Biochar: What to Know
You're not gonna want to miss this deep dive with biochar expert, Jared Conner M.S.
🔊Today is very special because we have our FIRST EVER GUEST on Soil is Sexy: biochar expert, Jared Conner.
Jared is a Soil Conservationist in the lower Shenandoah Valley in Virginia with an M.S. in Soil Ecology. His experience with biochar spans over 6 years of formal research plus several personal, transformed gardens - this combo lends itself to a refreshingly comprehensive perspective!
In talking with Jared for this piece, I followed him down a biogeochemistry rabbit hole and back out to the practical applications in the garden. Whether you hold strong opinions of biochar or this is the first time you’re hearing of it, Jared’s insights will give you something to think about.
The Intro
Biochar is, in its simplest definition, a material created from the partial burning of BIOmass to make a CHARcoal-like material.
The source of the biomass that is burned (in low-oxygen conditions) to make biochar can vary, but biochar is most often formed through the partial combustion of vegetation of some kind - be it leaves, stems, tree limbs, or other plant structures.
In natural ecosystems biochar is formed through wildfire. Jared says, “research indicates that an estimated 13.5% of all carbon in soils globally can be attributed to charcoal, and up to 60% of carbon in some grassland soils is pyrogenic ” – that is, carbon that was introduced to the soil through a fire event and has been stored long-term (Reisser et al., 2016) (Mao et al., 2012). Similarly, woodland fires form biochar, some of which is carried off and deposited in soils away from the fire site (Cotrufo et al., 2016).
As another example, if you’ve ever heard of terra preta – dark, fertile soils with large stores of carbon found throughout the Amazon basin – these were formed under human influence as ancient indigenous civilizations had a practice of adding charcoal from their stoves and land-clearing operations to surrounding soils, in addition to kitchen scraps, pottery shards, and other waste products (Kern and Kaempf, 1989).
What’s remarkable about all these examples is that biochar is evident in such soils today from events that occurred hundreds to thousands of years ago (Liang et al., 2010). In other words, biochar can be quite a stable and persistent carbon-based material - this can play an important role in soil structure and function! Which is why we see biochar products on the market today.
Jared will help us understand what we should know about biochar to make sense of products, applications, and practical at-home DIY methods.
Like most of my posts here, I will not be offering up prescriptions, rather I aim to provide you a framework to think through biochar, so you’re empowered to make or use it within your specific context. Comment any questions that come up for you, we love learning together here!
For clarity, aside from cited sources (parenthetical) text includes my added commentary.
The Interview
Jared, you did biochar research in your undergraduate and graduate studies, why is biochar an area of focus for researchers?
Due to its unique properties, biochar has ever-expanding uses in the agricultural and environmental fields.
Biochar can be used as a soil amendment, a filter media for water and air, or even as a building material. It’s been used to increase crop yields, treat acid mine drainage, absorb excess nutrients, provide organic matter to build soils where mining has destroyed topsoil, and it can also adsorb and immobilize heavy metals due to its negative charge (Atkinson et al., 2010) (Jeffery et al., 2011) (Bartoli et al., 2020) (Gao et al., 2022).
Another reason biochar is being studied is that you can make it from waste streams – making it an economically and environmentally advantageous material to produce.
In short, from building soil to aiding in the remediation of a contaminated site, biochar is a useful tool inspired by natural processes. Oh, and it can also sequester carbon in the soil (Lehmann et al., 2006)!
Like compost, not all biochar is created equal, what does the term “biochar” mean to you?
Biochar for agricultural use is charcoal that has been inoculated with microbes and/or nutrients as a tool for building structure and restoring functions to soils.
The char is made in the same process used to make charcoal: by burning or baking in a low oxygen environment - a process called pyrolysis. However, biochar differs from grilling charcoal in that it is made at higher temperatures (they sometimes leave a lot of volatile components in grilling charcoal to help it light better).
More technically, to be considered biochar, most of the hydrogen and oxygen must be driven out of the volatile organic compounds of the wood (International Biochar Initiative Standards). This leaves behind a material that is about 20% of the mass of the original material and two-thirds the original volume. The product is essentially a complicated lattice of carbon rings that retains the cellular structure of the original feedstock the biochar was made from.
An important distinction between biochar used for agriculture as a soil amendment and biochar used in environmental applications (remediation) is that biochar used as a soil amendment is usually inoculated with microbes and/or nutrients prior to application to soil. Biochar used for environmental applications is typically (intentionally) not inoculated.
