The Limits of Biochar

I recently did some basic equations about Biochar. If Biochar were the sole way that carbon is removed from the atmosphere, how much land would be needed to store the stuff? The calculations ended up being very pessimistic indeed. Armed with these calculations I sent off an email to the boffins at Real Climate. NASA scientist Gavin Schmidt responded and pointed out that I had wildly overestimated the amount of anthropogenic carbon in the atmosphere but, when taken with carbon that had been sequestered into the ocean, the final figure ends up being 400 billion metric tonnes - an amount that doesn't take into account future emissions.

How much Biochar can be used as a soil amendment? According to this, the amount is around 23.2 metric tonnes per hectare. It was just a matter of then dividing the amount of anthropogenic carbon by 23.2 and see how much land is required to sequester Biochar. The result is very depressing:

In short, 23.2 metric tonnes per hectare is not enough. Even if every single hectare of above ocean land mass is sequestered with 23.2 metric tonnes of biochar, the result is not enough to remove anthropogenic carbon. In reality, sequestering of biochar could not be achieved over the entire earth's surface, so I've given figures there for 10% of the earth's surface as well as 5%, which would require a Biochar sequestering of up to 23 times what is recommended.

So, the questions are:
  1. What amount of sequestered Biochar is too much? At what point will it turn from being a soil amendment and become toxic to plant growth?
  2. What would be the effects of deep Biochar sequestering, whereby Biochar is sequestered up to 10 metres underground rather than just existing within the 1-2 metres?
  3. Is it viable to use carbon as a resource to replace current commodities such as iron, aluminium, glass and so on?
The good news, I suppose, is that a cylindrical storage container 50 metres high and 18.25 kilometres wide could effectively store all 400 billion metric tonnes of carbon (at 2.267 grams per cm³ = 90,680,00 km³, volume of cylinder = πr²h) if necessary. NOTE: My spreadsheet let me down in its maths here. The real figure would be 500 metres high and 7600km in diameter: Not good news.