In the end concerns over methane production are irrelevant. We have doubled production in the last century and it is all gone. The reason is ultimately very simple. It migrates to the upper atmosphere and is consumed. This is something that is not an option for CO2.
Does the sharp increase in methane reaching the troposphere have any effect whatsoever? The quick answer is nothing that is obvious. It is a little like measuring the effect of the Mississippi on the Atlantic. The practical answer as always is to make as much as you desire and see were it takes you. My guess is nowhere.
That means that methane production concerns regarding all forms of biowaste combustion are misplaced. My real concern would be for well intentioned government regulation been actively imposed forcing a larger industrial price for the use of the method.
The second issue that has attracted comment is the association of pottery shards in the terra preta soils. I naturally postulated that this was partly to do with the disposal of kitchen waste in the corn stover stack kilns as we described in earlier postings in July. I also realized that a large bowl would have to be used to transport hot coals to the top of the stack and perhaps dumped into a prepared chimney.
These bowls are as primitive as you can get and very prone to heat breakage, so the presence of pottery is no surprise. My discomfort came from the fact that they would have normally taken broken pottery away with them for disposal elsewhere. So why not?
The answer came to me this morning. It is natural to take the bowl of coals to the top of the stack and to dump them there in the center and to let the coals slowly burn out a chimney. The problem is that you have to cover these coals with dirt to prevent flame out. The best way to do that is to upend the bowl on top of the coals and to throw dirt on top of that. Otherwise, the coals will end up been smothered by the dirt. The bowl would then migrate slowly to the bottom of the stack. In the process the high heat would cause this low quality pottery to breakup into very small pieces not worth recovering or causing any difficulty for cultivation.
Actually a pretty nifty solution to the problem of controlling the ignition coal mass. While this was progressing, the farmer would stand by to throw dirt on any emerging openings in the stack to prevent a flare up.