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…