When we talk about biochar, usually trees are considered as a usual candidate for production. However, even the fastest-growing tree needs at least 10 years to get to a decent size; among other things, this this is the reason I strongly oppose tree cutting, regardless of the purpose, except when it is absolutely necessary.
Instead, using tree leaves would be much more fitting (at least temporary), as they have a yearly cycle.
Apart from carbonisation or using leaves in different other products, there are multiple other things with the same yearly cycle we could use in a similar way without too much effort. Around the world, on daily bases, we use lots of things we then throw away. Those things usually end up in landfills, slowly decomposing via microorganisms, emitting carbon dioxide and methane into the atmosphere.
These are just a few examples:
Tea – worldwide, 5,304,500,000 kg of tea is produced per annum. Each time we make tea, we will soak the tea bag in hot water, take the herb essence, and then, after drinking the water, we throw the tea bag away.
Tea/herbs are also bio materials, very much like tree leaves, and they contain similar amounts of carbon; therefore, carbonising all that used tea would account for 1.6 MtC a year (5,304,500 t * 30%), which is 16 MtC over a 10 year period.
Coffee – production in 2016 reached 153,869,000 (60 kg) bags, or 9,232,140 tonnes per annum. If we would carbonise or convert it into some other non-decomposing product, a carbonisation rate of just 30% would give us ~28 MtC over a period of 10 years.
Human hair – very much like tree leaves, it is composed of more than 50% carbon. Typical growth rate is 1cm per month or 12 cm per year, with an average weight of 72 grams per year. That means that, by collecting and carbonising hair after haircuts, over 10 years, we could save 1.6 mega tonnes of carbon. Currently, in the UK, hair from salons is already collected in special ways (at the time of writing this, I have not found what they do with it afterwards).
Agriculture by-products – growing sugarcane (1.9 billion tonnes), corn (1.0 billion tonnes), rice (741.5 million tonnes in 2014), wheat (729 Mt), soybeans (324 Mt), barley (144 Mt), coconut (61.4Mt), oats (22.7 Mt), rye (14.6 Mt), and other grains and beans always produces some kind of stems, straws or fibre shell, and, if the main products are not used before expiration date (flours, grains, beans and other) will end up on landfills.
Taking wheat production as an example, in 2014, world production was 729 Mt of wheat. Taking that the average yield is 1500 kg per acre (56 bushels, 1 bushel = 60 lbs), that means that the world used around 480 million acres of land to grow wheat. Taking that one acre of wheat can easily produce 80 bales, weighing 50 pounds each, this would amount to 2 tons or 1.8 metric tonnes, so the entire world would produce around 874.8 Mt of wheat straw. If we would try to convert all that into carbon over 10 years, it would amount to 2.6 Giga tonnes of carbon from wheat straws (874.8*0.3(%)*10 y).
At this point, we should consider that there are other crops like corn and sugarcane (even larger footprint); converting a portion of it into biochar and storing it deep underground could potentially lock significant amounts of carbon.
Now, as we know that many of these products are used as animal feed or as bedding material, the question is “how much of that can we use?”
Stubble burning (or Slash-and-burn), for example, is still used in some parts of the world, although, since the 1990s, governments have restricted this practice. If we could prevent those manmade fires and instead turn those bio materials into biochar, we would prevent significant emissions of carbon dioxide.
What else can be used?
Expired herbs, flowers (floral industry), some spices, and many food ingredients would perform equally well.
Any hilltop covered with dry grass and hay—commonly seen in savannas of Africa, Australia, and many other parts of the world—could be harvested and turned to biochar, and it would be especially effective if we prevent people from burning it. Especially in Africa, this method could be quite useful, as activated carbon obtained this way could be used for water filtration, therefore giving access to clean water.
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