Well-made biochar (adequate T / low H/C_org) is much more persistent than raw biomass; fused aromatics are the mechanistic basis.
Sustainable feedstock (wastes/residues; no deforestation) is required for net climate benefit.
Biochar can raise yields especially in acid/degraded/tropical soils, but responses are context-specific.
N₂O reductions are real in many systems in year 1; magnitude and multi-year persistence less certain.
MRV can focus on mass × C fraction × permanence factor at the kiln/gate, with application verification—cheaper than full soil flux monitoring (reviews biochar climate mitigation · DOI).
Quality control for PAHs/heavy metals and process emissions (kiln CH₄) matters for safe deployment.
Open debates / uncertainties
Long-term decay model form: Two-pool fits from ≤few-year incubations may mis-estimate millennial recalcitrant pools; newer proxies (HyPy, conductivity, Ro) and policy commentaries argue for refined permanence metrics.
CH₄ in paddies: Direction and magnitude still study-dependent.
Priming ceiling: Decade-scale negative priming is promising but long-term saturation and organic-soil risks remain.
Energy vs CDR trade-off: Relative benefit depends on what energy is displaced (coal vs renewables); as grids decarbonize, soil biochar becomes relatively more attractive (Lehmann 2021).
Economic vs technical potential: Technical/sustainable multi-Gt ≠ near-term deployable; IAM/economic potentials often ≪1 Gt by 2050.
Non-soil uses (concrete, asphalt, animal feed, materials): permanence pathways differ; soil-amendment science does not automatically transfer.
Climate feedback on permanence: Warmer soils accelerate decay (models soil GHG from biochar · DOI Q10 framework)—future warming reduces F_perm relative to today’s tables.