Large source of nitrate, a nutrient and potential water contaminant, found in near-surface desert soils

University of California, Riverside: A UC Riverside-led study in the Mojave Desert, Calif., has found that soils under “desert pavement” have an unusually high concentration of nitrate, a type of salt, 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 groundwaters, posing an environmental risk. Study results appear in the March issue of Geology.

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, 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 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 (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 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 (and saltier) with time.

“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. 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,” Graham said. “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.”...

Desert pavement overlying soil. Image credit: Graham lab, UCR.