BRUSSELS — Much of the appeal of generating energy from plants was that they emit only as much carbon when burned in cars and power plants as they absorb while growing.

Lately, that appeal seems to be going up in smoke.

It turns out that the emissions from growing and processing some biofuels significantly diminish their benefits, when taking into account factors like the use of fertilizers manufactured with fossil fuels.

Concerns have also grown that large swaths of forest and grassland will be chopped down or burned to grow fuel crops — and to grow food that has been displaced by growing fuel crops elsewhere — thereby releasing additional stocks of carbon into the atmosphere.

Olivier De Schutter, the special rapporteur on the Right to Food for the United Nations, is among the experts who have said that pressure on farmland from demand for biofuels is a major factor in the food price spikes that have exacerbated hunger and social unrest in some of the poorest countries in the past three years.