Study Shows Increased Heat Boosts Biochar as Soil Additive

Caroline Masiello (Rice University)

A research team at Rice University in Houston, Texas has found that heating biochar — charcoal added to topsoil to enhance plant growth — to 450 degrees Celsius increases its ability to deliver water and nutrients to the targeted crops. Their findings appear online this week in the journal Biomass and Bioenergy (paid subscription required).

While practice of adding charcoal to soil goes back 2,000 years, interest in biochar has gained in recent years because of its ability to increase soil productivity without dangerous chemicals. Studies over the past decade have found that the charcoal-amended soil holds more water and nutrients and also makes the water and nutrients readily available to plants. Biochar can also hold carbon in soils for hundreds of years, making it a tool in the battle against greenhouse gasses and climate change.

Producing biochar involves heating biomass in the absence of oxygen, a process called pyrolysis, where biomass feedstocks are often agricultural wastes, such as wood chips, corn stover, or paper mill sludge. Making biochar on an industrial scale can often result in producing significant amounts of biofuels, making the process more commercially feasible.

The research team — led by Rice earth scientist Caroline Masiello (pictured at top), with student and faculty colleagues from Rice, Baylor University, Bard College, and a Bellaire, Texas high school science teacher — discovered that not all biochar is created equal. Their results show charcoal produced at temperatures of 450 degrees Celsius (842 Fahrenheit) or higher was most likely to improve soil drainage and make more water available to plants.

“People often tout the benefits of biochar; it can help clay-rich soils drain better, and it can help sandy soils hold water better,” says Masiello. “But we are finding that these hydrologic benefits vary widely with biochar production conditions.”

Those varying hydrologic benefits are shown in the study’s findings that biochar produced at temperatures lower than 450 Celsius could do more harm than good for the crops. Masiello’s team found that the lower-temperature biochar retained some organic compounds that can repel rather than attract water. The results also show that the lower-temperature biochar did not hold carbon as reliably as the higher-temperature varieties, and could return significant amounts of carbon to the atmosphere within a few hundred years.

Read more: Students Start Company to Commercialize Biochar Process

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