The concept of using biochar to sequester atmospheric carbon dioxide and as a beneficial soil amendment has become increasingly popular both internationally and in Australia. The current debate in Australia centres on whether the “science of biochar” is robust enough to include biochar in Australia’s proposed Carbon Reduction Pollution Scheme (CPRS).

This paper presents a critical review of the biochar sequestration process. The science can be classified into three areas, namely the production of biochar (pyrolysis), its recalcitrance to carbon dioxide mineralisation and biochar’s additionalities via fertility improvement and diffuse emission reduction.

The pyrolysis of biomass is a well-established science with considerable scientific literature on pyrolysis technologies, thermodynamic processes, techno economics and influences of biomass type. From a process technology perspective the key to proliferation is economic viability, particularly with respect to capital costs and energy efficiency.

The recalcitrance of biochar to carbon dioxide mineralisation in the soil determines its capacity as a carbon sink. Peer-reviewed studies indicate that biochar has millennial-scale stability verified using a number of independent measurement techniques. An improved understanding of the mechanisms of biochar mineralisation and the development of suitable techniques to predict mineralisation rates of newly formed chars will help accelerate the adoption of the biochar opportunity.

The science of additionalities is a complex area that involves biochar‘s interaction with soil chemistry, its physical modification of the soil character and providing active sites for reactions. The resolution of these complex issues will be emerge through detailed analysis of particular systems (soils, climate, char).