You will very often hear biochar referred to as “carbon negative” (including by yours truly!) and mostly that is correct. However not all biochar sequesters the same amount of carbon. There are a number of factors that influence just how much carbon can be sequestered with biochar. Let’s break this down a bit, shall we?
All biochar made from organic waste has some portion of organic carbon content. However the amount of carbon inherent in different type of feedstock varies significantly. The process used to create the biochar also has an effect on how much of the original carbon in the feedstock is carbonized.
The new IBI biochar characterization guidelines classify the organic carbon content into the following three categories:
- Class 1 => 60% (e.g. from plant and tree waste)
- Class 2 >30% < 60% (e.g. from poultry manure mixed with organic bedding material; paper sludge)
- Class 3 >10% < 30% (e.g. from cow manure, sewage sludge)
How far feedstock is shipped for processing and then shipped to the consumer before being buried in the ground also has a material effect on exactly how much carbon is really being sequestered. If you have to ship feedstock long distances to be converted into Char or you are shipping biochar across the country, then that should be factored into the overall carbon savings.
The math works like this. Dig out your old chemistry books and you will (re)learn that 1 lb of carbon is equal to 3.7 lbs of CO2. That is because (quoting James Bruges in “The Biochar Debate” here) the atomic weight of carbon is C-12; the atomic weight of oxygen: O-16. So to get the ratio of C to CO2 you need to add 1 carbon (12) to 2 parts oxygen (16 * 2 = 32) which gives you a net weight of 44. When divided by the ‘mere’ weight of carbon (i.e. 12) you get 3.7. [Who is smarter than a 5th grader?]
Let’s say a bag of blended char weighs 20 lbs and has half char plus half compost (this is not uncommon). At best a 20 lb bag then has 10 lb of char and if it is made from plant or tree waste could consist of up to 80% carbon or 8 lbs of carbon. This translates to 29.6 lbs of CO2 sequestered. Not too shabby but obviously buying locally made biochar which is made from high carbon content feedstock is going to help the most.
But biochar’s positive impact on GHG emissions doesn’t end there!
Often times the organic waste feedstock used for biochar was previously destined for the landfill. While it might intuitively seem like land-filling means sequestering ALL of the carbon absorbed during the plants lifetime, almost all of the carbon will biodegrade within a relatively short period and return to the atmospheric carbon cycle. Not only that but organic waste is responsible for large amounts of methane emissions. By averting the landfill, this methane generation is avoided. Methane, by the way, is a FAR more potent GHG than CO2, so avoiding sending more organic waste to the landfill is a very good thing!
Hang on… there is even MORE good news!
Recent research shows that applying biochar to soils helps to suppress the methane that is emitted at landfills, sometimes at a rate of more than 80%! Not only that but nitrous oxide (NOx), which is far WORSE than Methane or CO2 when it comes to GHG toxicity (we are talking 300 TIMES more damaging than CO2plus it kills the ozone layer), is also significantly curtailed when biochar is added to soils. NOx emissions are largely related to fertilizer use (roughly 2/3). Not only does using biochar mean less fertilizer needs to be produced and used which results in saving money, minimizing leaching and reducing NOx emissions, but it also has been shown to reduce NOx soil emissions by 80 – 100%.
So to summarize, biochar is a no brainer, right? Just make sure you know as much as possible about what its made from, and when possible buy it locally so you are maximizing the net carbon sequestered!