To optimize the agricultural and environmental benefits of biochar, a charcoal-rich soil additive, we need to overcome its potentially undesirable effects (see Nature 517, 258–260; 2015).

For example, it is uncertain whether biochar — effectively an underground carbon store — can help to mitigate carbon emissions. A ten-year study of boreal forests found that applying biochar led to soil degradation and increased the activity of soil microbes, causing carbon dioxide release (D. A. Wardle et al. Science 320, 629; 2008).

Adding blackened biochar can also lower the reflectivity (albedo) of the soil surface, potentially exacerbating climate warming (S. Meyer et al. Environ. Sci. Technol. 46, 12726–12734; 2012).

Tilling deep furrows in the soil would help to reduce the decline in reflectivity and increase the efficiency of applied biochar. However, this practice could also encourage carbon dioxide release.