A new study has found that soils under "desert pavement" have an unusually high concentration of nitrate - a potential water contaminant, close to the surface.
This study was carried out by a team of researchers from UC (University of California) Riverside.
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. It is also, however, 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.
Salts, including nitrate, are formed in deserts as water evaporates on dry lake beds. These salts then get blown on to the desert pavement by winds.
Other contributors of nitrate to desert pavement soils are atmospheric deposition and soil bacteria, which convert atmospheric nitrogen into nitrate that is usable by plants and other organisms.
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 with time.
In their study, Graham and his colleagues sampled three widely separated locations with well-developed desert pavement in the Mojave Desert. The researchers found that the nitrate they observed in association with desert pavement was consistent across the landforms.
Graham and his team of researchers found that nitrate concentration in soils under desert pavement in the Mojave reached a maximum 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 throughout the upper meter.
"After water, nitrogen is the most limiting factor in deserts, affecting net productivity in desert ecosystems," said Robert Graham, a professor of soil mineralogy in the Department of Environmental Sciences 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," he added.
According to the research, moister climates, increased irrigation, wastewater disposal, or flooding may transport high nitrate levels to groundwater or surface waters, which is detrimental to water quality.
"Deserts account for about one-third of Earth's land area," said Graham. "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," he added.