How long does Biochar benefit soil?

To start, I have often heard that biochar can last for hundreds if not even thousands of years in the soil as a stable source of carbon, however, I am more concerned with its use as a soil amendment. Especially in the context of temperate soil.

When looking into Terra Preta, I kept thinking about the viability of such a product in temperate regions. It has well-established benefits in the Amazon but my concern is the freeze-thaw cycle of the temperate regions. Both unglazed pottery shards and biochar are critically important due to their porous microstructure, which in turn allows for microbial housing as well as nutrient and water retention. My main concern is that the retention of water, combined with the expansion of the subsequently formed ice will rupture channels in the internal structure and degrade the structure with each freeze-thaw cycle. That being said, I know pottery can indeed crack and break with water freezing within it, however, my experiments have shown the effect is not as dramatic with smaller shards.

On the other hand, the soil may act as a thermal mass and reduce the severity of temperature swings, especially in warmer temperate regions where the soil does not freeze very deep. Even as a fine powder, biochar still retains its benefits as the microstructures are substantially smaller. Greater microbial access might also occur due to increased surface area. So perhaps even if the freeze-thaw cycle broke the pieces down repeatedly, it could still confer benefits as it becomes smaller. The established microbial community and built-up organic matter could act as a foothold, perpetuating soil fertility even with the diminished or used-up effects of biochar.

Does biochar break apart under the freeze-thaw cycle of the temperate region? If so, how long can it confer benefits as a soil amendment? Biochar would still likely remain stable for carbon sequestration, but how long can it benefit soil?

Any thoughts or resources are greatly appreciated,
Eric S.

There really isn't a good rubber stamped "this is the truth" answer out there quite yet that I have stumbled upon. I've made quite a few inferences that have served me some good. The freeze/thaw cycle most definitely has an effect but not one that I can say "After five years, you need reapplication" because I have been reapplying each yar so far. I do believe that the size of the biochar gets reduced as the freeze thaw cycle goes on. You dig in and the soil becomes more uniform after each year. I would recon to guess that the Biochar still benefits the soil for years but as you could guess probably with diminishing returns. We have a few biochar enthusiasts on the forum who product quite a lot, I wonder what their observations will be.


Some related literature
https://www.sciencedirect.com/science/article/abs/pii/S0341816219300086#:~:text=The%20results%20showed%20that%20the,capacity%20during%20the%20melting%20period. (Water Retention)

https://www.sciencedirect.com/science/article/abs/pii/S0301479723010903 (Microbe overwintering)

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0191246 (Grain size change after freeze/thaw cycle)

I read the abstract for Timothy Norton's link about freeze/thaw experiments. The final line,

The observed changes in biochar particle size and shape can be expected to alter hydrologic properties, and thus may impact both plant growth and the hydrologic cycle.

has left a lot of "wiggle room".

In the real world will it affect plants and the cycle positively, negatively, or both?
How much will that depend on the ecosystem? Since more freeze/thaw cycles break it down more, ecosystems that are slightly warmer and go through multiple freeze thaw episodes each winter, might have different results than an ecosystem where things tend to freeze solid late fall and stay frozen for months.

I do suspect that biochar helps some soils more than others, and I suspect that different soils may be benefiting in different ways. There's still so much we don't know, but from reading work from different people in different ecosystems, one example of that is that in sandy soil, biochar's ability to hold nutrients is very important, however in my clay soil, I'm suspicious that it's biochar's ability to lighten the soil, that is key, as my clay soil probably holds onto its microorganisms better than sandy soil to begin with. However, I could easily be wrong. We have a lot of constant rain and wet all winter, and maybe it will turn out that my microbes think biochar hotels are great places to hunker down during the Big Wet.

This is why I promote home-made biochar out of local waste material, and am cautious about it being made on an industrial level where a price tag is placed on it. We are a summer wild-fire risk area, so using waste from my neighbour's mill and branches that come down in storms to make biochar while reducing the fire risk makes sense to me. I use lots of material chipped and shredded as animal bedding or covered as small hugelkultures as well. I'm looking for "good enough", not perfection! Biochar is just one tool in my tool-box and permaculture is about using all the tools in all the ways!

