Iowa State research explores complexity of dissolved phosphorus loss

A research project led by soil scientists at Iowa State University provides new insight into the complex picture of phosphorus loss from agricultural land and evidence that phosphorus runoff is often underestimated.

One of the main objectives of the project was to determine the amount of dissolved phosphorus in runoff that cannot be accurately measured by common methods. The work was carried out over three years with funding from the Iowa Nutrient Research Center at Iowa State.

“Conventional views on phosphorus loss are that most of it is tightly bound to soil particles, so phosphorus is not a problem as long as erosion is controlled. Our study shows that it is not necessarily the case,” says Antonio Mallarino, professor of agronomy and ISU Extension and Outreach specialist in soil fertility and nutrient management. He led the project, working with agronomy research specialist Mazhar U. Haq and former graduate student John D. Jones, Jr.

Mallarino’s team used soil and surface runoff samples from several field experiments in Iowa. From these samples, they measured P loss in corn and soybean fields that had different soils, P sources, tillage management, and conservation practices. Their findings provide new insights and confirm a growing body of nationwide research that shows more phosphorus sheet fields are dissolving in runoff than previously thought. And that some conservation practices may not reduce – or even increase – the loss of dissolved phosphorus.

“Any P that leaves the field is potentially important,” Mallarino says. “However, dissolved P is in a highly bioavailable form, so it is of particular concern for its contribution to fresh and coastal water eutrophication and harmful algal blooms.”

The measure
Phosphorus is characterized in several ways, and there are different approaches to monitoring where and how it manifests in the environment. Another of the team’s main research goals was to compare measurement methods to better understand how P and its different forms can be estimated most accurately.

“Traditional methods for testing total bioavailable P from runoff have been complicated, time-consuming, and expensive to use,” Mallarino says.

An important achievement of this research was to show that the common measurement of dissolved reactive P in runoff often significantly underestimates the loss of total dissolved and bioavailable P, according to Mallarino.

Additionally, he says, research has demonstrated that a simple test of total dissolved P performed on filtered runoff with inductively coupled plasma spectrometry provides comparable results to other methods of measuring total dissolved or bioavailable P. , and it is much cheaper and easier to make.

“This part of our work should help inform — and improve — efforts to track the total loss of dissolved phosphorus from soils,” Mallarino said.

Management
Another major objective has been to refine knowledge and guidelines for best management practices to reduce the loss of dissolved phosphorus through runoff. Researchers examined the influence of different tillage systems and sources of P, as well as the potential benefits of cover crops and the use of two soil amendments, alum and gypsum, believed to reduce soil loss. by P.

What they found showed that applying P beyond the amount needed by crops significantly increases the loss of dissolved P. In addition, direct seeding management increased the proportion of total P loss composed of dissolved and bioavailable forms, mirroring the results of other long-term research efforts, particularly in the Great Lakes region.

With tillage to incorporate fertilizer or manure, the proportion of dissolved P in runoff was lowest where solid poultry manure was applied and highest after fertilizer or pig slurry. With no-till, however, the proportion of dissolved phosphorus loss was highest for fields where fertilizers were applied. Where manure was applied to no-till fields, loss did not differ significantly by type of manure — likely because pig manure was injected into the soil, according to Mallarino.

Use of a cover crop of winter grain rye reduced all forms of dissolved P in runoff – unlike a study in the Great Plains which showed increased loss of dissolved reactive P where cover crops were used. Iowa research showed the benefits of cover crops for fields managed with both tillage and no tillage.

Of the two soil amendments tested, only aluminum sulphate (alum) consistently reduced the loss of dissolved and bioavailable P.

“This work to better understand forms of phosphorus loss is critical as we work to reduce losses to downstream water bodies,” says Matt Helmers, director of the Iowa Nutrient Research Center. “It highlights the importance of dissolved phosphorus loss and the need to better understand the impact of conservation practices on dissolved phosphorus.”

Complex interactions
Mallarino and his team have a final report with more details posted on the Iowa Nutrient Research Center website. They are also working on other publications to highlight specific aspects of their long-term research and suggest next steps.

“Soils, rainfall, management and other factors interact in complex ways that mediate the movement of phosphorus from fields,” Mallarino explains. “Even then, the best advice is to avoid overloading soils with P. Reduce the amount of P in fertilizer and manure applications to the optimal levels needed by crops and use the Iowa P-Index to monitor the risk of loss when using nitrogen-based manure application rate. Otherwise, our research confirms that excess accumulated P will move out of the field. The amount will depend on rainfall and conservation practices.

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Sharon D. Cole