The belief that biofuels are inherently carbon neutral rests on an assumption that the CO2 absorbed when their feedstocks are grown balances out the CO2 emitted when the fuel is burned. That's why it's said that biofuels recycle carbon, in contrast to fossil fuels' one-way flow of carbon into the atmosphere.
However, the extent to which CO2 emissions actually get balanced needs to be verified, not just assumed, even as obvious as it may seem. A careful analysis shows that biofuel carbon flows balance out only under certain conditions. Moreover, those conditions are at best only partly met for the biofuels now being used. To see why, it is necessary to understand some fundamental concepts about CO2 flows between the biosphere and atmosphere.