Tell us more about the inoculation step, how does that affect the behavior of the biochar?
The process of inoculating biochar with microbes and/or nutrients is called “activating” or “charging” and is necessary in order to not remove soluble nutrients from the soil.
It sounds like “activated” or “charged” biochar is an especially important trait for use in cultivated soils, but may be less necessary or desirable for certain remediation contexts where you’re actively trying to remove substances.
Right.
Most of our readers are interested in cultivated or landscaped soils, so let’s focus there for now…
How can activated biochar help build better soil?
Say you’re working with sandy soil and you just cannot get organic matter to stick around, even after adding lots of compost. If you add a one time application of biochar in the right amount, you can take your organic matter from 1.5% to 3% overnight. Just by adding biochar. That doesn’t even account for the potential accumulation of organic matter you might get after you have the biochar in place.
Technically speaking, biochar contributes to / behaves like MAOM – mineral-associated organic matter (which I wrote about in this post!), it is incredibly resistant to decomposition (Wang et al., 2016). In layman's terms, biochar acts like a savings account of carbon and nutrients in the soil.
Biochar acts like a savings account of carbon and nutrients in the soil.
It has an affinity for nutrients and water – it has high surface area and can hold onto water and nutrients due to its negative charge (Bruun et al., 2014) (Hagemann et al., 2017). The surfaces of biochar are like a never-ending game of musical chairs; it’s like a holding center for nutrients and water.
Additionally, biochar provides an incredibly stable building block for other organic matter to cling to and accumulate on, allowing the continued enrichment of the soil. This tendency for biochar to stabilize organic matter in soil actually allows for biochar to increase the carbon sequestration capacity of the soil to which it is added (Weng et al., 2022).
We can see this in action in the rich, deep terra preta, or “black earth” soils in the Amazon. This region is known for its incredibly poor, acidic, and weathered clayey soils, however in the areas where indigenous Amazonians threw out ash, charred pieces of wood, bones, pot shards and other kitchen waste, the soils to this day stabilize organic matter at higher rates than similar soils nearby, that did not receive charred waste.
We often hear this idea that biochar is like a “condo” or “hotel” for microbes in the soil, what do you think of that visual?
This may come as a surprise, but I do not consider microbial habitat to be among the top benefits of biochar. My thoughts on this are purely based on anecdotal observations and there is not sufficient scientific work to confirm or deny those observations.
Understood! Would you mind sharing your thoughts / anecdotal observation with us?
Sure. From what I’ve observed, biochar is incredibly resistant to decomposition by fungi and bacteria. When I dig up some biochar in my garden that’s been there for a few years I may find plant roots, fungi, and soil attached to the surface of the biochar, but when I break it open the inside surfaces appear clean (to the naked eye) and usually free of any noticeable biofilms or mycelium - perhaps you could scrape a sample out and look at it on the microscope for a more accurate look at the microbial world!
Anyway, does biochar create habitat for microbes? I’d say, to an extent. I suspect microbes are finding habitat in the outer surface of the biochar, and likely taking advantage of the nutrients, water, and air being exchanged between the biochar and soil.
The microbes are also likely living on the organic matter that accumulates on the biochar. I have seen one study that can back up the idea that the biochar-soil interface is where the microbial habitat is rather than throughout the inner pore structure of the biochar, which has a relatively closed-cell structure (Quilliam et al., 2013) (Jaafar et al., 2015).
If I may, thinking like a microbe for a second, it sounds like biochar may serve as a substrate that the microbial world can start building upon. Which can be incredibly useful in a soil that has poor structure or lacks aggregation. Microbial processes naturally aggregate soil and build structure and organic matter, but the addition of biochar may be an appropriate solution in certain contexts to help accelerate that natural process of soil building. It’s a tool.
I think that’s a good practical summary, yes.
I would add that the effect biochar has on soil chemistry can be just as important a factor for microbes as the habitat provided, be it directly or indirectly.
Can you elaborate on this relationship - how biochar effects soil chemistry and therefore the microbiome?
Biochar will affect soil aeration, pH, and the nutrients and organic compounds floating around in soil solution. This can have a huge effect on the microbial community (who’s there) and their abundance (microbial biomass).
Studies have shown that adding biochar to soil - especially acidic soil - can increase the populations of ammonia oxidizing and nitrifying bacteria, and essentially speed up the rate at which ammonium is converted to nitrate (Abujabhah et al.,2018).
