[mnw21cam]

Neat idea.

My main concern is about whether you're sealing all the plant nutrients deep underground too, or are they separated out at some stage?

As a world, we're going to need to put all those nutrients back into the soil to be able to keep growing stuff. Particularly phosphorous has limited mineable stocks.

[photochemsyn]

I'd guess the ash/gunk remaining from the pyrolysis would be a mixture of carbon, phosphorous, maybe some nitrogen. It'd probably make a decent fertilizer/soil amendment, particularly for degraded low-carbon soils.