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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
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The answer is: Unknown.

So try it. We need more data points.

In general if energy can be extracted from a reaction, some bacteria does it for a living. Given the nature of certain ecozones, notably various pines that have a strong firesuccession (Lodgepole, Jackpine) then a series of low temmperature burns should leave substantial carbon in the soil. High temp fires burn the soil back to mineral soils.

However, in my travels over the jack pine regions of the Canadian Shield, I don't see black soils. An area right after a fire has lots of charcoal, but 5-10 years later (christmas tree sized pines) it's not readily apparent. Mind you, I didn't look that carefully for it. This is a mark against it working.

On the other hand, the Terra Preta in the amazon has a large charcoal component. This is a mark in favour of it working.

Carbon sequesterization may be in part controlled by pH. Sphagnum moss seems to last for thousands of years if in soggy anaerobic circumstances. The moss generally creates a pH of around 4-5.

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 (modular shipping container) 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 distillates) 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.

Attempts to use biofuels have foundered on transport costs. It just costs too much to move them. A dump truck full of coal has a lot more energy than that same truck full of corn husks. And coal can be close to "just in time" where ag waste is very seasonal. You have to be able to store 18 months or so stock at a time.

Producing biochar/methanol/heat from ag waste on site means that the sellable product is cheaper to ship.

A similar thing happened with the frontier in America. Shipping grain back east wasn't very practical until railroads and steam boats came in. But a backwoods farmer could turn corn or barley into whiskey. THAT was worth transporting.

  • 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
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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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There are many problems with the article you cited, but I agree with the main point it makes about the need for extensive research before any global scale solution should be attempted. Nevertheless, extensive field trials are not required to definitively prove the effectiveness of biochar to sequester carbon. Simply dig a hole in poor soil one foot wide and three feet deep. Fill it with charcoal and sample it at any given time over the next few millennia and you will be guaranteed to find 100's of times more carbon than that which was in the dirt you removed. Fortunately for the debate, that's not the end of the story.

Most people who successfully use biochar are making it themselves on a very small scale. They crush it, infuse it with healthy microbes and organic nutrients, and then blend it with soils as an amendment. They know that too much carbon in the soil will rob it of nitrogen so they add plenty of compost, apply some manure tea, and mulch heavily to prevent erosion from wind and water. This is profoundly different from modern large scale agriculture.

If you want to grow things, the ideal way to store carbon in the soil is in the form of humus which is created over time by the activity of microorganisms breaking down organic material to form organic compounds. Biochar helps this along by providing a consistent habitat for the bacteria that take up residence in the many tiny pores that are retained from the original structure of the wood you burned. Carbon is provided to the system by roots of plants in the form of carbohydrates that are photosynthesized in the leaves and exuded through the roots in a symbiotic relationship with specialized bacteria which are attracted by other exudates according to the nutrient requirements of the plant.

A steady supply of nitrogen is required to keep the biological activity thriving, so leguminous trees and cover crops are planted to drive the process. Unfortunately, most farmers will shoot you if you try to plant trees in the fields they intend to plow. Your mission, if you choose to accept it, is to provide the components nature needs to help you create active anthrosols that continually sequester carbon in whatever land you are steward of. Biochar, although inactive, can be one of many important components of your living system in much the same way that sand, silt, and clay are considered components of good healthy soil.

Sadly, most farmers till their soil deeply destroying most of the biology in the topsoil while releasing tons of CO2 into the atmosphere. Notice how the highest emissions of CO2 occur when most farmers are plowing their fields https://www.youtube.com/watch?v=x1SgmFa0r04 Biochar won't help them much if they continue to grow crops abiotically using chemicals to supply the nutrients they need, but if they ever decide to start restoring the soil instead of depleting it, then biochar can play an important role in that effort. Soil can hold much more carbon than the atmosphere can, or it can continue to release it as it has been doing for quite some time now.

A global effort to produce massive quantities of biochar is a frightening prospect on several fronts, and the article did a good job of pointing some of those out, but I still douse my cooking fires before they turn to ash and harvest the charcoal to blend with my soils. A good layer of mulch alleviates my concerns regarding albedo, erosion, and air quality, and my nitrogen concerns are adequately addressed with manure, compost and nitrogen fixing plants. I can't ask farmers to do things the way I do, but they would let me play in some of their fields if I didn't have enough of my own to tend to. I know this because I've asked them.

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