Studies have even shown that the type of bacteria in soil that consume methane, a powerful greenhouse gas, are more abundant in biochar-amended soil, which helps explain why we see decreased methane emissions sometimes from biochar amended soil (Abujabhah et al.,2018) (Kubaczyński et al., 2022).
Biochar can also increase the amount of mycorrhizal fungi and the overall biomass of microbes in soil, reinforcing the idea that biochar promotes the success of soil biota that crops rely on (Xu et al., 2021).
A common concern with using biochar is that it might absorb a lot of nutrients in the soil, making them unavailable to plants – what are your thoughts on that?
Seeing as biochar is a highly porous material made up of 60 – 90% elemental carbon, it acts similarly to activated charcoal, soaking up a lot of things, nitrogen being one of them. However, the nitrogen attached to properly activated biochar is more plant-accessible than originally thought (Hagemann et al., 2017b) (Haider et al., 2020). So, it’s not that nitrogen is being removed from the soil solution, but rather that it’s being held in place, less likely to leach into run-off, when bound to (activated) biochar.
And again, it’s important that biochar be activated or charged when used in agricultural contexts (as opposed to bioremediation) for this reason. There is even evidence to suggest that the composting of biochar with food waste results in an activated or charged biochar with the ability to slowly release captured nitrogen into the soil (Kamman et al., 2015) (Hagemann et al., 2017a).
How can biochar contribute to carbon sequestration and storage?
In a few ways, it…
Fixing carbon long-term: Vegetation that decomposes is part of a very active carbon cycle – the carbon is constantly moving between living organisms and CO2 released from respiration. When vegetation is charred, some carbon is released as volatile organic compounds into the atmosphere or is burned in the fire, but much of it can be stored as char, allowing that carbon to persist for decades to centuries, especially in the case of charcoal-making kilns such as the Oregon kiln. This also happens naturally in slow-burning wildfires. The hotter and more quickly a wildfire burns, the more carbon that is lost to the atmosphere and the less stable char will be left behind.
(I hope it’s obvious that decomposition isn’t a bad thing, on the contrary; it’s just in terms of carbon, some burning can be good…)
Generates more carbon storage: biochar boosts soil’s ability to accumulate organic matter (and support more plant life) effectively storing more carbon.
Protects OM Loss: Lastly, biochar protects organic matter that comes from modes of decomposition, compost, or other inputs through adsorption to its charged surfaces.
It sounds like you can definitely apply too much of a good thing when it comes to biochar, is that right?
Yes. Ideally, biochar should be a minor component of your growing soil. The application rate will vary depending on soil type and other site conditions.
Biochar, if applied once in a high enough application rate - up to 10% of the volume of the soil down to 6 inches - it never needs to be applied again, at least not for a lifetime or two.
What are the different methods of making biochar and can we successfully make some at home?
You can definitely DIY biochar.
Some sources of biochar can include…
the char that’s left over in a firepit
the biochar you can make in your backyard using an Oregon style kon-tiki kiln or a homemade retort
the fancy biochar made in industrial retorts you can buy from companies like Wakefield biochar
Here is a helpful video demonstrating two simple ways to make biochar using a 55 gallon drum that you could use in your backyard.
I understand you add biochar to your compost – tell us why!
Two reasons:
It’s the easiest way to inoculate the biochar with microbes, a process usually referred to as “activation” or “charging”
It really speeds up the maturation process of composting and is useful for maintaining proper moisture and air mix and bulking (Nguyen et al., 2022) (Agyarko-Mintah et al., 2022) (Steiner et al., 2010).
I personally believe that the finished compost is also better than if I hadn’t added biochar. It also adsorbs excess nutrients and retains off-gassed ammonia that might be otherwise lost from the compost (Agyarko-Mintah et al., 2022) (Steiner et al., 2010).
So if I wanted to make biochar at home, I could just salvage the wood coals from my firepit and toss them in my compost? Take me through some pointers!
It’s important to know the biochar must be cooked enough, or else it is only slightly burnt wood which will readily break down and can be acidic. On the other hand, you don’t want the biomass burning so much that it just becomes ash.
So, what I do is I keep a metal bucket full of water near my firepit and use a shovel to scoop out glowing embers into the bucket, quenching the coals and stopping the burning process. From there, once the coals are cool I’ll break them up, aiming for flecks that range from sand to pebble size, and place them into the center of my maturing compost bin.
