Soil scientists believe one of the reasons that former prairie soils are so productive is because of the fires that raged through the grasslands over many millennia.

“It’s a legacy of perhaps 10,000 years of prairie fires. Every time a fire went through, a small fraction of that above-ground biomass was turned into char. Over time this char built up in the soils because it is very stable,” says David Laird, Iowa State University agronomy professor.

Char is a general term for charcoal-like materials made by heating biomass such as grass, corn stocks or wood. “It’s called charcoal when burned to provide heat for a barbecue, and called biochar when added to the soil,” Laird notes.

As a USDA-Agricultural Research Service researcher at Iowa State in the early 1990s, he became aware of the importance of biochar in soils. Laird and other soil scientists were determining the fate of the pesticide atrazine in soil and discovered that the biochar was absorbing more than its share.

“We knew it was playing an important environmental function for some time. What really clicked is when we realized that biochar could be a byproduct of bioenergy production,” he notes.

Iowa State scientists are leaders in developing a process called fast pyrolysis, which rapidly heats corn stover or other biomass to high temperatures without oxygen to produce bio-oil, which can be refined into liquid transportation fuels, and a powdery biochar material as a byproduct. Laird believes this creates the opportunity to return the biochar to the soil, and make biomass harvesting more sustainable.

Most crop residues are either plowed into the ground or left on the soil surface. As the residue decomposes, nutrients, such as nitrogen, potassium and phosphorous, are released into the soil where they are available for the next crop.

Crop residues, such as corn stover, are important for building humus, the rich, dark soil organic matter. “If we harvest corn stover year after year without putting anything back, both the humus and the nutrients will be depleted and the soil’s productivity will decline,” Laird says.

Laird’s research is driven by the hypothesis that amending soils with the biochar co-product of fast pyrolysis will not only return nutrients that are removed when corn stover is harvested but will also help build soil quality, because biochar is an important component for building soil organic matter.

Laird is working with Robert Brown, director of Iowa State’s bioeconomy institute, on what’s called the Initiative for a Carbon Negative Economy, a concept that would generate economic value while removing carbon dioxide from the atmosphere. Brown leads a 33-member National Panel for a Carbon Negative Economy, which includes Laird and several other Iowa State researchers.

“We want to do is capture economic value from a figurative prairie fire. When a prairie fire came through, a lot of energy went up in smoke, literally, but left behind plant nutrients and a little bit of biochar that enriched the soil. If we could just capture the energy in the fire and use that for human economic activity, and still return the biochar and the nutrients to the soil, it would be a win-win-win system,” he says.

Iowa State’s BioCentury Research Farm has a pyrolyzer and the scientists are working on integrating the production of biomass and bioenergy in ways that will maintain soil quality.  

“The research goal is to find a way to make the harvesting of biomass for bioenergy economically and environmentally sustainable,” Laird says.

Several field studies are ongoing. The largest one was started in 2007 with a goal of measuring the effects of harvesting corn stover and adding biochar to soils on crop productivity and soil quality. In its first year there was a small increase in grain yield due to biochar, but it’s not been statistically significant since. Laird notes one reason for the small effect of biochar on grain yields might be that Iowa soils are already high quality. Poor quality soils seem to benefit more from biochar than productive ones.

Researchers also are measuring several soil properties and are seeing positive results from putting biochar in high-quality Iowa soils — soils are less dense and have a higher nutrient and water retention capacity, Laird says. Biochar also functions as a liming agent raising the soil pH and reducing the amount of lime that needs to be applied.

Lab work has included studies to determine the effect of biochar on nutrient leaching, soil quality, soil density, the ability of soils to hold nutrients, soil organic carbon content, and how the biochar interacts with other carbon sources like manure or crop residue.

“It’s a young field, and it’s old. People in the Amazon have been using it as a soil amendment for thousands of years. But the science is very new. Our understanding of its potential role in both remediating degraded soils, in boosting agricultural efficiency, nutrient and water use efficiency and in maintaining soil quality while producing bioenergy — these are all relatively new topics being investigated,” Laird notes.

Source: Iowa State University