Increasing carbon dioxide (CO2) levels in the atmosphere could reduce crops' protein content by 20 per cent, according to scientists, who say that new fertilisers may be needed to counteract the effects.

Researchers found that plants lose the ability to take up so much nitrate — the most common form of nitrogen in agricultural soils — and convert it into organic compounds, such as proteins, when growing in CO2-enriched environments.

The problem is that "most crop plants ... use nitrate as their main form of nitrogen," said Arnold Bloom, lead author of the study — published in Science last week (14 May) — and a researcher at the US-based University of California, Davis. Increasing the levels of CO2 leads to "nitrogen starved" crops that contain less protein for human consumption, he said.

Bloom estimated that the increased CO2 levels predicted for the next 20–50 years could reduce the amount of protein in crops by up to a fifth because of this phenomenon.

"Wheat grain that has been exposed to the conditions that we expect in the next few decades declines about 20 per cent," he said.

Bloom's team tested the two major forms of soil nitrogen available to plants (nitrate and ammonium) and how they affect two major groups of plants — monocotyledons and dicotyledons living in a high-CO2 atmosphere. Results revealed that those plants exposed to nitrate have difficulty in producing nitrogen-containing compounds, such as proteins, while those exposed to ammonium do not.

"This study is alerting us about the need to develop new fertilisation techniques and to improve crops' nitrogen use efficiencies," said Marta Lopes, a wheat physiologist at the International Maize and Wheat Improvement Center, Mexico.

Lopes said fertilising with ammonium could be an alternative, but highlighted that care should be taken to avoid its toxic effects on the environment.

Researchers have known that increases in CO2 — a key molecule that plants use in photosynthesis to grow — improve crop productivity at early stages, and many models have taken this into account for their production scenarios.

But this effect is diminished because plants eventually acclimatise to CO2. Now, Bloom's study suggests that the inhibition of nitrate assimilation may be the reason why plants do not thrive in CO2-rich environments as expected.

"We don't know how this will affect productivity," Lopes said.

Gerald Nelson, an agricultural economist at the International Food Policy Research Institute, United States told SciDev.Net that the new study "reinforces the point that we cannot count on CO2 fertilisation to offset the negative productivity effects of climate change on agriculture".

Link to full paper in Science