If I wanted to purchase biochar, what are some things to consider?
The factors that most influence the physical and chemical characteristics of biochar are:
- the material it is made from (feedstock)
- the way it was made (pyrolysis conditions)
- the activation or charging process (inoculation of microbes)
You’ll find that some biochar manufacturers produce biochar by heating the feedstock in the absence of oxygen, allowing for the capture of the off-gassed volatiles for use as fuel, and that other manufacturers simply burn feedstock in kilns and then extinguish the coals before they turn to ash. Both types of biochar are valid, and both should perform well if properly activated and applied at the proper rates.
(Note: Look for this key information from the manufacturer. If you can’t get a straight and sound answer, move on to a maker that is more transparent about their methods.)
Biochar can sometimes be a controversial topic – can you explain why?
There are lots of controversies involving biochar, and not all of them are resolved. Some are based on individual interests, while others are a result of not comparing apples to apples.
Biochar Purists vs Practical Hobbyists: they mostly debate over the “best” methods of pyrolysis and other technical aspects of making biochar (i.e. pyrolysis in a sealed retort in the absence of oxygen vs burning in a metal kiln and quenching the coals)
Negative Effects: there are accounts of people seeing a decrease in soluble nutrients and/or decrease in yield when using biochar (Jeffery et al., 2017). You have to make sure biochar is well activated, suited to your growing situation (soil and crop), and not overapplied. There are even some research studies that use char that wasn’t activated and make claims about biochar as a whole, the oversight of this important factor can be frustrating as non-activated biochar will behave very differently compared to activated biochar.
Deforestation concerns: In the 2000’s and 2010’s, there were lots of people coming out against biochar claiming it was snake oil and warning that whole forests were going to be ripped out to be turned into biochar. Obviously, it’d be counterproductive to harvest forests to create biochar; it’s more about finding the opportunities of existing waste streams to turn biomass into biochar. Biochar is not going to save the world but it can definitely serve an important part of the soils-based component of fighting climate change.
Source of Contamination: a group of people have concerns that biochar is a source of contaminants like PAH’s (polycyclic aromatic hydrocarbons). This largely depends on what you’re burning, and how complete the pyrolysis process is (Odinga et al., 2021). For instance I wouldn’t recommend burning treated wood for biochar. After a while in compost and soil, compounds that result from burning such as PAH’s aren’t a concern (de Resende et al., 2018) . Remember, people have been growing healthy food in charcoal-amended soils for thousands of years.
Pollution: Another concern is that biochar is seen as polluting due to the need to burn wood or fuel to make it, but as long as smoke is minimized and especially if you burn wood and then quench it to make biochar rather than burning wood to heat a retort full of wood, the pollution is not important in the long run.
Your are a wealth of knowledge and perspective! 💫
My last question for you: what are some things you wish more people knew about biochar?
Biochar fundamentally increases the carbon sequestration capacity of a soil – in other words, it boosts the soil’s ability to accumulate organic matter and store carbon to fight climate change (Weng et al., 2022).
Biochar sequesters carbon in three major ways.
the carbon in the biochar can remain in soil for decades to centuries
biochar adsorbs and protects organic matter from compost, root decay, and other inputs.
biochar enhances the formation of Mineral-Associated Organic Matter (MAOM), the soil’s savings account for carbon (Weng et al., 2022).
Biochar, if applied once in a high enough application rate (up to 10% of the volume of the soil down to 6 inches) never needs to be applied again, at least not for a lifetime or two.
You can make biochar in a firepit and just use a shovel to scoop out glowing embers and dump them in water before incorporating it into compost
Biochar isn’t a cure all or silver bullet, and is just supposed to be a part of a larger heterogenous healthy soil
THE TAKEAWAYS
Do:
Co-compost or activate/charge your biochar prior to applying to soil
Make it yourself if you can, it is currently expensive to buy
Make sure your biochar is fully cooked by breaking it apart. If it is brittle and ceramic-like, it is done
Incorporate the biochar into the soil profile if possible. It is most beneficial when incorporated in the top few inches
Crush biochar to the size of sand to pebbles
Wet biochar after its made to reduce the amount of dust
Apply biochar at an application rate of 2-20 tons/acre (context dependent)
Do NOT:
Apply biochar raw to soil without charging/activating or composting it
Make biochar from treated wood, pallet wood, or other chemical-laced feedstocks
Apply too much biochar - biochar should be a minor component of your growing soil
Apply biochar to alkaline soil
Make biochar from nitrogen-rich feedstocks
Apply large chunks of biochar
Crush biochar to fine dust - the fine dust can blow or wash away
Biochar isn’t a cure all or silver bullet, and is just supposed to be a part of a larger heterogenous healthy soil
-Jared Conner M.S.
