Plants have been hailed as possible saviors of the planet as it continues to warm up, especially considering that they can absorb more harmful carbon dioxide than previously thought. However, now new research says soil nutrients may hinder this plan, keeping plants from slowing down climate change.

"Humanity so far has greatly benefited from plants removing carbon dioxide from the atmosphere," researcher Will Wieder said in a statement. "But if a lack of nutrients limits their ability to keep soaking up CO2, then climate change becomes an even bigger problem than we thought - unless society can cut back on emissions."

A team of University of Montana researchers looked at 11 leading climate models to examine changes in soil levels of nitrogen and phosphorus, important nutrients for plants. They found that decreased soil nitrogen will actually lessen plant uptake of CO2 by 19 percent, while limiting both nitrogen and phosphorus will reduce plant uptake by 25 percent.

Many of the world's leading climate models assume that the growing level of CO2 in the atmosphere will in fact accelerate plant growth - known as the CO2 fertilization effect. That is, the more the plants grow, the more CO2 they absorb from the atmosphere, thereby slowing climate change.

"But CO2 is far from the only determinant of plant growth," explained lead author Cory Cleveland. "Soil nutrients - especially nitrogen and phosphorus - also are critical. Because the supply of such nutrients is limited, scientists have warned that plant growth will be less than indicated in climate models."

"If society stays on its current trajectory of CO2 emissions and the growth rates of plants don't increase as much as many models project, the result by the end of the century could be more extreme than we predicted," he added.

In fact, there may be more than an additional 10 percent of CO2 in the atmosphere by the end of the century, which would accelerate climate change. (Scroll to read on...)

So if soil nutrients play such a major part in determining whether plants and other vegetation help or hurt the planet, why haven't scientists considered this factor before? Apparently, it's because such biogeochemical processes are difficult to simulate and vary greatly from one type of terrestrial ecosystem to another.

In this study, Cleveland and his colleagues decided to take a closer look. They studied the world's leading climate models, focusing on how they represented plant growth in specific geographic regions. They then compared that data to changes in nitrogen and phosphorus availability caused by deposition of airborne particles and other factors.

"We found that instead of acting as a carbon sink and drawing down CO2, the terrestrial biosphere could become a net source of the greenhouse gas to the atmosphere by the end of the century, with soil microbes releasing more carbon than growing plants could absorb," Cleveland said.

However, it should be noted that scientists are still unsure exactly how soil microbes will respond to warming temperatures, which free up nitrogen in the soil, but also release CO2 into the atmosphere.

Not to mention it is unclear whether or not plants will become more efficient at absorbing additional nutrients from the soil. If not, plants won't be able to keep up with humanity's CO2 emissions.

"To store that much carbon on land, plants will need more nitrogen and phosphorus," Wieder said. "If they can't get it, we're going to go from terrestrial ecosystems sponging up CO2 to actually having them contribute to the problem."

The results were published in the journal Nature Geoscience.

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