America’s rich farmland was created by prairie plants, capturing carbon from the air and sinking it into the earth. Generally, the darker the soil, the more carbon it contains. As pioneers broke the sod the sequestration of carbon into our soils nearly stopped. Plowing increased the rate at which carbon left the soil.

We can restore the carbon capturing ability of our soils and tap into a carbon negative form of energy. Chiefly, by coupling new understandings of two old concepts: prairies and agrichar.

University of Minnesota (U of M) researchers have been studying a highly diverse prairie grown on degraded land as a potential source of sustainable bio-mass fuel. U of M ecologist David Tilman published his team’s findings in the Dec 8, 2006 issue of the journal Science. He reported that mixed native prairies produce more biomass per acre on degraded land than any other single monoculture crop, including switchgrass, by a whopping 238%. The researchers estimate that mixed prairie would yield 51% more energy per acre than ethanol from corn. Corn grown under high input conditions on good land.

On degraded soils, prairie roots sequester 1.2 to 1.8 U.S. tons of carbon dioxide per acre per year, and the researchers estimate that this can continue for about 100 years before the soil carbon level begins reaching equilibrium. The above ground parts of the plants make excellent bio-fuel, leading the researchers to estimate that if the entire world’s degraded lands were planted with mixed prairie they would produce enough biomass fuel to displace 13% of the worlds petroleum use and 19 % of the world’s electricity use. This research was funded by the National Science Foundation and the University of Minnesota Initiative for Renewable Energy and the Environment.

Prairie Study Article

Energy from prairie bio-mass can be extracted in various ways. Power plants may directly burn it, or distillers may make it into cellulosic ethanol. I hope to see some of it pyrolysed (chemically decomposed by heat in the absence of oxygen) into syngas and agrichar.

Pyrolysis is similar to gasification. During WWII Europeans used wood as feedstock for pyrolysis to make wood gas. Any carbonaceous material including wood, grass, corn stover, logging slash or sewage works as feedstock. The feedstock is heated in the absence of oxygen, and uses temperatures of 400-500 degrees C. Long molecules decompose into shorter ones, and produce a combustible gas called syngas. It is composed of hydrogen, carbon monoxide, tars and particulates. The syngas may be directly used as fuel to generate electricity, or cleaned and further refined into products such as bio-diesel. The remaining solid feedstock is called char, biochar, or agrichar. If you are interested in learning more about the technical aspects of pyrolysis and subsequent bio-energy, Engineer Poet has an excellent diary posted at The Oil Drum.

Char is black, porous, finely granular, carbon material. Think of smashed charcoal briquettes. It may hold up to 50% of the carbon originally contained in the bio-mass feedstock, and the carbon in the char is stable for thousands of years (it is sequestered). It is used in various applications, but most importantly in agriculture. Because of its agricultural significance it is best known as agrichar.

Agrichar’s story begins thousands of years ago in the Amazon River basin. Large areas of dark fertile soil called Terra Preta (“dark earth”) exist there, heavily amended with agrichar 500-2500 years ago. Early agriculturalist knew how to make agrichar, and that it made their crops grow better. Probably a lot better.

Agrichar returns carbon to the soil as it is applied to fields. It increases crop yields by improving the soil’s tilth, ability to hold nutrients, store water, and maintain pH. Sometimes the crop growth increase is radical. Agrichar also increases the soils ability to hold onto agrochemicals, nitrate, and phosphate thus reducing leaching and improving water quality. It also reduces gaseous nitrous oxide emission (nitrous oxide is a very damaging greenhouse gas).

Research is being conducted around the globe to examine potential new applications of agrichar. Can agrichar benefit other weathered soils typical of the tropical regions? Can it be used to reduce methane release from rice paddies? (methane is another powerful greenhouse gas). Which waste streams should be diverted into feedstock? Will agrichar increase the safety of pesticides, and thus increase our use of no-till farming? The potential is very exciting, and I encourage anyone interested in agrichar to visit the International Biochar Initiative for current research and articles. Find something I missed. Bring it to the discussion.

I have digressed from my original idea. Bio-mass from mixed prairies is carbon negative because the plants sequester carbon with their massive prairie roots. Bio-energy produced in the making of agrichar is also carbon negative because 50% of the carbon from the feedstock remains in the agrichar. Therefore, let’s consider making agrichar from prairie! We will benefit from carbon negative energy and beautiful prairies. In the mean time, include some native plants in your garden. Replace some of your lawn with native flora. I will keep an eye on agrichar developments and report back with the latest.