You can find Jared on LinkedIn.
Jared helps farmers get government funding and conservation plans to protect and improve the quality of water and soil resources on their lands. Help me thank him for his important work and for sharing his knowledge with us! To hear MORE interesting bits from my conversation with Jared and for the scientific sources cited (all thanks to Jared’s diligence) continue on.
🪱BONUS
What was your area of focus in your academic research?
At the University of Georgia, I did three years of undergraduate research using biochar to filter out nutrients from the wastewater produced by factory farms. After graduating I pursued an MS in soil ecology at Virginia Tech where I studied the effects of biochar on soil nitrogen and the accumulation of organic matter.
And?!
From my undergraduate research, I learned that biochar can adsorb lots of ammonium from wastewater, but not as much phosphate (due to biochar’s negative charge repelling negative phosphate ions) (Conner et al., 2019).
From my graduate work, I found that at 1 month after biochar application, there isn’t a huge effect on inorganic nitrogen availability. And when co-applied with an organic nitrogen fertilizer, there is a short-term release of N. However, after 11 months, inorganic nitrogen was lower in biochar soil compared to control soil. While this seems like a bad thing, studies have shown that microbes and plants can access this biochar-attached N (Conner, 2022).
I also applied dairy manure effluent to soil and biochar-soil and found that while the manure stimulated the loss of stable organic matter (MAOM) from soil, biochar reversed this effect and actually allowed the manure to accumulate in the soil (Conner, 2022).
How did you get interested in biochar to begin with?
I first learned about biochar when I was 15 when I was just starting out as a gardener. I grew up in Georgia and had really compacted clay soil and was struggling to get it to produce. I saw some youtube videos about biochar and how it could really improve acidic, clayey soils and thought well I need to try this out. So I made some charcoal in my backyard and after treating it with compost tea, I mixed it into my soil and added compost as well. And I was hooked. I cared a lot about fighting climate change and it really fascinated me that I could help sequester carbon (biochar doesn’t break down for decades to centuries) AND improve my garden at the same time.
What has your experience been in your gardens using biochar?
I’ve had over 7 gardens in my lifetime that I’ve added biochar to. I’ve witnessed the power of biochar to build soil and accelerate the transition of just ok garden soil to really great soil. I can also attest to the long-lived nature of biochar as I’ve gone out to my first garden from high school and can still find the biochar I applied to it over 10 years ago.
References
Agyarko-Mintah, E., Cowie, A., Van Zwieten, L., Singh, B. P., Smillie, R., Harden, S., & Fornasier, F. (2017). Biochar lowers ammonia emission and improves nitrogen retention in poultry litter composting. Waste Management, 61, 129-137.
Atkinson, C. J., Fitzgerald, J. D., & Hipps, N. A. (2010). Potential mechanisms for achieving agricultural benefits from biochar application to temperate soils: a review. Plant and soil, 337, 1-18.
Bartoli, M., Giorcelli, M., Jagdale, P., Rovere, M., & Tagliaferro, A. (2020). A review of non-soil biochar applications. Materials, 13(2), 261.
Bruun, E. W., Petersen, C. T., Hansen, E., Holm, J. K., & Hauggaard‐Nielsen, H. (2014). Biochar amendment to coarse sandy subsoil improves root growth and increases water retention. Soil use and management, 30(1), 109-118.
Conner, J., Favero, L., Nzengung, V. (2019). Modification of Peanut Shell Biochar With Different Metal Oxy-hydroxides for Improved Removal of Ammonium and Phosphate From Wastewater. Proceedings of the 2019 Georgia Water Resources Conference. Link: https://gwri.gatech.edu/sites/default/files/files/docs/2019/4.6.4.pdf
Conner, Jared P. (2022). The Effects of Biochar and Reactive Iron Additions on Soil Carbon and Nitrogen Retention. https://vtechworks.lib.vt.edu/handle/10919/110436
Cotrufo, M. F., Boot, C. M., Kampf, S., Nelson, P. A., Brogan, D. J., Covino, T., ... & Hall, E. (2016). Redistribution of pyrogenic carbon from hillslopes to stream corridors following a large montane wildfire. Global Biogeochemical Cycles, 30(9), 1348-1355.
de Resende, M. F., Brasil, T. F., Madari, B. E., Netto, A. D. P., & Novotny, E. H. (2018). Polycyclic aromatic hydrocarbons in biochar amended soils: long-term experiments in Brazilian tropical areas. Chemosphere, 200, 641-648.
