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Some studies are quite skeptical. For example,

https://asi.ucdavis.edu/news/new-study-biochar-helps-yields-...

https://journals.plos.org/plosone/article?id=10.1371/journal...

Some studies even show that biochar soil amendments accelerate the breakdown of humus and release net positive CO2 to the atmosphere:

https://onlinelibrary.wiley.com/doi/full/10.1111/gcbb.12376

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> However, soil CO2 fluxes were suppressed when biochar was added to fertilized soils, indicating that biochar application is unlikely to stimulate CO2 fluxes in the agriculture sector, in which N fertilizer inputs are common.

> Our results showed that biochar application increased soil CO2 fluxes by 43.33% in unfertilized soils, but decreased by 8.61% in N-fertilized soils, consistent with the meta-analysis of Liu et al. (2016).

The metastudy did go into detail about N2O and CH4 as well in both fertilized and unfertilized soil.

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