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An unexpected outcome from rising CO2

December 3, 2009


Peter Reich and students at Cedar Creek.

Elevated carbon dioxide levels may mitigate losses of biodiversity from nitrogen pollution, according to work by Regents Professor Peter Reich.

Photo: Patrick O'Leary

Higher carbon dioxide levels may help reduce some biodiversity losses

By Deane Morrison

For years, a global rain of nitrogen from fertilizers and exhaust fumes has been linked to losses of species diversity among communities of plants. And there are fears that the rising carbon dioxide (CO2) level could also reduce biodiversity, especially when it acts in concert with nitrogen pollution.

Now, work by University of Minnesota researcher Peter Reich shows that in fact, rising CO2 could mitigate the loss of biodiversity brought about by nitrogen pollution. He published his findings in the December 4 issue of Science.

A complex combo

According to Reich, studies indicate that nitrogen pollution today could lead to a 25 percent reduction in plant biodiversity. There is also worry that together, rising CO2 and nitrogen pollution could cut diversity by up to 50 percent. But whether these two factors will join forces to wreak that kind of havoc cannot be predicted, and so Reich decided to find out.

By the numbers

Here are the levels of elevated CO2 and added soil nitrogen, averaged over 10 years, to which plots of prairie plants were exposed. For CO2: 560 ppm, a level likely to become the global mean between 2050 and 2080 at current rates of increase. Today it's about 380 ppm. Nitrogen: four grams per square meter per year, or about 4-6 times the usual nitrogen pollution in Minnesota. Control plots received no extra CO2 or nitrogen.

In a 10-year grassland study, Reich, a Regents Professor of forest resources, examined the effects of increasing CO2 and soil nitrogen singly and together. He found that adding nitrogen to the soil reduced the number of plant species by 16 percent, and growing plants under an elevated level of CO2 reduced diversity by about two percent.

But the drop was only eight percent when plants were grown with higher levels of both nitrogen and CO2.

That was a far cry from the worst-case scenario, in which the loss of species under both treatments would have been 18 percent (16 from nitrogen, 2 from CO2) or higher. But neither was it a big surprise, given that environmental forces often combine their effects in complex and unpredictable ways.

So what led to this result?

A battlefield out there

As an example, consider the ways elevated CO2 and nitrogen affect the way plants use water. Under enriched nitrogen, certain species grew faster, used more water, and thus reduced the availability of water for other species. This hurt the less dominant plant species and reduced the overall species number.

But elevated CO2 led to a higher percentage of water in the soil; this counteracted the effect of nitrogen enrichment, allowing more species to survive.

"This gave plant biodiversity a boost," Reich says. But he cautions that this doesn't give one carte blanche for optimism. What his work illustrates more than anything else is the difficulty of predicting the outcomes of interactions among the atmosphere, the climate system, and terrestrial ecosystems.

"While it is a relief to find out that rising CO2 and nitrogen together may not cause enormous losses of diversity, all losses of diversity are troubling, and in any case this finding does not detract from the urgent need for us to curb CO2 emissions, given the other critical CO2 effects," he says.


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