Accounting for all forms of phosphorus

Leaving a buffer strip of uncultivated land next to streams is a proven way to keep nitrogen and phosphorus fertilizers on farms from running off into surface waters. But land managers looking to protect stream health may want to give these “riparian,” or streamside, buffers a closer look. New research suggests they themselves can be a source of phosphorus to waterways.

Eroding streambank in Vermont

That’s an important conclusion of a study in the Sept.-Oct. issue of the Soil Science Society of America Journal, which examined the amount of both inorganic and organic forms of phosphorus in the soils of two restored riparian areas bordering streams in northwestern Vermont.

Organic phosphorus isn’t routinely measured in agronomic soil studies, says lead author, Eric Young, of the William H. Miner Agricultural Research Institute in Chazy, NY. “The focus usually is inorganic phosphorus.” Yet, to the researchers’ surprise, more than half the phosphorus in the 14 riparian soils they investigated turned out to be organic.

Moreover, 75% of the phosphorus most at risk of running off into streams—so-called water-extractable phosphorus—was also organic, and about half of it was potentially “bioavailable.”

In other words, “If this organic phosphorus gets into the stream, there are naturally occurring enzymes produced by bacteria that can transform it into an inorganic form that would be immediately available to algae,” Young explains. “So that’s a water quality concern.”

The findings have important implications for Vermont. Despite two decades of efforts by farmers to keep agricultural phosphorus from entering waterways in the state’s Lake Champlain Basin, the lake’s water quality hasn’t improved to any significant degree.

In the meantime, people have long suspected that eroding stream banks in the watershed were adding phosphorus to Lake Champlain, but the levels and types of phosphorus in these stream bank soils hadn’t actually been measured. Based on the team’s findings, Young now hypothesizes that erosion of phosphorus-rich riparian soils is indeed confounding attempts to boost lake water quality through approaches like nutrient management planning and planting of buffer strips.

That doesn’t mean farmers should give up on these practices, of course. What it does mean is that recommendations for improving water quality and optimizing fertilizer use should consider all forms of phosphorus, Young asserts.

For example, the soil tests farmers use to decide much fertilizer to apply only measure inorganic phosphorus, even though scientists know some agricultural soils contain large amounts of organic phosphorus, as well. Scientists further understand that soil microorganisms transform much of this organic phosphorus into inorganic forms, making it available to plants.

“But there are currently few if any agronomic phosphorus recommendations, at least that I know of in the United States, which take this into account,” Young says.

One tool for predicting organic phosphorus availability might be the soil map. By combining the findings of their current research with previous work, Young and his collaborators demonstrated that some types of soils in riparian areas naturally contain higher levels of organic phosphorus than others. So, to identify those riparian buffers most at risk of adding phosphorus to streams, land managers might simply need to look for these particular soils on a map.

Once riparian areas with high-phosphorus soils were pinpointed, these areas could be prioritized for restoration activities that reduce stream bank erosion. Knowing how much phosphorus different riparian soils contain and what forms the phosphorus takes could also help scientists and practitioners predict how effectively a buffer strip will intercept phosphorus from adjacent croplands, adds Young.

First, though, people need to move away from their traditional focus on phosphorus in its inorganic forms. “I think this paper highlights the importance of starting to look at phosphorus more holistically and taking a more dynamic approach to predicting phosphorus availability,” Young says. “And that includes taking into account organic phosphorus.”

The study’s other authors are: Donald Ross, Barbara Cade-Menun, and Corey Liu. The work was supported in part by the Vermont Water Resources and Lake Studies Center through funding it receives from the U.S. Geological Survey.

View the abstract of the paper: Phosphorus Speciation in Riparian Soils: A Phosphorus-31 Nuclear Magnetic Resonance Spectroscopy and Enzyme Hydrolysis Study

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