Thanks for posting this! Down here in the humid subtropics, where my acid "soil" is a fine white quartz sand, I've been dragging fallen branches to my burn barrel from all over the neighborhood for a year, to turn them into charcoal, as a base layer for each new garden bed: I dig the bed, put in 3-4 inches of charcoal, then dump a few weeks' worth of kitchen compost on top and cap it with store-bought composted manure. Where I've dug the holes deep enough, this seems to be working pretty well. I'm counting on the charcoal layer to keep the added compost from washing right out, hold onto some water, and maybe preserve some organic matter there for the years to come. I love the idea of hugelkultur, but it doesn't work here. You'd hesitate to pile up that much wood anywhere near your house, because of termites. And it wouldn't last more than two years-- you'd have to keep piling it up year after year, and where would you get all that wood from? If there's anywhere biochar is appropriate, it's here-- and I'm glad the study agrees!

My understanding is that biochar is not necessarily useful as a fertilizer by itself, but rather it provides charge sites for cations to attach to. It increases your soils TCEC (total-cation-exchange-capacity), so you would still need to add whatever your soil is missing, and replenish those things when your plants have removed them. And some things are water soluble and are going to leach away anyway. I read the book "The intelligent gardener" and in it he likened it to "how big is your food pantry". The larger your TCEC the less often you have to refill it and the longer it takes to deplete the soil. My questions about biochar are more about if they help to maintain and balance soil nutrients, assuming you balance those nutrients to begin with, and if in the long run you have to use fewer external inputs because more of your nutrients are captured and subsequently made available to the plant.

I'd like to see the studies plotted on a map. They say that tropical soils are avid and low fertility and temperate ones aren't, but in Australia most soils, even in temperate areas, are acid and low fertility. Is it the climate zone that makes the difference, or the soil type?