In contrast to a popular conservative argument, a new study has found that increased atmospheric carbon dioxide isn’t necessarily a boon to plant growth — instead, it causes plants to have a more difficult time absorbing nitrogen, a nutrient critical to plant growth and health.
Published in the journal Global Change Biology, the study found that as carbon dioxide levels in the air increase, the concentration of nitrogen in plants decreases, thus decreasing the plant’s protein levels and growth ability. The team of international researchers studied the impact of increased atmospheric carbon across multiple types of ecosystems — from grasslands for forests — by looking at large-scale field experiments conducted in eight countries across four different continents.
“For all types of ecosystem the results show that high carbon dioxide levels can impede plants’ ability to absorb nitrogen, and that this negative effect is partly why raised carbon dioxide has a marginal or non-existent effect on growth in many ecosystems,” Johan Uddling, senior lecturer at the Department of Biological and Environmental Sciences at the University of Gothenburg and lead researcher on the project, said in a press statement.
Among conservatives — and some scientists — there has been a long-held hope that climate change could actually stimulate plant growth in the short term, as the atmosphere becomes more rich with carbon dioxide. Sen. James Inhofe (R-OK) has said that climate change has “contributed to increasing agricultural productivity,” arguing that “CO2 is a fertilizer.” And while some studies have supported Inhofe’s claim, others — like this most recent one — have found the opposite to be true.
“The findings of the study are unequivocal. The nitrogen content in the crops is reduced in atmospheres with raised carbon dioxide levels in all three ecosystem types. Furthermore, we can see that this negative effect exists regardless of whether or not the plants’ growth increases, and even if fertiliser is added. This is unexpected and new,” Uddling said.
The study found that for both wheat and rice, increased carbon dioxide in the atmosphere led to less nutritious crops. Wheat and rice are two of the most important crops globally — alongside maize, wheat and rice provide over 50 percent of the world’s plant-derived energy, according to the International Development Research Center.
Past studies have also seen reductions in nitrogen content for plants grown in a high-carbon environment, but have traditionally attributed it to a kind of dilution — the idea being that as carbon stimulates plant growth and the rate of photosynthesis increases, nitrogen uptake simply wasn’t able to keep up. That theory, Uddling said, has now been called into question.
“The findings of this study show that this interpretation is simplified and partly incorrect. We are seeing reduced nitrogen content even when growth has not been affected. Moreover, the effect is there in trials with powerful fertilizer, which indicates that it is not down to limited access to nitrogen in the soil,” Uddling said. “Future studies should look at what is causing the effect, but it appears to be linked to plants’ capacity to absorb nitrogen rather than to changed levels in the soil.”