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7nov03

by George Zamora

SOCORRO – A New Mexico Tech hydrology professor and U.S. Geological Survey scientist who earned her doctorate at Tech have teamed up with five other researchers to co-author a groundbreaking scientific article in the latest issue of the prestigious journal Science, which details the discovery of large reservoirs of nitrates that had previously been overlooked in studies of global nitrogen distribution.

The study titled “A Reservoir of Nitrate Beneath Desert Soils” was originated by Michelle A. Walvoord as part of her doctoral dissertation study conducted at New Mexico Tech. Her dissertation was conducted under the direction of her faculty advisor and now coauthor, Fred M. Phillips, a professor of hydrology at New Mexico Tech.

“Up until now, desert soils were considered nitrogen limited, and nitrogen availability was thought to be a significant limiting factor on desert biomass production,” Phillips says. “But, most of the previous studies conducted on the nitrogen content of desert soils were done to a maximum depth of about a meter.”

In contrast, Phillips and Walvoord’s study was based largely on data obtained from soil cores that were drilled 10 meters in depth into various desert terrains.

Walvoord had initially been analyzing the soil cores for chloride content, using an ion chromatograph, and since nitrogen comes out fairly close to chloride in such tests, she soon noticed the unexpected presence of high levels of nitrogen in the desert soils.

“What she found were very high levels of nitrate in the soil cores she was sampling — along the lines of thousands of parts per million — particularly down in depths of three to eight meters,” Phillips says. “These nitrate concentrations are up to ten times greater than those used for fertilizing water used in hydroponic agriculture.”

The data obtained further supports Phillips and Walvoord’s theory that this previously undetected reservoir of nitrate has actually been accumulating in desert soils from anywhere from a few hundred years to upwards of many thousands of years.

The amounts of nitrogen measured in this new study have been determined to be as large as 10,000 kilograms per hectare sitting below the surfaces of deserts throughout the world, significantly increasing the impact that deserts were thought to have on the global nitrogen cycle.

“This could mean a more than 50 percent increase in the amount of available nitrate thought to have been present in desert soils, and a more than 15 percent increase in the amount of available nitrate worldwide,” Phillips explains.

Even though the new study has the potential of helping scientists better understand the global nitrogen cycle, Phillips concedes that, like many other scientific studies, the research article he co-authored raises more questions than it answers.

“We’re at a loss to explain why desert vegetation doesn’t use up most or at least some of the nitrogen in this reservoir of nitrate,” Phillips says. “Here we have an ecosystem that was previously thought to be nitrogen limited where we’ve now found plant roots virtually soaking in pools of nitrogen, yet they’re not using it. That’s become the paradox of our study.”

The study could also have implications in cases where desert terrains are stripped of their native vegetation and are converted over to agricultural uses — situations that are becoming more and more common in some places.

“If a pool of nitrate gets displaced, it could seep down and contaminate the groundwater underneath it,” Phillips points out.

Desert soils, the article in Science states, cover approximately one-fourth of the conterminous United States and one-third of the land surface worldwide.

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