Leading the Soil Carbon Revolution
by Jackie Swift
Johannes Lehmann, School of Integrative Plant Science at Cornell University, Soil and Crop Sciences, is leading a revolution. Over the past two decades, he has been instrumental in overturning a long-held scientific belief regarding the fundamental nature of soil, while at the same time exploring innovative ways to mitigate climate change.
“There is much more carbon in the soil than in the atmosphere and in all the plants together on the globe,” he says. “It’s a conundrum why there is so much. If you give a leaf to microorganisms to eat, they very quickly eat it all the way down until all that’s left is carbon dioxide. Yet, in the soil, we still find remains of leaves even after hundreds and thousands of years.”
Debunking the Theory of Humification
The persistence of organic matter in soil traditionally has been explained by the theory of humification, which says that as microorganisms break down pieces of organic matter, such as leaves and roots, they resynthesize the small pieces into large, extremely complex molecules called humic substances or humus. These molecules are very difficult for microorganisms to eat, the theory goes, thus they remain in the soil for centuries. “My colleagues and I have been working for close to 20 years to debunk this notion of humification,” Lehmann says.
Looking to understand soil organic matter at the most fundamental level, Lehmann joined with David A. Muller, Mechanical Engineering, and Lena F. Kourkoutis, Applied and Engineering Physics, as well as physics colleagues at the State University of New York, Stony Brook, to investigate the makeup of soil using state-of-the-art microscopy and spectroscopy. “We published our first paper in 2008 where we said we cannot find these humic substances,” Lehmann explains. “They do not exist.”
The researchers have replaced the theory of humification with the Soil Continuum Model, which says that organic matter is broken down into ever smaller parts that do not recycle into a super, humic molecule but, instead, are distributed throughout the soil matrix on a very fine scale. “The reason we have so much carbon in soil is because these ever smaller molecules have a lot more opportunities to hide,” Lehmann says. “They can adsorb onto a piece of mineral or clay or they can be sandwiched between two pieces of clay, and the microorganisms can’t get at them.”
Mitigating Climate Change with Soil Carbon
In the summer of 2020, Lehmann and his collaborators followed up with another paper in which they took their model one step further. “We’re saying that carbon is not really locked into a stable pool where it never comes out,” Lehmann explains. “It’s in constant flux, and the reason there’s so much of it is because it’s in such a complex environment. Soil is cold, dry, wet, hot; even when it’s full of organic carbon, microorganisms can’t make enough enzymes to eat it all because there are so many different molecules. We call this functional complexity.”
“If we put just a tiny proportion more carbon into the soil than is there right now, climate change would go away.”
Functional complexity is key to figuring out how to use the sequestration of organic carbon in soil to fight climate change, an overarching consideration for Lehmann. “If we put just a tiny proportion more carbon into the soil than is there right now, climate change would go away,” he says.
“But functional complexity means that we have to think about soil and environmental management under the paradigm of constant care, rather than thinking we can shove the carbon in there and lock it away for good,” he continues. “We have to make sure the soil is plowed correctly, or plowed less, and that we add carbon in the right way, and so on, so that all of these mechanisms we are starting to discover are operating together. At the same time, microorganisms have to eat some of the carbon because the nutrients need to be released or plants can’t live. It’s a tight balance between eating the carbon and storing it.”
Biochar to the Rescue
In another study linked to carbon, Lehmann has been looking at new ways to produce biochar — organic matter heated with the exclusion of oxygen into high-carbon residue through a process known as pyrolysis. Charcoal is a type of biochar that humans have known how to make for thousands of years using wood heated in kilns, but Lehmann and his colleagues are working to upgrade that ancient technology.
Lehmann first became interested in biochar during postdoctoral work in Brazil in the late 1990s. At that time, he learned of an extremely fertile area of the Amazon called Terra Preta de Indio. “The Terra Preta soils are carbon-rich,” he says. “The carbon is very old, mostly composed of biochar. There’s so much biochar that there had to be purposeful charring happening. People must have intended to make these soils.”
Talking with anthropologists and archaeologists, Lehmann and other soil scientists began to realize that the soils must have been made by a sophisticated, complex society that depended on agriculture to sustain a large population in an environment that normally wouldn’t be conducive to cropping. Even though the advanced civilization is gone, the soils they created remain. “We learned from this,” Lehmann says. “This biochar works amazingly well to improve the soil, and it lasts from one to two orders of magnitude longer in soils than anything else does. This makes it a very good candidate for climate change mitigation.”
Using the Leland pyrolysis kiln at Cornell — one of the most sophisticated slow-pyrolysis units in the world — the researchers are working out how to produce biochar from biowaste such as chicken droppings, cow manure, cherry pits, and vineyard prunings. At the same time, the process for converting the waste needs to be environmentally sustainable. Put a piece of wood in a traditional kiln, Lehmann points out, and two-thirds goes up in smoke leaving only one-third as biochar.
“It’s a challenge to engineer a new way to process biochar,” he says. “Traditional charcoal kilns release a lot of dirty smoke, but there’s a community of engineers who only want to work with that smoke. They make all kinds of things out of it: liquid transportation fuel, food flavorings, bioplastic. So we soil scientists got together with them to work out how we can make this system a win-win for all of us and use every molecule in the biomass to create something useful.”
Next Up: Soil Nitrogen Cycles
Lehmann isn’t content to rest on the scientific breakthroughs he’s helped foster so far. In another current research project, he’s investigating the microbial and abiotic nitrogen cycles in soil. Both of these are also relevant to climate change. “We’re on to a really interesting story,” he says. “I think we’ll be able to change some very long-held beliefs about how nitrogen ends up in the soil and how it moves from the soil into the plant. There will be a lot of new knowledge created in the next few years, and we want to contribute to it.”
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