In an article published last summer in Science, researchers at the Great Lakes Bioenergy Research Center (GLBRC) reported on ten years of work assessing the potential climate benefit of producing dedicated bioenergy crops such as switchgrass, poplar, or restored prairie. The mood? Cautiously optimistic.
"The climate benefit of cellulosic biofuels is actually much greater than was originally thought," said Phil Robertson, University Distinguished Professor of Ecosystem Science at Michigan State University (MSU) and lead author on the study. "But that benefit depends crucially on several different factors, all of which we need to understand to get right."
According to GLBRC scientist and University of Wisconsin-Madison professor of agronomy Randy Jackson, one of those factors will be determining how and where bioenergy crops are capable of enhancing carbon storage in soils and helping to stabilize the climate. "There is this assumption that if we plant native prairie grasses it's automatically going to lead to carbon sequestration in the soil," says Jackson. "But what we're seeing so far is that sequestration can be site-specific."
There is this assumption that if we plant native prairie grasses it’s automatically going to lead to carbon sequestration in the soil. But what we’re seeing so far is that sequestration can be site-specific.Randy Jackson, UW–Madison professor agronomy
By more closely studying the relationship between soil carbon storage and bioenergy crops, Jackson and his team hope to identify ways to manage perennial bioenergy crops that can benefit the climate and the environment. Jackson's most recent research examines how grassland composition - specifically the ratio of different types of grasses - and underlying soil characteristics affect carbon accumulation.
For the study, Jackson's team calculated the "net ecosystem production," a commonly used measure of carbon storage, of 60 grassland plots varying in composition and growing across a range of high-productivity and low-productivity soils. Net ecosystem production is the difference between plant growth above ground and below (carbon input) and the amount of carbon released from the soil surface due to decomposition (carbon output).
The researchers found that having prairie grasses such as big bluestem, indiangrass, or switchgrass resulted in carbon gains when planted on productive soils. However, the same was not true for marginal soils, where all grasses, regardless of species or type, resulted in net carbon losses to the atmosphere.
Since GLBRC's vision is to develop sustainable biofuels and bioproducts from bioenergy crops grown on marginal, or non-agricultural, lands, the carbon loss finding on marginal soils may have important ramifications for future GLBRC research.
"I think these findings add some nuance to our marginal lands focus," says Jackson. "Marginal land can be marginal for different reasons and we have to be cautious about what we plant where and what our expectations are for those crops. Looking forward, we're going to need a longer-term perspective to understand how carbon sequestration is achieved or not over a broad range of soils, climate, and crop types."