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June 2, 2016

Fertilizer use could reduce climate benefit of cellulosic biofuels

According to a new study from the Great Lakes Bioenergy Research Center and Michigan State University, the use of nitrogen fertilizer on switchgrass crops can produce a sharp increase in emissions of nitrous oxide, a greenhouse gas up to 300 times more harmful than carbon dioxide and a significant driver of global climate change.

Switchgrass is one of several crops poised to become a feedstock for the production of “cellulosic biofuels,” fuels derived from grasses, wood or the nonfood portion of plants. Though touted for being a clean energy alternative to both fossil fuels and corn ethanol, cellulosic biofuel comes with its share of complexities. Many of its environmental benefit depends, for starters, on how its crops are grown.

“We’ve established that the climate benefit of cellulosic biofuels is much greater and much more robust than people originally thought,” said Phil Robertson, University Distinguished Professor of Ecosystem Science at MSU and coauthor. “But what we’re also seeing is that much of that climate benefit is dependent. It’s dependent on factors such as land use history and – as we’re seeing with these results – it’s dependent on nitrogen fertilizer use.”

Led by former MSU graduate student Leilei Ruan and published this week in Environmental Research Letters, the study reports nitrous oxide emissions from switchgrass grown at MSU’s Kellogg Biological Station when fertilized at eight different levels.

“What we discovered is that there’s not a one-to-one relation between adding fertilizer and producing nitrous oxide,” Ruan said. “It’s not a linear relationship. After a certain amount of fertilizer is added, there is, proportionately, much more nitrous oxide produced than what you might expect.”

The cause of that nonlinear relationship can be traced to the soil microbes responsible for converting nitrogen fertilizer to nitrates and then to nitrous oxide. Unlike humans, when some soil microbes are short on oxygen they have the option of using nitrate in place of oxygen. As the microbes respire these nitrates they produce nitrous oxide. Ruan says that fertilizing beyond what the plant can use and needs is likely providing an opportunity for these soil microbes to take up excess nitrate and produce nitrous oxide.

The disproportionately adverse results of over fertilizing have the potential to effectively change the math on biofuel crops’ net climate benefit. An over fertilized switchgrass crop can reduce its climate benefits as much as 50 percent once the fertilizer’s production, use, and nitrous oxide emissions are subtracted from the crop’s carbon benefit.

The study also measured the relationship between fertilizer and nitrate leaching, and found – also for the first time – that nitrate leaching is also disproportionately greater at high fertilization rates. Soil nitrate not converted to nitrous oxide is also available for loss to groundwater and then eventually to streams, lakes, rivers and wetlands, where it’s once again eligible to be converted into nitrous oxide.

“If we’re ever going to realize the environmental potential of biofuels, we will need to have smart strategies for fertilizing cellulosic crops,” Ruan said.

Potential strategies include developing nitrogen use calculators to help farmers determine how much fertilizer to use, or paying farmers for the perceived risk of yield loss as a result of lower fertilization.

Robertson says future research in this area could focus on identifying which soil microbes are responsible for the nitrous oxide increase in order to develop management strategies that suppress them, or – sidestepping the microbes entirely – simply designing a plant capable of more efficient nitrogen use.

MSU’s Stephen Hamilton and Ajay Bhardwaj also contributed to the paper.

GLBRC is one of three Department of Energy Bioenergy Research Centers created to make transformational breakthroughs and build the foundation of new cellulosic biofuels technology.

By: Layne Cameron

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