throw everything that already exists “overboard”, as it is called in a UFZ announcement. Rather, they demonstrate a new method to better determine the global productivity of plants.

With regard to global warming, Welp's results do not mean the all-clear. A faster carbon turnover means not only more CO2 uptake by plants, but also higher CO2 emissions. So the amount of greenhouse gas in the atmosphere continues to rise. nes

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