How much bio-char can be added to soil?

Using published projections of the use of renewable fuels in the year 2100, bio-char sequestration could amount to 5.5–9.5 PgCyr−1 if this demand for energy was met through pyrolysis, which would exceed current emissions from fossil fuels (5.4 PgC yr−1).

Assuming large bio-char sequestration over long periods of time, a bio-char sequestration of 140MgCha−1 would calculate to 224 PgC storage capacity globally for the 1,600 Mha of cropland worldwide and to 175 PgC storage capacity globally for the 1,250 Mha of temperate grasslands (IPCC 2000), not including forest land.

From:
Bio-char sequestration in terrestrial ecosystems – a review, Lehmann et al. (2006)

Note:
1 PgC (petagram Carbon) = 1 Gt (gigatonne Carbon) and corresponds to ~3.67 Gt CO2.
1 ppmv CO2 in the atmosphere corresponds to 2.12 GtC or ~7.78 Gt CO2.
9.5 PgC per year corresponds to 4.5 ppm per year, or 112 ppm in 25 years.


Image from: The way back to 280 ppm - by Sam Carana

Agrichar

Agrichar can be produced by means of pyrolysis from organic material. In that case, it's often referred to as biochar. Agrichar can also be produced from carbon that is captured from the air, as indicated on the picture below.