A couple of things to note: biochar's ability to hold water and provide aeration stays the same down to quite small particle sizes (as long as it's incorporated into the soil and not laying on top). The porous structure is on the microscopic level and pieces that are barely visible still have lots of internal surface area.

The other thing is that to appreciate the long-term attributes in temperate soils we have some excellent examples in the prairies of the US midwest, plus the chernozem soils of Ukraine and the Argentine pampas. All of these places had a natural fire cycle that saw fast, relatively cool fires come through regularly, and then when people showed up they just tweaked the burning times and frequencies to suit their need to drive game and improve habitat for the things they wanted to hunt.

The effect of all this burning meant that pyrogenic carbon was continuously being added to the soils, so that over time this built up huge reserves and it's one of the reasons these are the most fertile regions in the world. As an answer to the "how long" question, there have been radiocarbon studies on the fire-derived component of these soils, and it dates back 8,000 years in Iowa and 14,000 in Ukraine. By the time it's that old, it's probably been modified to the point where it doesn't perform to the same level as "fresh" biochar, as the pore structures get filled in and minerals start to occupy the attachment points. But it's still there, like ancient trees on a mountainside, and still storing carbon.

Where I live there is a spot that was used to burn the results of building the house, nothing has grown there since we bought the property in 2013.

So I would say benefits are complicated.  

Also if I made a fresh pot of biochar and put it in the garden I bet it would either wash away or blow away, the last one is more probable.

Anne, it's pretty likely that your burn pile cooked the soil and wrecked it. Hot, intense bonfires in one spot, especially if they're repeated, are the polar opposite of prairie fires that move through faster than a horse can run.

If you try making biochar and adding it to your garden, harrow it into the top layer so it doesn't blow away. Or cover it with mulch and let the worms do the work. Or (stay with me here) dig it into that burn pile dead zone, cover with mulch, and sow a cover crop. I've remediated some really sad spots this way.

As Phil said, biochar crunching down doesn't harm it.  Most people crunch it every time before inoculating it and applying it. Also, it should be covered when applying, especially in dry or windy conditions. You don't want the life in it to dry out. It is primarily a hotel for microbes, facilitating all kinds of life processes throgh the soil, including retention and passage of moisture.
John S
PDX, OR

Actually, I count on the freeze-thaw cycle to break down coarse biochar and make it increasingly useful in my soil. Why spend a lot of time crushing when nature will do the work for me?

Thank you all for the excellent and insightful comments.

So clearly the particle size of biochar does get reduced through repeated exposure to the freeze/thaw cycle, but since the porous structure is microscopic, it can still confer benefits to the soil despite this size reduction. Temperate soils with fire regimes are an excellent point and they demonstrate the presence of pyrogenic carbon dating back thousands of years. These cooler fires don't cook the soil and incinerate the seed bank like uncontrolled wildfires from years of fire suppression policies. The regular cycle of these burns allows for a large build-up of pyrogenic carbon and stores huge amounts of carbon.

There seem to be some diminishing benefits to biochar as it weathers in the soil, not only from the freeze/thaw cycle but also from the pores filling and the attachment points being occupied by various minerals. As for how long it is until biochar transitions from a useful soil amendment to solely carbon storage is likely far longer than I initially thought. Biochar is known to last for thousands of years in the soil but I was curious as to whether it is a beneficial soil amendment for those thousands of years. While it certainly is not a negative influence due to the carbon storage, I think there is a threshold where biochar becomes simply carbon storage. That being said, most fire regimes or garden users replenish the supply of biochar in their land regularly, consistently supplying fresh biochar for optimal benefits.

It certainly depends on numerous environmental factors but I do wonder when that transition takes place.

