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

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.”

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 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.”

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 explained.