Nature. 14 January 2015. On a drizzly day in autumn, Ben Flanner tends a sea of red and green lettuce on a 6,000-square-metre rooftop farm.
The soil beneath the plants looks ordinary, but Flanner grabs a handful and holds it up for inspection. Amid the brown clods of dirt are small black particles — remnants of charcoal fragments that were mixed into the soil two years ago. Flanner thinks that this carbon-rich material, known as biochar, has helped the crops to thrive, possibly even increasing their yield, and he hopes for more impressive results over the next few years.
Across the United States, sales of this long-lasting soil additive have surged over the past few years, tripling annually since 2008, according to some estimates. The Biochar Company in Berwyn, Pennsylvania — which supplied Flanner’s Brooklyn farm — sells it both wholesale and direct to consumers, through outlets including Amazon and some Whole Foods stores. And countries ranging from China to Sweden are using biochar on agricultural fields and city lawns.
Proponents see big potential for the soil enhancer, which is produced by heating biological material — such as husks and other agricultural waste — in a low-oxygen chamber. Biochar can be made as a by-product of biofuel generation, so some companies are hoping to cash in on both products as demand grows for greener forms of energy.
Interest in biochar is also growing among scientists, who are quickly ramping up studies to test its potential. They are particularly interested in how the chemical and physical properties of biochar particles affect water moving through soil, remove pollutants, alter microbial communities and reduce emissions of greenhouse gases. The hope is that biochar can help farmers around the world, particularly those in Africa and other developing regions, who often struggle with poor soils.
Johannes Lehmann, a crop and soil scientist at Cornell University in Ithaca, New York, says that different types of biochar “have unique potential to mitigate some of the greatest soil-health constraints to crop productivity — for example, in highly weathered and sandy soils”.
But there are still many questions about biochar, particularly in terms of making sure that it is affordable and has positive effects. In some studies, the material has actually reduced yields. Part of the difficulty is that biochar can be produced from all kinds of biomass and at different temperatures and speeds, which leads to huge variation in the substance — and in results. “I always say we should not even use the singular for biochar,” says Lehmann. “There are only biochars.”
Although it is just starting to catch on with farmers today, biochar has ancient roots. Hundreds to thousands of years ago, residents of the Amazon produced it by heating up organic matter to create rich, fertile soils called terra preta. But the practice was abandoned around the time that European nations invaded South America, and relatively few farmers elsewhere have routinely used biochar. Scientists first took a big interest in the material about a decade ago, when growing concerns over global warming led some to tout biochar as a way to store huge amounts of carbon underground. Hope for that application has faded somewhat, but soil scientists are now exploring its use in agriculture and remediating pollution.
A particular focus has been explaining how biochar affects water movement through soils. Rebecca Barnes, a biogeochemist at Colorado College in Colorado Springs, and some of her colleagues tested that by adding biochar to different materials1. In sand, through which water typically drains very quickly, biochar slowed the movement of moisture by an average of 92%. In clay-rich soil, which usually retains water, biochar sped up movement by more than 300%.
The researchers suggest that the biochar alters how water moves through the interstitial space — the gaps between grains in the soil.
Workers at the Villa Carmen Biological Station in Peru turn soil containing black flecks of biochar, produced by burning bamboo in metal drums.
“Clays tend to be flat grains and sand tends to be circular grains, but biochar is very amorphous — and so it’s not only creating these crazy pathways through the biochar, but it’s also creating crazy pathways in that interstitial space,” says Barnes. She and her colleagues suggest that these convoluted pathways help to slow down drainage in sand and speed it up in clays.
That is significant, Barnes says, because even though clays can hold large amounts of water, that moisture has a hard time moving through the grains and reaching plant roots. Some studies have shown that plants grow better in soil with added biochar than in plain soils or those treated just with compost2.
Researchers are also teasing apart how biochars influence microbial activity in soil. Microbes typically act as a community; for example, many pathogenic bacteria attack a plant’s roots only when they have sufficient numbers to overwhelm the host’s immune response. Caroline Masiello, a biogeochemist at Rice University in Houston, Texas, and her co-workers have found3 that biochar can inhibit this by binding to the signalling molecules that bacterial cells secrete to coordinate their activity. read more