Are our forests full of biochar?

During last month's podcast episode, postdoctoral researcher Yuge Bai discussed how biochar – a product of burnt biomass – can provide many benefits to soils including increased carbon sequestration.

As part of the discussion, Yuge had mentioned how forest fires help fix carbon sequestration in the soil which brought to mind a question.

If forest fires burn organic matter, and biochar is created by burning of organic matter shouldn’t forest soils be full of biochar?

Not exactly. There is a difference between the burnt product of wildfire and biochar. Forest fires do produce charcoal, but while both charcoal and biochar are types of pyrogenic carbon, they’re not quite the same thing.

Charcoal vs Biochar

A study by Cristina Santin et al. (2017) examined the differences between these two types of pyrogenic carbon.

Biochar is produced through pyrolysis, which is heating of organic matter without oxygen, meaning combustion does not occur. Biochar is most commonly produced by burning organic matter over several hours at temperatures between 300-700 degrees celsius.

Charcoal is produced during wildfires during which temperatures can exceed 700 degrees and, due to the presence of oxygen, combustion occurs.

Santin et al. found that both kinds of pyrogenic carbon produced under higher temperatures could sequester more carbon, however biochar outperformed charcoal produced at even the highest temperatures.

That’s not to say that charcoal cannot be beneficial to soil health in forests but, of the two, biochar is more effective at improving carbon sequestration in forests.

So the short answer is no, the burned biomass that results from forest fires is not biochar, but forests could still be a source of biochar.

Forests as biochar fuel

Forests in Alberta have been affected by the mountain pine beetle, leaving behind dead trees that act as easy fuel for fires.

These dead, dry trees are extremely flammable. The Canadian government has looked into using these dead trees as biofuels and some companies do use them to create biochar.

Another source of organic matter for biochar is the material removed from forests as part of fuel management. “Fuel” refers to highly flammable vegetation that may contribute to fire risk.

Both of these methods help manage wildfire and could potentially increase the carbon sequestration of forests if the resulting biochar was added back into the forest's soil.

Challenges

As discussed by Yuge in “Soil Health, Biochar and the Changing Climate,” biochar has a few drawbacks:

Price

The production of biochar is expensive. Using less heat and therefore less energy is one way to keep the cost of biochar production down, however, according to Santin et al., biochar produced at lower temperatures has a lower capacity for carbon sequestration than biochar produced at higher temperatures. For this reason it’s difficult to make biochar widely available.

Health concerns

There is some concern about the presence of polyaromatic hydrocarbons (PAHs)in biochar enhanced soils. PAHs are chemicals released through burning of various substances and are therefore present in biochar. Research into PAHs in biochar have found that PAH concentrations vary depending on the type of biochar and that exposure to PAHs from biochar in soils a foods posed a low risk of cancer

Long term effects on soil

Right now, we don’t know what the long term effects of biochar on soil are since studies rarely exceed five years.

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To learn more about how Biochar could be used as part of sustainable agriculture, check out the Land Use Podcast.