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Research Finds Arid Areas Absorb Unexpected Amounts of Atmospheric Carbon

Posted: April 10, 2014

UNLV researchers were part of a team that found that arid areas, among the most widespread ecosystems on the planet, take up an unexpectedly large amount of carbon as levels of carbon dioxide increase in the atmosphere.

The findings give scientists a better understanding of the earth’s carbon budget – how much carbon remains in the atmosphere as CO2, contributing to global warming, and how much gets stored in the land or ocean in other carbon-containing forms.

“It has pointed out the importance of these arid ecosystems,” said lead researcher R. Dave Evans, a Washington State University (WSU) professor of biological sciences specializing in ecology and global change. “They are a major sink for atmospheric carbon dioxide, so as CO2 levels go up, they’ll increase their uptake of CO2 from the atmosphere. They can’t take it all up, but they’ll help.”

The findings, published in the journal Nature Climate Change, come after a novel 10-year experiment in which researchers exposed plots in the Mojave Desert to elevated carbon-dioxide levels similar to those expected in 2050. They then removed soil and plants down to a meter deep and measured how much carbon was absorbed over the decade.

“We just dug up the whole site and measured everything,” said Evans.

The idea for the experiment originated with scientists at Nevada’s universities in Las Vegas and Reno, led by Stan Smith (UNLV) and Bob Nowak (UNR). Evans was brought in for his expertise in nutrient cycling and deserts, while researchers at Desert Research Institute, the University of Idaho, Northern Arizona, Arizona State, and Colorado State University also contributed.

The work addresses one of the big unknowns of global warming: the degree to which land-based ecosystems absorb or release carbon dioxide as it increases in the atmosphere.

Receiving less than 10 inches of rain a year, arid areas run in a wide band at 30 degrees north and south latitude. Along with semi-arid areas, which receive less than 20 inches of rain a year, they account for nearly half the Earth’s land surface.

Forest soils have more organic matter and, square foot for square foot, hold much more carbon. But because arid soils cover so much area, they can have an outsize role in the Earth’s carbon budget and in how much the earth warms as heat-trapping gases accumulate in the atmosphere.

Working on the Nevada National Security Site (formerly Nevada Test Site), the researchers marked off nine octagonal plots about 75 feet in diameter. For 10 years, three of the desert plots received no extra air; three were pumped with air containing carbon dioxide concentrations of 380-400 parts per million, the current level; and three received concentrations of 550 parts per million, the level expected in 2050. The CO2 was fed through PVC pipes ringing the plots and had a specific chemical fingerprint that could be detected when the soil, plants, and other biomass were analyzed.

Overall, rising CO2 levels may increase the uptake by arid lands enough to account for 4 to 8 percent of current global CO2 emissions.

The experiment did not account for other possible changes stemming from climate change, like varying precipitation and warming temperatures.

Still, said Smith, “We were surprised at the magnitude of the carbon gain and that we were able to detect it after 10 years, because 10 years isn’t very long in the life of a desert ecosystem.”

While forest ecosystems tend to store carbon in plant matter, the Mojave researchers found most carbon was being taken up by increased activity in the rhizosphere, a microorganism-rich area around plant roots.

From an optimistic point of view, the research suggests that by mid-century arid ecosystems will be doing more than their fair share of taking earth-warming carbon out of the atmosphere. But a potential cause for concern is what happens to these ecosystems as the planet’s population grows and people look for places to develop and live. Smith noted, “Maintaining sustainable desert landscapes may thus be increasingly important as global climate continues to change in the future.”

Funding came from the U.S. Department of Energy’s Terrestrial Carbon Processes Program and the National Science Foundation’s Ecosystem Studies Program.