A coalition of environmental and taxpayer advocacy groups told the Treasury Department its members have “serious doubts about the manure biogas claims of ‘negative’ emission” and want to ensure that other biofuels don’t make “similarly dubious claims.”

The coalition, representing Friends of the Earth, the R Street Institute, Taxpayers for Common Sense, Environment America and the U.S. Public Interest Research Group, said it worries about potential abuse of credit if biofuel producers exaggerate the carbon benefits of their products.