Biochar is an additive used along with fertilizer to manage waste and store carbon in soil. Advocates say its use can slow climate change. But, a recent study casts doubt on its efficacy — at least in certain climates.

Northern Arizona University professor and study author Bruce Hungate said biochar increases  fertility and crop yields — in certain climates.

“It does depend where you are, what the climate is and what crop you are growing and what kind of agricultural system you are using, which benefit and how much benefit the farmer will enjoy," Hungate said.

His study finds tropical climates benefit most from using biochar. Previous studies had thought all climates benefited equally.

Hungate said biochar is still important to support sustainable agriculture.

“It stays in the soil for a long time. It’s resistant to microbial decomposition,” Hungate said.

His study was published last month. His team is now looking at the effects of biochar on emissions of other greenhouse gases like nitrous oxide and methane.