AMES, Iowa – Iowa State University (ISU) researchers are studying nitrogen and phosphorus losses to water and the atmosphere, pinpointing nutrient fluxes using an innovative new monitoring system – and refining their knowledge of where and when to expect nutrient-loss hot spots.   

“Even with our modern drainage infrastructure, we still see substantial ponding in many of these areas over most years,” said Steven Hall, ISU assistant professor of ecology, who is leading the work to investigate nutrient loss at farmed wet spots in central and north-central Iowa.  

“This often results in high crop mortality,” he added. “From a producer perspective, these areas tend to represent net loss of income.” 

Hall said the poorly drained, cropped depressions in the tile-drained areas of the state make up a significant portion of Iowa’s landscape. “From an environmental perspective, we’re finding these low spots are especially susceptible to environmental losses of nutrients, and this is exacerbated when the crop doesn’t survive,” he said.

According to the researchers, the areas’ clay-rich soils are naturally high in nutrient-rich organic matter, which often accumulates in depressions over time. 

In addition, high crop mortality means that readily available nutrients, which would have been taken up by the plant, may instead be lost to the environment. 

Hall said the common thought has been the dominant form of nitrogen loss from wetland-type areas is probably as a harmless form of gas, dinitrogen (N2), produced by denitrification; N2   makes up much of the earth’s atmosphere.

“This is really difficult to measure, though,” he said. “This is happening, but we also see significant nitrate leaching to water and additional nitrogen losses in the form of the nitrous oxide (N2O).” 

Contrasting with natural wetlands, the researchers found farmed depressions tend to lose more nitrogen in what they referred to as “environmentally harmful forms.”

The new monitoring tool the team developed has made it possible to more accurately track emissions of N2O, he said. Their system measures N2O emissions from research plots every few hours, providing tens of thousands of data points over the year, many more than are possible from people traveling to the fields taking manual measurements. 

He said the set-up is reasonably inexpensive, compared to existing technologies, and works in all kinds of weather. The main developer of the new measurement system is Nathaniel Lawrence, a Ph.D. student working with Hall. 

Lawrence said the higher-frequency measurements are critical to accurately tracking the dynamic biological processes that drive nitrogen cycling. 

“During brief periods of days, or even hours, when the moisture circumstances are just right, we’re finding you may have much of the nitrous oxide produced for the whole year,” he said. “With standard monitoring techniques, you would have to get very lucky to catch this.”

The new monitoring system also tolerates high moisture conditions, allowing the researchers to refine understanding about when and where to expect higher losses of undesirable N2O. 

When the wet areas are inundated, harmless denitrification tends to be dominant, Hall said. When soils are at intermediate moisture levels, they generally emit more N2O.

Under wet or flooded conditions, the soil particles are swollen and tight, limiting infiltration. 

But when conditions are dry, the soils shrink, causing surface cracks that can quickly transport water during rainstorms.

“Overall, a lot of nitrogen slips through the cracks of these depressions literally and figuratively, moving into tile water, or being lost into the atmosphere,” he said.

“We also appear to have greater amounts of soluble phosphorus loss due to leaching in the cropped landscape than has been expected, and the loads coming from the depressions tend to be the highest,” he added.

The team’s research also suggested there may be benefits to increasing drainage in some of the areas, as the intermediate drainage levels of many farmed depressions make them more likely to function as hot spots for nutrient loss.

“At this point, they’re crummy farmland and crummy wetlands,” he said. “If they were more intensively drained and offset with better wetlands at strategic locations, we could likely optimize desired outcomes for farmers, and for the environment.”

He said reducing or eliminating application of fertilizer in and near landscape wet spots would be a great place to start. 

“Excess nutrients are accumulating in these landscape depressions,” he said. “There’s an economic benefit right off the bat to reducing nutrient inputs in areas where they are not needed.”