Researchers find losses of nutrients, greenhouse gases from poorly drained depressions higher than predicted

wet spot in farm field with cloudy sky above
Researchers are investigating nutrient loss at poorly drained, cropped depressions in the tile-drained areas of Iowa. Photos by Nate Lawrence. 

AMES, Iowa — With the help of an innovative new monitoring system, Iowa State researchers looking at nitrogen and phosphorus losses to water and the atmosphere are able to pinpoint nutrient fluxes better than many previous studies. In the process, they are refining knowledge of where – and when -- to expect nutrient-loss hot spots.   

Steven Hall, assistant professor in the Department of Ecology, Evolution and Organismal Biology, is leading the work to investigate nutrient loss at farmed wet spots in central and north central Iowa. Funding for several related projects Hall is working on has come from the Iowa Nutrient Research Center, the Leopold Center for Sustainable Agriculture and the U.S. Environmental Protection Agency.

The poorly drained, cropped depressions in the tile-drained areas of the state make up a significant portion of Iowa’s landscape. “Even with our modern drainage infrastructure, we still see substantial ponding in many of these areas over most years,” Hall said. “This often results in high crop mortality. From a producer perspective, these areas tend to represent net loss of income. 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.”

There are several reasons for this, according to the researchers. The areas’ clay-rich soils are naturally high in nutrient-rich organic matter, which often accumulates in depressions over time as erosion and runoff move soil and nutrients downslope. Also, high crop mortality means that readily available nutrients which would have been taken up by the plant may instead be lost to the environment. Even so, the common thought has been that 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,” Hall 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), a potent greenhouse-inducing gas.” Contrasting with natural wetlands, the researchers found that farmed depressions tend to lose more nitrogen in environmentally harmful forms.

The new monitoring tool the team developed has made it possible to more accurately track emissions of N2O. 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. 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 PhD student working with Hall. Their tool is described in a recent article in the peer-reviewed journal, “Atmospheric Measurement Techniques.”

The higher-frequency measurements are critical to accurately track 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,” Lawrence 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. When soils are at intermediate moisture levels, they generally emit more N2O.

These same conditions can also increase leaching of water-borne nitrogen and phosphorus to below-ground tiles and groundwater. This happens in part due to the shrink-swell clay soils that are typical of poorly drained areas. Under wet or flooded conditions, the soil particles are swollen and “tight,” limiting infiltration. When conditions are dry, the soils shrink, causing surface cracks that can quickly transport water during rainstorms.

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

“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.”

Hall is involved with other research at Iowa State looking into ideas for conservation and cropping practices that could help reduce nutrient loss from poorly drained wet areas. These studies are exploring the potential impacts of conservation tillage, growing perennial crops that may be able to survive better in wet soils, incorporating winter cover crops and using alternative drainage.

The team’s research also suggests there may be benefits to increasing drainage in some of 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. 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,” Hall said.

“If nothing else, 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. There’s an economic benefit right off the bat to reducing nutrient inputs in areas where they are not needed.”

The Iowa Nutrient Research Center pursues science-based approaches to evaluating the performance of current and emerging nutrient management practices, providing recommendations on implementing the practices and developing new practices.

Monitoring equipment in area of field where emerging crops have been drowned out.
An innovative, new monitoring system developed by the research team collects measurements of nitrous oxide emissions from study plots every few hours, greatly increasing the frequency of data collection relative to traditional manual sampling methods. Nutrient leaching was measured using buried collectors installed near each gas measurement apparatus.