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Desert Dirt Hoards Nitrogen
6 November 2003 (All day)
Desert plants face a hardscrabble life. Their soil is parched, and it's poor in bioavailable nitrogen, an essential nutrient. A few meters down, though, lies a potential bonanza. In the 7 November issue of Science, a team reports that desert subsoils in the southwestern United States contain much more nitrogen than previously estimated, a finding that raises questions about how desert ecosystems work.
The nitrate was discovered by accident. Hydrologist Michelle Walvoord, then a Ph.D. student at New Mexico Institute of Mining and Technology in Socorro, and hydrologist Peter Hartsough of the University of Nevada, Reno, were drilling cores at the Nevada Test Site to study chloride in the subsoil. They found a peak of chloride several meters below the surface, which indicates that the climate became drier some 16,000 years ago. Oddly, nitrate concentrations also peaked at that depth. "We thought that was weird," recalls Walvoord, now a postdoc at the U.S. Geological Survey in Lakewood, Colorado, because nitrate in soil is usually concentrated in the uppermost meter.
The matching peaks suggested that nitrate, like chloride, was leaching from the soil without being taken up by plants or microbes. To check out the pattern, Walvoord, Hartsough, and colleagues studied four more desert sites in the Southwest. Although only two other cores showed a tight match between chloride and nitrate, the researchers found subsoil nitrate in almost all the cores, in amounts ranging from 2000 to 10,000 kilograms per hectare. That's up to 10 times more than is found in the topsoil.
Because deserts make up one-third of the dry land on Earth, their subsoil reserves could add up to a lot of nitrogen, Walvoord and her colleagues say. They estimate that it could mean up to 16% more nitrogen in Earth's soil than previously thought and as much as 71% more in deserts. In the future, Walvoord and colleagues say, that huge nitrate reservoir could percolate into aquifers and contaminate groundwater if Southwestern deserts are irrigated or the climate becomes wetter.
How widespread that problem could be isn't clear. "The variation they saw across cores makes regional and global extrapolations a lot more questionable," cautions Robert Jackson of Duke University in Durham, North Carolina. At the same time, Jackson wonders if the pool of nitrate could help explain why deep-rooted woody plants have invaded the Southwest over the past century or so: "The paper is helpful because it makes us rethink the way deserts work."