One of the benefits of cellulosic biofuels that has been touted is that they are considered more environmentally friendly and may contribute less to global climate change. However, a new study released last week indicates crops grown for cellulosic biofuels may actually contribute up to higher emissions of nitrous oxide, a greenhouse gas that contributes to global warming.
The study, conducted by the Great Lakes Bioenergy Research Center and Michigan State University, found that using nitrogen fertilizer on switchgrass crops sharply increases the amount of nitrous oxide emitted. Switchgrass has been heralded as a significant feedstock for producing cellulosic biofuels, which are derived from grasses, wood or other nonfood portions of plants. It has been believed that the crop was a clean energy alternative for both fossil fuels and corn ethanol. However, the crop's environmental benefit depends on how it's 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 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 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.