A 12 year study in China shows that biochar boosts microbial carbon storage in the top 20 centimeters of cropland. This effect is particularly strong in carbon deficient soils, where fungal residues increase significantly over time. However, the research found a trade off. In soil layers between 20 and 40 centimeters, biochar application actually decreased microbial carbon by up to 30.4%. This suggests that surface gains may be partially offset by losses deeper in the soil profile.
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This is exactly why carbon accounting for soil is so messy. If credits are issued based on topsoil samples alone, we are likely overestimating the net sequestration. We need a standard for depth profiling before these credits hit a major market.
Agreed. Most projects just scrape the surface. If the subsoil is losing carbon due to metabolic stress or nitrogen shifts, the net benefit might be way lower than the pitch deck claims.
Interesting biological data, but what is the energy cost of the pyrolysis and transport for this biochar? If the subsoil offset is real, the carbon payback period for the energy used to produce it might be too long to be practical.
The meta analysis part is the most useful bit here. 83.5 percent of cases showing topsoil increase is a strong signal, but the 10.2 percent average is modest. Not a silver bullet.
