Here we review the relationship between emissions reductions and CO2 removal by biochar systems, which are based on pyrolysing biomass to produce biochar, used for soil application, and renewable bioenergy. Half of the emission reductions and the majority of CO2 removal result from the one to two orders of magnitude longer persistence of biochar than the biomass it is made from. Globally, biochar systems could deliver emission reductions of 3.4–6.3 PgCO2e, half of which constitutes CO2 removal. Relevant trade-offs exist between making and sequestering biochar in soil or producing more energy. Importantly, these trade-offs depend on what type of energy is replaced: relative to producing bioenergy, emissions of biochar systems increase by 3% when biochar replaces coal, whereas emissions decrease by 95% when biochar replaces renewable energy.
The lack of a clear relationship between crop yield increases in response to fertilizer and to biochar additions suggests opportunities for biochar to increase crop yields where fertilizer alone is not effective, but also questions blanket recommendations based on known fertilizer responses. Locally specific decision support must recognize these relationships and trade-offs to establish carbon-trading mechanisms that facilitate a judicious implementation commensurate with climate change mitigation needs.
Biochar technology shows promise in mitigating climate change and improving soil quality.
Effective climate change mitigation requires both reductions of greenhouse gas (GHG) emissions and withdrawal of atmospheric carbon dioxide (CO2) to achieve the net zero emissions required to meet the Paris Agreement goal
1 . Use of biochar as a soil amendment to both reduce GHG emissions and deliver CO2 removal (CDR) was first proposed as a global strategy for climate change mitigation only 15 years ago and has been intensively studied over the past decade
2 . Biochar is produced by the anoxic thermochemical conversion of biomass through pyrolysis processes that generate recoverable heat and fuels, such as gases and condensable volatiles, besides the solid biochar, an environmentally persistent material characterized by high carbon and low oxygen and hydrogen contents
3 . This Review provides an in-depth overview of the scientific progress in understanding the biogeochemical mechanisms of biochar persistence, its effects on CO2, nitrous oxide (N2O) and methane (CH4) emissions from soil, and on plant growth and concomitant CO2 uptake, and explores the trade-offs between energy generation and carbon sequestration. Of particular importance is the ensuing balance between GHG emission reductions and CDR, which mainly depends on prioritizing either energy generation to offset fossil fuel use or sequestering biochar, through choices of feedstock type and production conditions, which lead to different systems-level climate mitigation outcomes.
Optimization of emissions reduction, CDR and non-climate effects, such as crop yield enhancement, is needed, for which we lay out research priorities and policy mechanisms.
