Of all the approaches to cutting carbon emissions, making charcoal and putting it in the ground as fertilizer would seem one of the least controversial. But a report published today offers words of caution around expecting too much from biochar.

Biochar, also called man-made charcoal, is made by decomposing plants and other organic materials into charcoal through pyrolysis, or slowly burning biomass at high temperatures with no oxygen. The resulting biochar can be used as a soil fertilizer, a technique used by ancient civilizations in the Amazon.

Unlike naturally decomposing organic materials, biochar holds onto carbon dioxide for hundreds or even thousands of years. For that reason, it's been touted by everyone from Virgin CEO Richard Branson to environmentalist James Lovelock as a promising method for fixing carbon dioxide in the ground. Biochar has also attracted detractors in the past few years who say that growing plants to create biochar would be a "false solution" to climate change.

Environmental advocacy group the National Resources Defense Council today published an analysis that seeks to put some perspective on the potential on biochar. The study also proposes first steps for a U.S. policy to promote production of biochar on a commercial scale (click for PDF).

An extensive study earlier this year estimated that as much as 12 percent of human-caused greenhouse gases could be offset by making biochar.

But that number best represents the technical potential of biochar, not a realistic expectation, according to the NRDC. It notes that, because there are no commercial biochar production systems, it's difficult to assess the environmental benefits and financial cost.

"The truth as to whether biochar is a cure-all or a scourge is apt to lie between the extremes, but we cannot say exactly where at present," according to the NRDC, which said it isn't in a position to endorse or discourage its development without further technical development and tests.

Among some of the concerns is the environmental impact from diverting land for biochar and the energy footprint of transporting biomass. The NRDC said that using waste biomass, such as plant residue or manure, looks like the most promising feedstock. Uncertainty around carbon markets makes it difficult to assess the costs of these systems.

Among its recommendations, the NRDC says that commercial-scale biochar facilities using slow pyrolisis use waste such as manure. It said that 5 to 10 demonstration facilities would cost between $100 million and $150 million.