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To the root of the matter – keeping nitrogen out of small streams


Feb. 25, 2014

GAINESVLLE, Fla. – For years, scientists tried to find out why some small streams carry only minute concentrations of nitrogen.

Now Stefan Gerber, a University of Florida researcher with the Institute of Food and Agricultural Sciences, and Jack Brookshire, an assistant professor of biogeochemistry from Montana State University, believe they have solved the mystery.

Nitrogen is a necessary nutrient for vegetation and occurs naturally from decaying plant matter. But many waterways worldwide carry high levels of nitrogen, which can often be attributed to pollution from fossil fuels, fertilizer, sewage and animal waste.  Too much nitrogen can lead to overgrowth of aquatic plants and algae, fish kills and even the death of water bodies.

Decaying plant matter also adds nitrogen to water in the soil in large quantities, but some streams in undeveloped areas only contain small concentrations of the nutrient. It is a phenomenon that has long puzzled scientists.

Gerber and Brookshire worked together to put the pieces together. They created a mathematical model that describes the motion of dissolved nitrogen molecules in soils through a process called diffusion, and the root absorption of these molecules.  The researchers showed that the diffusion and root absorption mechanisms work together to substantially reduce soil nitrogen concentrations. As a result, little is left to be carried into streams.

“Working through the equations makes you realize that plants have an amazing capability to scavenge for nutrient molecules in soils,”said Gerber, who works with UF’s Water Institute as a professor with the Soil and Water Science Department.  “Our results support evidence that healthy, intact ecosystems can help to keep our streams and waterways free of nutrient pollution.”

The duo compared the solutions of their mathematical framework with data from small watersheds and found a remarkable similarity between their prediction of nitrogen concentration and the actual nitrogen levels in streams.

The researchers’ work, published online in February in The American Naturalist, will help others developing ecosystem models and large models that integrate the complex interactions between climate, land vegetation and oceans on a global scale. It will also help improve predictions of how ecosystems respond to climate change and pollution.

“We can show how pristine ecosystems – for example forests – can really keep our streams clean. We show that these systems can do an amazing job in keeping streams virtually nitrogen free,” Gerber said. “One thing that it may be useful for in the future, is that it can serve as a reference, against which nitrogen pollution in streams can be evaluated.”

By Kimberly Moore Wilmoth, 352-294-3302,

Source:  Stefan Gerber, 352-294-3174,