I don't know what fraction of growth is agricultural, but one can project from The Billion-Ton Vision that some 300+ million tons/yr of carbon (equivalent of 1.1 billion tons CO₂) is available from various non-food plant matter already produced in the USA alone, and it may be possible to increase this substantially (1.3 billion dry tons @ 45% carbon is 585 million tons carbon, equivalent to ~2.1 billion tons CO₂).

Note that there are several versions of "The Billion-Ton Vision" on the web, with different covers.  I'm not sure how the contents differ; I'm citing the version linked from GCC (which also lists the authors prominently).

And how does spreading all this char into soil affect the oxidation state of the soil?

Almost not at all; the stuff appears to be stable on a scale of thousands of years.  Normal soil carbon is lost far more easily.

Hi Euan,

I'm working on your first question now. I'll give you the estimates from the Climate Code Red report (on p. 58), these are worldwide I think. Citing Marris in Nature News 2006 442 624 they give 9.5 billion tonnes of carbon per year: http://www.nature.com/nature/journal/v442/n7103/full/442624a.html with a citation there to:
Lehmann, J., Gaunt, J. & Rondon, M. Mitigation Adapt.Strateg. Global Change 11, 403–427 doi:10.1007/s11027-005-9006-5 (2006). http://www.springerlink.com/content/etm7526m07672103/
This is more than we currently emit annually.

For crop wastes, another estimate they give is 1 billion tonnes per year from here: http://planetwork.net/climate/emergency.html
but this estimate does not seem be well supported.

On your second question, soils oxidize mostly through microbial action. One usually wants as much carbon in soil as possible because it grows plants very well, but agricultural activity provides oxygen to the soil and so the carbon decays and you need to add more. Grasslands continue to build up carbon through their continuing root formation.
http://www.sciencemag.org/cgi/content/abstract/314/5805/1598
This difference is the reason why biofuels produce more emissions that fossil fuels. If you need to disturb previously untilled soils because biofuel crops are displacing food crops, then all that soil carbon decays and enters the atmosphere. The thing about biochar is that it does not seem to be eaten much by microbes so that it remains in the soil without decaying as quickly as other biomass (manure, compost, dead roots etc.). This appears to also set up an ecology in the soil where carbon will be built up in deeper layers at least in the Amazon.

The amount of air in soil is dependent on its porosity. Biochar should add to this since it retains some of the structure of the original plant. This is why it buffers nitrogen and retains water. Since it is not itself decomposing much, it should not be creating anaerobic conditions though I think this could occur if the land converts to wetlands.

Not a full answer, but I'm still learning myself. One of the co-chairs of our EcoAction committee works with Danny Day so I'll be picking his brain as I go along.

I thought camels were better for milk and blood than for barbecue?

Chris