Experiments with biochar – carbon sequestration and soil enrichment with biochar

By Emma Kloes

Oftentimes within restoration projects and routine land care we have significant amounts of brush and wood on our hands. Some wood will decompose in the field and provide habitat for a plethora of insects. Some is burned in brush piles to make room for native species growth and to prepare a unit for a prescribed burn.

The traditional brush pile burn undergoes wood combustion. Wood is exposed to heat and oxygen, gases are released then combust, and what remains is a pile of ash. This ash can be great for soils with its high mineral composition, rich in elements such as calcium, potassium, phosphorus, and magnesium – trees and plants need these nutrients to thrive. However, inevitable byproducts of wood combustion are carbon monoxide and carbon dioxide. We experience two losses in this movement of carbon: a decrease in soil organic matter and an addition of carbon emissions to our atmosphere.

This year we began to wonder if our wood reduction system could be adapted to reduce greenhouse gas emissions. The answer is yes, and the solution is biochar. Biochar is a form of stable carbon that can live in the soil for thousands of years. Making biochar is a win-win in its ability to both sequester carbon and enrich soil. It may seem incredulous, but this form of carbon can stay in soil for thousands of years as a semi-permanent compost.

Now, how is biochar made? You may be familiar with biochar even if the name is new to you. Recall that thin dusty layer of black following a prescribed burn of woodland, savanna, or prairie? That’s biochar, and it’s been accumulating in our soils for thousands of years thanks to the indigenous practice of controlled burns. Tall grasses and forbs burn quick and hot, but close to the ground air is excluded and the base of the plants will char and not burn to ash. The upper Midwest prairie states have some of the most valuable agricultural soils in the world because of the carbon-rich outcome of prairie burns.

We have mimicked this system of creating char but utilize different feedstock: wood instead of grasses and forbs. Our system uses a thick steel kiln as our which chokes out oxygen from the bottom and sides. We begin with a small but mighty brush pile, then tilt the kiln on top. Continuous, quick feeding with brush and smaller pieces of wood ensures a constant, strong flame on top. The flame-capped top is essential as it consumes all nearby oxygen and prevents our feedstock from burning into ash. Combustion requires oxygen, but carbonization requires the absence of oxygen. The biomass undergoes a high temperature thermochemical conversion: wood dehydrates and water vapor is driven off, the volatile gases composed of oxygen and hydrogen are driven off, then those volatiles are the gas that burns into flame. We are left with char that’s protected from air, and it builds up and up the 4ft tall, 5 ft diameter kiln. When the kiln is full, we quench with water, tip the kiln over, and gawk at the sea of black gold. Each kiln sequesters 1 ton of CO2 and amounts to 600 pounds of biochar.

Biochar creates healthy air and healthy soils. In biochar amended soils we see improved water holding capacity, nutrient retainment and cycling, support to microbial life and biodiversity, and plant resilience against disease and pathogens. Soil organic matter has taken a hit through mass tillage and erosion, and unhealthy, depleted soils is the result. The solution to improving soils is multifaceted, but biochar incorporation is one avenue is to build up organic matter and restoring overall soil health.

This winter at Holy Wisdom we’ve loaded nine kilns full of biochar and hope to run at least 4 more. Nine kilns accumulate to nine tons of carbon dioxide sequestered and 5,400 pounds of biochar. Sometimes it won’t be the best option to use a kiln when burning brush, but we hope to strategically choose the biochar route when it makes sense. Maybe you’re thinking, what in the world will you do with several tons of biochar? Biochar is advantageous to our savanna and woodland soils and thus plant and tree communities. Our garden plots and orchards will also receive a fair amount. Much like our prairie seed that is harvested on site then planted all around Dane County, our biochar will likely reside in many gardens and farms in our area.