A new study in the Mojave Desert, Calif., has found that soils under “desert pavement” have an unusually high concentration of nitrate close to the surface. Vulnerable to erosion by rain and wind if the desert pavement is disrupted, this vast source of nitrate could contaminate surface and ground water, posing an environmental risk, according to the University of California-Riverside-led research.
Desert pavement is a naturally
occurring, single layer of closely fitted rock fragments. A common land surface
feature in arid regions, it has been estimated to cover nearly half of North
America’s desert landscapes.
Nitrate, a water-soluble nitrogen
compound, is a nutrient essential to life, however, as drilling contractors
know, it also is a contaminant. When present in excess in aquatic systems, it
results in algal blooms. High levels of nitrate in drinking water have been
associated with serious health issues, including methaemoglobinaemia (blue baby
disease, marked by a reduction in the oxygen-carrying capacity of blood),
miscarriages and non-Hodgkin’s lymphoma.
Salts, including nitrate, are
formed in deserts as water evaporates on dry lakebeds. These salts then get
blown on to the desert pavement by winds. Other contributors of nitrate to
desert pavement soils are atmospheric deposition (the gradual deposition of
nutrient-rich particulate matter from the air), and soil bacteria, which
convert atmospheric nitrogen into nitrate that is usable by plants and other
Ordinarily, in moist soils, plants
and microbes readily take up nitrate, and water flushing through the soils
leaches the soils of excess nitrate. But desert pavement, formed over thousands
of years, impedes the infiltration of water in desert soil, restricting plant
development and resulting in desert pavement soils becoming nitrate-rich (and
saltier) with time.
“After water, nitrogen is the most
limiting factor in deserts, affecting net productivity in desert ecosystems,”
says Robert Graham, a professor in the Department of Environmental Sciences at
University of California-Riverside and the lead author of the research paper.
“The nitrate stored in soils under desert pavement is a previously unrecognized
vast pool of nitrogen that is particularly susceptible to climate change and
human disturbance. Moister climates, increased irrigation, wastewater disposal
or flooding may transport high nitrate levels to ground water or surface
waters, which is detrimental to water quality.”
In their study, Graham and his
colleagues sampled three widely separated locations with well-developed desert
pavement in the Mojave Desert. The locations were
selected to represent a variety of landforms commonly found in the desert. The
researchers found that the nitrate they observed in association with desert
pavement was consistent across the landforms.
“Deserts account for about
one-third of Earth’s land area,” Graham says. “If our findings in the Mojave
can be extrapolated to deserts worldwide, the amount of nitrate – and nitrogen
– stored in near-surface soils of warm deserts would need to be re-estimated.”
Graham and his team of researchers
found that nitrate concentration in soils under desert pavement in the Mojave
reached a maximum (up to 12,750 kilograms per hectare) within 0.1 to 0.6 meter
depth. In contrast, at each location they studied, the soils without desert
pavement had relatively low nitrate concentrations (80 to 1500 kilograms per
hectare) throughout the upper meter. “In these nonpavement locations, water was
able to infiltrate the soil and transport the nitrate to deeper within the
soil,” Graham explains.
The researchers note that desert
land use – road construction, off-road vehicle use and military training –
often disrupts fragile land surfaces, increasing surface erosion by rain and
wind. According to them, nitrogen-laden dust transported by wind from disturbed
desert pavement soils may impact distant nitrogen-limited ecosystems, such as
Furthermore, the researchers note
that increased soil moisture resulting from climate change increases the
potential for “denitrification” – a naturally occurring process in soil, where
bacteria break down nitrates to return nitrogen gas to the atmosphere.
“Denitrification also produces nitrous oxide, a major greenhouse gas,” Graham says.