Biofuel crops destroying US grasslands
The biofuels boom has boosted US corn and soybean cultivation to record levels, but has this expansion come at the cost of damaging more land? According to the US Department of Agriculture (USDA), the net acreage of cropland has increased negligibly since biofuels became popular. However, a new study shows that USDA's net measure masks the fact that farmers are shifting crops onto undisturbed grasslands at high rates while retiring existing cropland (Environ Res Lett 2015; doi:10.1088/1748-9326/10/4/044003).
To prevent the biofuel gold rush from destroying natural habitat and releasing carbon stored in soils, the US Renewable Fuels Standard (RFS) withholds “renewable credits” for feedstocks grown on land that had not been farmed before 2007, says Tyler Lark, lead author of the study and a land-change scientist at the University of Wisconsin–Madison (Madison, WI). The US Environmental Protection Agency relies on the USDA's net annual cropland acreage to monitor compliance with the RFS. Yet Lark suspected that there were undetected changes underlying the static net USDA figures.
Lark and his team analyzed a new USDA satellite-based map that recorded nationwide crop cover for each year since 2008. Integrating other land-cover databases, the researchers tracked the expansion and abandonment of farmland from 2008 to 2012, coinciding with the biofuel boom.
The scientists found that 7.34 million acres of land nationwide, an area roughly the size of Maryland, had been converted to crop production during that time period. Nearly 80% of the new cropland came from prairie, pasture, and hay fields. “If you convert native prairie to cropland, it's nearly impossible to regain the biodiversity and carbon-storage capacity that's lost upon tilling”, Lark says. Over the course of the study, the researchers estimated that the carbon emissions associated with converting the land to grow corn and soybeans could be equivalent to a year's worth of CO2 releases from 34 coal-fired power plants. Lark explains that the widespread expansion of farmland could not be seen from the total net figures because while farmers expanded cultivation, they abandoned 4.36 million acres of existing cropland, much of it destined for a conservation program.
“The study hints that renewable fuel policies may be generating an unintended, but substantial, carbon debt”, says Chris Wright, a landscape ecologist at the University of Minnesota–Duluth (Duluth, MN). The findings support the need for enforcing the RFS feedstock provisions through a spatially explicit framework and broadening policies that discourage grassland conversion, he concludes.