Researchers at the Department of Energy's (DOE's) Argonne National Laboratory are using computational modeling to predict which U.S. counties could see increases in soil organic carbon from cultivating crops like switchgrass for biofuels.

In an effort to lower transportation-related greenhouse gas emissions, the U.S. Environmental Protection Agency's Renewable Fuel Standard calls for increased production of advanced biofuels. DOE says that most bioenergy crops can grow on a range of crop and marginal lands across the country, taking advantage of acreage that may not be viable for traditional crops. Because Earth's soil stores about three times as much carbon as the atmosphere, these perennial plants might also help reduce greenhouse gas emissions before they are harvested for fuel by increasing the amount of carbon stored in the soil.

(Read "One Billion Tons of Biomass for Energy by 2040: DOE.")

Most bioenergy crops can grow on a range of marginal lands, taking advantage of acreage that may not be viable for traditional crops. Image credit: ORNL.Most bioenergy crops can grow on a range of marginal lands, taking advantage of acreage that may not be viable for traditional crops. Image credit: ORNL.In the Argonne study, researchers combined county-level crop yield and weather data, as well as soil data at depths relevant to bioenergy crops. The team used a soil carbon model to calculate sequestration rates—the rate at which carbon is transported in or out of the soil—for five plants: corn, switchgrass, poplar, willow and Miscanthus.

The production of each species was modeled on four types of land, including cropland, cropland pasture, longstanding undisturbed grassland and forest—totaling 20 land scenarios depicting different initial and final land uses or covers. Researchers also considered changes to soil carbon stocks when 30% of the corn stover remaining after corn harvest is collected, which is likely to contribute to future biofuel production and can also affect carbon stocks.

By modeling soil carbon at a depth of 100 centimeters rather than the standard 30, the study results represent the deeper root systems of crops such as switchgrass and poplar trees that transport carbon below the topsoil, unlike more shallow-rooted row crops like corn. The team also collected detailed data on local weather patterns, soil conditions, historical land use and local crop yields for each county, as well as data from bioenergy crop field trials conducted by other agencies and national laboratories.

"By doing this type of analysis, we can find areas where bioenergy crops can have positive environmental effects—but also hotspots where growing bioenergy crops may cause a decline in soil carbon," says Argonne postdoctoral researcher Zhangcai Qin.

Supported by local data, county-by-county modeling results confirmed that grasslands and forests generally have richer soil with larger carbon stocks and will contribute to greenhouse gas emissions if transitioned to bioenergy crop production. On the other hand, converting some cropland or cropland-pasture to bioenergy crops such as switchgrass and Miscanthus can help reduce greenhouse gas emissions. Notably, results show soil carbon on cropland can benefit from bioenergy crops in large areas of the United States, particularly by planting Miscanthus in the Midwest and Pacific Coast and switchgrass in the Southeast.

The results will be included in Volume 2 of the DOE's third "Billion-Ton Report," expected to be published later in 2016.

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