The issue we’re working to address is how to design bioenergy systems that are sustainable. It’s not idealistic. We wanted to show that it’s doable; if we design for specific outcomes, we’ll see real results.

—Cristina Negri, principal agronomist and environmental engineer at Argonne

Negri and her team created a pilot farm site that balances the priorities of economic feasibility, bioenergy and environmental health.

We developed an approach to design such landscapes at a field scale to minimize concerns of land use change, water quality, and greenhouse gas emissions associated with production of food and bioenergy. This study leverages concepts of nutrient recovery and phytoremediation to place bioenergy crops on the landscape to recover nutrients released to watersheds by commodity crops. Crop placement is determined by evaluating spatial variability of: 1) soils, 2) surface flow pathways, 3) shallow groundwater flow gradients, 4) subsurface nitrate concentrations, and 5) primary crop yield. A 0.8 ha bioenergy buffer was designed within a 6.5 ha field to intercept concentrated surface flow, capture and use nitrate leachate, and minimize use of productive areas.

—Ssegane et al.

Rather than looking at whole fields as the unit of planting decisions, researchers analyzed subareas of the cornfield. They found that subareas with the lowest yield also had the lowest nitrogen retention. These sections of land are doubly taxing—unprofitable for the farmer and damaging to the environment.

Negri explained what happens in the underproductive land: “Imagine pouring a nice, nutrient-rich solution through a fertile soil with plants growing in it,” she said. These nutrients would be retained by the soil long enough to be taken up by plants, minimizing any leakage. “Now imagine pouring this same solution through a colander: If nutrients filter through the soil too quickly, they’re no longer available for plants. The corn grows less, and more nitrogen is leached into groundwater.”

Planting bioenergy crops in the colander-like soil could solve both problems—environmental and economic—as the Argonne team showed with the Denitrification Decomposition simulation.

Willows and switchgrass are perennial bioenergy crops; these plants have a more extensive root system than annual plants, which start their growth from scratch every year. Deeper roots are better able to absorb nitrogen as it seeps deeper into the soil.

The loss of nitrogen from agricultural land is a major environmental concern. If not retained by soil or taken up by plants, nitrogen escapes into air or water. It is released into the atmosphere as nitrous oxide, a greenhouse gas with 310 times the warming potential of carbon dioxide. Nitrate leaking into water spurs oxygen depletion that harms aquatic ecosystems and can lead to toxic algal blooms, as seen in Lake Erie. The Fairbury cornfield is located within the Indian Creek Watershed, draining to the Vermilion River and eventually to the Gulf of Mexico, which for years has been suffering from oxygen depletion caused by nutrient runoff.

While scientists may be invested in energy and environment, the team recognized that farmers have to think first and foremost about their economic bottom line.

Across the entire field your farm might be profitable, but by collecting more specific data we can identify subareas where the farmer is not recovering his or her investment.

—Argonne postdoctoral researcher Herbert Ssegane

The money lost comes from farmers cropping and applying expensive nitrogen fertilizers to patches of the field that are just not producing enough. Inserting rows of bioenergy crops where there is low corn yield means the farmer is not sacrificing substantial profit from row crops. As a cost-saving bonus, the deep-rooted bioenergy crops naturally accumulate the lost nitrogen as a free fertilizer.

Argonne scientists planted willows at the Fairbury site in 2013 and will continue collecting data through next year to see how results compare to their predictions. “We’ve already reached a 28 percent reduction in nitrate, even with two full growing seasons still ahead of us,” Ssegane said. Willow growth has also been good, without the researchers applying any fertilizer.

According to Ssegane, this project is about proving a concept. It shows farmers that strategic planting of bioenergy crops can increase productivity and save money, while demonstrating to the scientific community that bioenergy will be sustainable if we match plants to their optimal position within a landscape.

Before this work, the popular idea was ‘dedicated fields,’ where you might convert a large area from corn to switchgrass. But dedicated fields of bioenergy crops are currently inviable in an agricultural setting where the economy is tied to grain. What does pass the cost-benefit test is converting underproductive subareas to an alternative crop.

—Herbert Ssegane

A multifunctional landscape finds an efficient medium between a dedicated bioenergy field and a farm growing continuous acres of the same cash crop.

The scientists are exploring how these design principles can be scaled up to the entire watershed. Eventually, they hope this research informs agricultural planning for scientists and farmers alike.

The most recent paper analyzing data from Fairbury was published in the journal Biomass and Bioenergy. Funding for this research comes from the DOE’s Office of Energy Efficiency and Renewable Energy, Bioenergy Technologies Office.

Resources

• Herbert Ssegane, M. Cristina Negri, John Quinn, Meltem Urgun-Demirtas (2015) “Multifunctional landscapes: Site characterization and field-scale design to incorporate biomass production into an agricultural system,” Biomass and Bioenergy, Volume 80, Pages 179-190 doi: 10.1016/j.biombioe.2015.04.012