Michelle Walvoord, Hydrology Program Alumna
by John Wilson, Professor of Hydrology, March 2004

One of the papers resulting from Michelle’s PhD dissertation was published in Science Magazine this last year (Walvoord et al., A reservoir of nitrate beneath desert soils, Science, 302, 1021-1024, 2003; also see the NMT press release). I knew she had submitted the paper but had not realized it had been accepted and published. I first learned of the publication when it came out in early November 2003. I was at the National Science Foundation, where I serve on an Advisory Board. Two NSF program directors and a couple of other Board members brought it to my attention, commenting in glowing terms and congratulating me (as if working in the same field and building deserves congratulations). Less than a month later I was sitting in the audience at the Fall 2003 meeting of the American Geophysical Union, when in two different sessions the plenary speaker went to special lengths to mention this paper. Like me, the speakers had trouble pronouncing the name of the author, whom they had never heard about before this. Why does this paper have this kind of impact?

There is a USGS web site that summarizes this work. In her field work Michelle, working with Fred Phillips and other colleagues, found unexpectedly large concentrations of nitrate in desert subsoils. “This previously overlooked accumulation of nitrogen is surprising because desert ecosystems are known to have become adapted to a lack of nitrogen. However, the nitrate recently found is within a few meters of land surface and below the biologically active root zone. Contrary to conventional wisdom, small amounts of naturally occurring nitrate appear to have been leaching from soil layers and accumulating for thousands of years in the vadose zone of arid regions. Scientists are concerned about this high level (thousands of kilograms per hectare) of nitrate because irrigating desert soils, a change to a wetter climate, disposal of liquid wastes, or construction of dams could release large quantities of nitrate to ground water, which in turn could potentially threaten drinking-water supplies (high concentrations of nitrate can cause human health problems). These accumulations will cause scientists to rethink the long-term movement of nitrate and how nitrate is used (or not used) by plants in desert environments.” http://toxics.usgs.gov/highlights/nitrate_desert.html

Why does nitrogen accumulate in this subsoil zone? The major thrust of Michelle’s work was aimed at understanding recharge through desert soils. The same field studies which led to the observation about nitrate found that southwestern deserts are still drying out since the ice age. Using field sites in the Mojave, Sonoran, and Chihuahuan Deserts, as well as the High Plains region, and northern New Mexico, Michelle discovered that sediments beneath the Nation’s southwestern deserts have been drying out for 16,000 years, since the cold and wet conditions of the ice age began to change to the hot and dry conditions of today. Aided by computer models she and her colleagues have learned that the drying out of the vadose zone has resulted in moisture moving upward rather than downward as expected. “Normally precipitation percolates downward and replenishes ground water. However, the arid climate and desert plants draw moisture upward from hundreds of feet down in the subsurface.”

What Michelle learned explains the reason for the large pool of nitrate. It also has “… implications for long-term waste disposal in the desert’s extremely dry unsaturated zones, because it means that removal of native vegetation, disposal of liquids, or changes to a wetter climate could potentially allow water and contaminants to percolate downward.”

http://toxics.usgs.gov/highlights/drying_deserts.html

I looked for confirmatory evidence of impact in the literature and on the web. Her work is mentioned on a variety of science news and views sites, including ones at USGS, NASA, LANL, and the Center for the Study of Carbon Dioxide and Global Change.

The best is contained in the news article in the front pages of the same issue of Science. Eric Stokstad writes that this work shows 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. ‘This paper clearly will cause people to think about arid lands and where they fit into the global nitrogen cycle,’ says ecologist Ross Virginia of Dartmouth College in Hanover, New Hampshire.’” Walvoord estimates that it could mean 16% more nitrogen in Earth’s soil than previously thought and as much as 71% more in deserts. In commenting on this newly-found store of nitrogen, Stokstad calls it a "potential bonanza" and says that Duke University ecologist Robert Jackson actually “wonders if the pool of nitrate could help explain why deep-rooted woody plants have invaded the Southwest over the past century or so.”