Haider, G., Joseph, S., Steffens, D., Müller, C., Taherymoosavi, S., Mitchell, D., & Kammann, C. I. (2020). Mineral nitrogen captured in field-aged biochar is plant-available. Scientific reports, 10(1), 13816.
Hagemann, N., Joseph, S., Schmidt, H. P., Kammann, C. I., Harter, J., Borch, T., ... & Kappler, A. (2017a). Organic coating on biochar explains its nutrient retention and stimulation of soil fertility. Nature communications, 8(1), 1-11.
Hagemann, N., Kammann, C. I., Schmidt, H. P., Kappler, A., & Behrens, S. (2017b). Nitrate capture and slow release in biochar amended compost and soil. PloS one, 12(2), e0171214.
International Biochar Initiative Standards: https://biochar-international.org/wp-content/uploads/2023/01/IBI_Biochar_Standards_V2.1_Final-2.pdf
Jeffery, S., Verheijen, F. G., van der Velde, M., & Bastos, A. C. (2011). A quantitative review of the effects of biochar application to soils on crop productivity using meta-analysis. Agriculture, ecosystems & environment, 144(1), 175-187.
Kammann, C. I., Schmidt, H. P., Messerschmidt, N., Linsel, S., Steffens, D., Müller, C., ... & Joseph, S. (2015). Plant growth improvement mediated by nitrate capture in co-composted biochar. Scientific reports, 5(1), 11080.
Kern, D.C., Kaempf, N., 1989. The contribution of Indian prehistoric settlements to Archaeological Black Earth formation at Oriximina-PA (in Portuguese). Rev. Bras. Cienc. Solo 13, 219–225
Lehmann, J., Gaunt, J., & Rondon, M. (2006). Bio-char sequestration in terrestrial ecosystems–a review. Mitigation and adaptation strategies for global change, 11, 403-427.
Liang, B., Lehmann, J., Sohi, S. P., Thies, J. E., O’Neill, B., Trujillo, L., ... & Luizão, F. J. (2010). Black carbon affects the cycling of non-black carbon in soil. Organic Geochemistry, 41(2), 206-213.
Mao, J. D., Johnson, R. L., Lehmann, J., Olk, D. C., Neves, E. G., Thompson, M. L., & Schmidt-Rohr, K. (2012). Abundant and stable char residues in soils: implications for soil fertility and carbon sequestration. Environmental science & technology, 46(17), 9571-9576.
Nguyen, M. K., Lin, C., Hoang, H. G., Sanderson, P., Dang, B. T., Bui, X. T., ... & Tran, H. T. (2022). Evaluate the role of biochar during the organic waste composting process: A critical review. Chemosphere, 299, 134488.
Odinga, E. S., Gudda, F. O., Waigi, M. G., Wang, J., & Gao, Y. (2021). Occurrence, formation and environmental fate of polycyclic aromatic hydrocarbons in biochars. Fundamental Research, 1(3), 296-305.
Reisser, M., Purves, R. S., Schmidt, M. W., & Abiven, S. (2016). Pyrogenic carbon in soils: a literature-based inventory and a global estimation of its content in soil organic carbon and stocks. Frontiers in Earth Science, 4, 80.
Steiner, C., Das, K. C., Melear, N., & Lakly, D. (2010). Reducing nitrogen loss during poultry litter composting using biochar. Journal of environmental quality, 39(4), 1236-1242.
Terra preta: https://en.wikipedia.org/wiki/Terra_preta
Wang, J., Xiong, Z., & Kuzyakov, Y. (2016). Biochar stability in soil: meta‐analysis of decomposition and priming effects. Gcb Bioenergy, 8(3), 512-523.
Weng, Z., Van Zwieten, L., Tavakkoli, E., Rose, M. T., Singh, B. P., Joseph, S., ... & Cowie, A. (2022). Microspectroscopic visualization of how biochar lifts the soil organic carbon ceiling. Nature Communications, 13(1), 5177.