Fertiliser use could reduce climate benefit of cellulosic biofuels, US researchers claim

The use of nitrogen fertiliser 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, according to a new, three-year study from the Great Lakes Bioenergy Research Centre and Michigan State University (MSU).

Switchgrass is one of several crops poised to become a feedstock for the production of cellulosic biofuels. For example, fuels derived from grasses, wood, or the non-food 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. A lot of its environmental benefit depends, for starters, on how its crops are grown,  according to researchers from the MSU and the Great Lakes Bioenergy Research Centre.

“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 a principle on the study.

Robertson said: “But what we’re also seeing is that a lot 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 fertiliser use.” 

Nitrous oxide emissions

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 fertilised at eight different levels.

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

The cause of that non-linear relationship can be traced to the soil microbes responsible for converting nitrogen fertiliser 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 said that fertilising 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.

While little nitrous oxide is lost to the atmosphere when the plants are fertilised at appropriate levels, Robertson said the message is quite simple: “a little bit of over fertilising hurts a lot.”

In fact, the disproportionately adverse results of over fertilising have the potential to effectively change the math on biofuel crops’ net climate benefit, according to the researchers. An over fertilised switchgrass crop can reduce its climate benefits as much as 50% once the fertiliser’s production, use, and nitrous oxide emissions are subtracted from the crop’s carbon benefit.

Consequences

The study also measured the relationship between fertiliser and nitrate leaching, and found – also for the first time – that leaching is also disproportionately greater at high fertilisation 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.

For Ruan, the implications of the study are clear. “If we’re ever going to realise the environmental potential of biofuels, we will need to have smart strategies for fertilising cellulosic crops.”

Those strategies could include developing nitrogen use calculators to help farmers determine how much fertiliser to use, or paying farmers for the perceived risk of yield loss as a result of lower fertilisation.

Robertson said 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.  

GLBRC is one of three Department of Energy Bioenergy Research Centres created to make transformational breakthroughs and build the foundation of new cellulosic biofuels technology. MSU's Stephen Hamilton and Ajay Bhardwaj also contributed to the paper.