Biochar is charcoal, typically produced from waste wood, manure or leaves, and used as a soil additive. Studies have found biochar can improve both the nutrient and water-holding properties of soil, which has led to its increasing popularity in recent years.

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Biochar proponents claim wider use of the additive has the potential to help mitigate climate change, through carbon sequestration which can last many hundreds of years. It can also increase soil fertility, agricultural productivity and, as a team of scientists in Texas have illustrated, offer protection against certain foliar and soil-borne diseases.

In what they claim is the first study of its kind, scientists from Rice University in Houston used synthetic biology to study how popular soil amendment biochar can interfere with the chemical signals that some microbes use to communicate. The class of compounds studied includes those used by some plant pathogens to coordinate their attacks.

They said theirs is the first study to examine how biochar affects the chemical signaling routinely used by soil microorganisms that interact with plants.

Study co-author Joff Silberg said, "A potted plant may look tranquil, but there are actually a lot of conversations going on in that pot. In fact, there are so many different conversations going on in soil that it was impractical for us to determine exactly how biochar was affecting just one of them."

Professor Silberg and colleagues used the tools of synthetic biology (the construction and study of artificial biological systems) — and a refined experimental setup that Silberg initially drafted with his son's spare Lego bricks — to establish a situation where just one microbial conversation was taking place and where biochar's effects on that conversation could be measured.

Rice University, where the study was conducted, is home to an interdisciplinary Biochar Research Group, formed in the wake of Hurricane Ike in 2008 when the city of Houston needed to dispose of an estimated 5.6 million cubic yards of fallen trees, broken branches and dead greenery left behind by the storm. Houston dubbed the call-out 'Recycle Ike' and awarded its grand prize to Rice's biochar programme.

Professor Silberg's cell-signaling study grew out of a previous investigation by one of the group's founding members, Carrie Masiello, an associate professor of Earth science. Masiello investigated the combined effects of adding biochar and nutrients to soils. In all but one case, the biochar and nutrients seemed to enhance one another. In the lone exception, a soil fungus that was typically beneficial to plants began growing so rapidly that it impeded plant growth.

Commenting on the group's work to date, Silberg said, "All of these organisms, to a much greater extent than we probably understand, are talking to each other all the time. Microbes talk to microbes. Microbes talk to plants. Plants talk to microbes. And they each make decisions about their behaviour based on those conversations. When we started talking about these results, my first thought was, 'You're probably interfering with a conversation.'"

Biochar experiment

Silberg began working with a biochemist colleague to make use of two tailored forms of E. coli bacteria. One strain of the bacteria 'spoke' with a type of chemical communication commonly used by soil microbes, and the other 'listened.' Unlike the fungi that use this communication method in soil, the E. coli could be grown in clear agar gels in a petri dish, allowing researchers to study their behaviour.

The team next inserted florescence genes into each organism, which caused them to glow different colours — red for speaking and green for listening. "We needed a way to conduct two experiments in the same dish, one where biochar had a chance to interfere with a conversation and another where it didn't," Silberg said.

Working with his son's Lego bricks, Silberg constructed a pair of rectangular platforms that sat parallel in the dish, about one inch apart. Agar was added to fill all parts of the dish except for the areas blocked by the bricks. Once the agar gel had set, the rectangular platforms were removed to create two empty parallel troughs. One of these was filled with clear agar, and the other was filled with agar containing biochar. 'Speaker' organisms were added to the middle of the dish, and “listeners” were placed on the opposite side of each trough.

Over the course of dozens of subsequent tests, the Lego design was refined and researchers were able to see how different formulations and amounts of biochar affected cell signalling.

"In every case, we observed significantly less green light from the opposite side of the biochar, which meant the E. coli on that side had trouble hearing the sender," Silberg said. "That upheld our hypothesis, which was that biochar could interfere with cell signalling, most likely by binding with the fatty-acid molecules that the speakers were using to broadcast their message."

The team found that biochar that was created with higher temperatures was as much as 10 times more effective at shutting down conversations. Earlier studies had found that biochars created at higher temperatures were no more effective at holding water and nutrients.

"Biochar can be made in processes that range from 250 to 1,000 degrees Celsius, and there's mounting evidence that the temperature can dramatically affect the final properties," said Professor Masiello, Silberg's colleague and founder of the Biochar group at Rice. "Ultimately, we'd like to create a simple guide that people can use to tailor the properties of their biochar."

Discussing the potential for future research, Silberg added, "Some microbes help plants and others are harmful. That means there's good communication and bad communication going on in the soil at the same time. We think it's likely that some biochars will knock out some conversations and not others, so we want to test that idea and, if possible, come up with a way to tailor biochar for the microbial diversity that’s desired."