Biochar is often heralded as a great combination of soil improvement and carbon sequestration. However, at least on the carbon sequestration front, it seems to be a mixed bag. Biofuelwatch cites a paper from Nature (2011):

“Persistence of soil organic carbon is primarily...an ecosystem property.”' This is why some types of soil carbon which are highly 'unstable' in laboratory conditions have been found to remain in some soils for long periods of time whereas black carbon, which under laboratory conditions appears particularly stable, has been found under some circumstances to decompose quicker than other types of carbon. The authors therefore warn: “Sequestration strategies based on adding recalcitrant material to soils, whether through plant selection for recalcitrant tissues or through biochar amendments must be reevaluated.”

The linked text also claims that black carbon is not inert, but the exact mechanisms under which black carbon degrades are not well understood.

They also mention several field trials where after 2 or 4 years, the carbon content of soil treated with biochar did not differ significantly from soil without biochar. So the claim that black carbon in the form of biochar has a very long lifetime in all soils seems to be false. As mentioned in the nature article, black carbon degradation seems to be very much a function of the soil ecosystem, which in turn depends on soil type; mineral, oxygen and moisture content; vegetation and farming practices etc.

Given the uncertainty of the lifetime of biocahr-C in the soil, is biochar actually a viable way of carbon sequestration? Are there specific application, e.g. biochar + specific soil type + specific farming style, that are shown to work as long term carbon sequestration?

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    The frequency and depth of tilling will also have a significant effect, especially as the depth of the subsequent analysis is fixed but not specified. Similarly particle size and invertebrate activity will affect the depth that the biochar reaches. There are plenty of counter examples (e.g. terra preta) so it works in some soils, perhaps when combined with nutrients, but not in others. That's not really surprising. – Chris H Jun 28 '16 at 16:22
  • I'm not sure if there's exactly one conter example (terra preta) and we don't know yet how to replicte it. – mart Jun 28 '16 at 19:08
  • I've read articles suggesting viability, but don't have journal access on mobile (or the patience for a search). Besides you may well have seen them. I could only comment rather than answer as I only had a few ideas for things to look at – Chris H Jun 28 '16 at 19:45
  • It might be worth a note or possibly even a separate question on other possible benefits of biochar, since the material I've seen often finds the combined benefits to be worthwhile, without necessarily splitting out soil carbon from farm productivity. If you're adding carbon as biochar, then quickly taking it out as crop carbon there's still a benefit, it's just not appearing as added soil carbon (or necessarily as "reduced carbon depletion" if crop yields go up at the same time). – Móż Jun 30 '16 at 0:49
  • Whether there's a net benefit from soil-plant-biochar-crop is where "ask a different question" comes in, I think. From what I can tell the answer is, as always, "it depends" :) – Móż Jun 30 '16 at 0:51

The answer is: Unknown.

So try it. We need more data points.

One of the mechanisms that would have a net benefit even if biochar doesn't work at all:

Process ag waste for energy. Suppose you have a unit the size of a standard 53 foot seacan that you can load bales of straw/cornstocks, wood chips into, and then this is turned into a mix of energy, distillates and biochar.

Units like this can be dropped off at a farm. Farmers can bale farm waste, and use it during the winter for heating barns. Methanol (one of the main distilates) can be used either to run a generator, or it can be sold as a crop. Char can be sold or returned to the field.

Depending on the unit, it can be easy to push the output mix one way or another. Worst case, it's all turned into energy, well that's energy he didn't get from fossil fuel.

  • I had to look up seacan... looks like a Canadian slang term for shipping container, from a brand name (like kleenex/tissue)? Also, why do you assume the farmer is a he? – LShaver Mar 3 '17 at 20:57
  • Hmm. Knew seacan was slang, but thought it was as universal as 'semi' for a road-tractor/trailer rig. As to farmer gender, while there are female farmers, locally the vast majority of farms near me are quite traditional: HE does the farming, and works an outside job. SHE runs the household, and has an outside job. Yah, during seeding and harvest everyone big enough to reach the pedals takes shifts on the equipment. And everyone over the age of 6 has chores. – Sherwood Botsford Mar 4 '17 at 13:14

There is certainly one way how to do carbon sequestration with biochar: fill empty coal open pit mines with that charcoal, and cover with soil. That's how the fossil fuel in that pit was stored for a very long time before humans thought it's a good idea to dig it up and burn it.

When wanting to give biochar simultaneously tasks for soil amendment, it becomes more complicated of course. But I'd argue that is optional.

To give some data points about biochar stability in soils, here are multiple studies with both good (centuries, millenia) and bad (decades) results:

Found most of these in Biochar as a Geoengineering Climate Solution: Hazard Identification and Risk Management. It also contains two sections dealing with biochar stability specifically:

  • 15.2.1 Biochar Stability
  • 15.3.4 Turnnover of [Soil-Organic Carbon] SOC
  • The list or resources is useful, but would be better if you could include a finding or data point from some of the more notable ones. – LShaver Feb 15 at 22:15

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