Background
The authors write that "biofuels are receiving growing negative attention," because "direct and/or indirect land-use changes that result from their cultivation can cause emissions due to carbon losses in soils and biomass and could negate any eventual greenhouses gas reduction benefit."

What was done
In further exploring this perplexing subject, Achten and Verchot evaluate "the implications of land-use change emissions on the climate-change mitigation potential of different biofuel production systems in 12 case studies in six countries," calculating "carbon debts created by conversion of different land-use types, ranging from annual cropland to primary forest," while evaluating "case studies using three different biofuel crops: oil palm, Jatropha, and soybean."

What was learned
The two researchers identified carbon debts ranging from 39 to 1744 Mg CO2/hectare, with oil palm case studies creating the largest debts (473-1744 Mg CO2/hectare), because, as they learned about this crop, "most of the area expansion came at the expense of dense tropical forest." And given numbers such as these, they found that only soybean in certain parts of Brazil needed less than one human generation (30 years) to repay the initial carbon debt, while "highest repayment times were found for Jatropha (76-310 years) and oil palm (59-220 years)."

What it means
Achten and Verchot say their findings "raise serious questions about the sustainability of biofuel production," and they say that "due to direct land use change carbon implications following the conversion of (semi-)natural ecosystems with medium to high carbon content, and to indirect land-use changes following conversion of agricultural or pasture land, the potential of biofuels to contribute to climate-change mitigation is questioned" ... as well it should be (see more about the sorry state of Biofuels in our Subject Index).