Here is a summary of the optional reading entitled "Land Clearing and the Biofuel Carbon Debt" from Science.
Biofuel production is expanding to meet demand for alternatives to petroleum and as a result, new land is being converted to grow palm, soy, sugarcane, and corn. This land conversion releases carbon dioxide through direct combustion of biomass and decomposition of organic carbon. The authors label the amount of carbon dioxide released over 50 years after conversion as "carbon debt". If biofuels are to cause a net reduction in emissions, they would then need to cause emissions reductions of larger than the carbon debt created to enable production of the fuels.
The authors then evaluate the carbon debt created by biofuel production under a number of scenarios covering different crop types and different geographic regions. They find that in all cases where new land is converted to agriculture for biofuels, it takes decades or even centuries for the reductions from biofuel use to "repay" the carbon debt created to produce the biofuel in the first place. This is true in the case of clearing tropical forests for palm, soybean, and sugar cane and it's true in converting grassland in the US to corn for ethanol. Of the scenarios evaluated, only the use of abandoned or marginal farmland in the US for the production of biofuel from prairie biomass offers a quick repayment of the carbon debt.
The conclusion of the article is that, unless biofuels are produced without land conversion and with alternative feedstocks, use of biofuels is unlikely to result in a net decrease of carbon emissions in the short to medium term.