I am going to disagree with some of the points here.  My friend, a PHd Materials scientist, explained what I've heard many times elsewhere.  Biochar is crushed on purpose because it is more effective after being crushed. It has that much more surface area.  It can shelter more microbes and enable more healthy chemical reactions. Biochar lines up in the soil and allows mycelium to flow minerals and moisture through it.  When it is dispersed, it is more effective.  There is no evidence that I've seen that it diminishes its power over time.  Ice and heat crunching it down will make it more effective.  The terra preta soils were made at least 500 years ago and are still astonishingly productive soils. The people who made it were wiped out quickly but the soil lives on.  When the soil is removed and placed in a temperate climate, it somehow seems to improve the new soil in the temperate location.  The old location, from where the terra preta was taken, seems to heal itself over time, and make up for the loss.  How this happens hasn't been exactly figured out yet. The carbon we put into the Earth as biochar is remarkably stable.  There is some data about the point at which you gradually have diminishing returns on continuing to add biochar. That isn't about the biochar dissolving or becoming ineffective.  That is just saying that plants grow in soil, not pure biochar.  Minerals don't really "use up" the biochar.  The biochar is a highway through which the mycelium moves the minerals to other locations where they are needed.  Trees and other plants output exudates, which set up these soil food webs as well and encourage the growth and movement of microbes and minerals to where they are needed.

John S
PDX OR

I live in northern Wisconsin so we get the freeze thaw cycles you worry about.

This summer I found two old fire pits.  One was right on the surface in the woods and I assume it is at least 20 years old it still had marble sized pieces of charcoal that were easy to identify as charcoal.

The other one was about 15 inches below the surface in a field and was in a small collection of rocks that stood out in an area that is otherwise sand deposits and didn't appear to be naturally deposited there.  That one had BB sized chunks of charcoal and fine ash.  I don't know if it was from modern times and someone dug a pit and lined it with rocks, Or if it was ancient and from before the sand was deposited above it by a flood.

But either way they both had recognizable charcoal and are at least 20+ years old.

Those are some good points! I feel as if I am coming to a better conclusion.

The weathering of biochar through, heat, freeze/thaw, and other elements does seem to make it smaller in size in most cases. I am curious about the fire pits mentioned by J Hillman, but perhaps there are vastly different rates of weathering given local conditions.

But as you mention this size reduction greatly increases available surface area for microbial interaction and the like, so it increases its benefit to the soil. Initially, I was worried that at some point the biochar would be weathered so much that this microscopic pore structure would be eliminated entirely. To be clear, it still can be eliminated over time, it is not immune to entropy, but I think this process is ridiculously slow. In other words, the complete destruction of the pore structure only occurs over the entire lifespan of the biochar or close to it.

I was initially hesitant to include examples of Terra Preta in this discussion because they do not experience the freeze/thaw cycle which was my main concern. That being said, they are not static and still experience weathering to a great deal in this scenario, the lack of seasonal inactivity of the soil life may contribute more to the weathering of biochar. Any further statements on that are beyond my current knowledge base.

The regeneration of Terra Preta is also fascinating! The biochar and unglazed pottery shards are not regenerating in the soil, so I have always theorized that the organic matter content is the key. Almost like a foothold, the former two elements allow for the accumulation of stable organic matter and a robust microbial community. Once that community is established, it can ideally be self-perpetuating and incredibly resilient.

Eric S.

J Hillman wrote:This summer I found two old fire pits.  One was right on the surface in the woods and I assume it is at least 20 years old it still had marble sized pieces of charcoal that were easy to identify as charcoal.

The other one was about 15 inches below the surface in a field and was in a small collection of rocks that stood out in an area that is otherwise sand deposits and didn't appear to be naturally deposited there.  That one had BB sized chunks of charcoal and fine ash.  I don't know if it was from modern times and someone dug a pit and lined it with rocks, Or if it was ancient and from before the sand was deposited above it by a flood.

Interesting! I recall that local archaeological digs have used radio carbon dating of charcoal from ancient fires to show that humans were active in my area more than 10 millennia ago -- 10,000 years! (I hope I have my facts straight.)

I wonder about chemical composition and longevity -- charcoal from an open fire is a mix, ranging from clean "biochar" to torrefied wood with a charred outer layer, and everything in between. Where along that spectrum is the signal that will persist when someone digs up my garden ten millennia from now? (And should I leave some ceramic pottery with a message?)