Tilling the soil actually releases quite a bit of CO2, some of the best practices are "no till" methods. But to your point, biochar retorts can and should be used at the source of the biomass, instead of burning fossil fuels to transport the biomass to a dedicated facility.

Many small scale gasifier generators are designed to do just that. The problem with all solid fuels is that material handling is difficult (you need to chip wood to the correct size and shape to have it feed through a machine effectively, this gets more complicated when you're trying to process corn cobs, sugar cane bagasse, and fruit pits also), not to mention that the fuel needs to be the proper dryness to burn without producing excessive tar.

Then there is the problem with the PAH (Poly Aromatic Hydrocarbons) that are produced, and their carcinogenic on any of your workers...not to mention any other toxic chemicals that may be produced out of your smoke stack when using mixed fuels.

All this boils down to details that have to be worked out in a mobile unit.