Despite popularity among producers, retailers, media, and farmers, inconsistent effects of biochar amendment on yields and greenhouse gas (GHG) fluxes due to variations in properties and interactions with diverse soils limit broad-scale advocacy for its use. We measured the effect of biochar amendment on GHG flux, soil carbon change, and the carbon equivalencies of agricultural inputs/outputs to conventional food and zero-tillage fuel cropping systems in two soils with contrasting fertility in Hawaii.  

The best prospect for a sustainable biochar system was amendment of a tropical perennial bioenergy crop in the high fertility soil coupled with decentralized, on-farm production of biochar. In this case, the combined effects of biochar amendment on GWP, crop yields, and production costs increased net present value by as much as 71% over the control treatment.  

Because of the large financial investment involved with biochar amendment at the field scale, a positive yield effect is critical in attaining an economically viable system.  Any future interactions with climate change that negatively affects yields and increases GHG flux in the tropics will need to be balanced by the increasing societal costs of continued climate change.