Profile of Johannes Lehmann

Abstract

Soil scientist Johannes Lehmann has spent his career examining how to improve soil quality to secure agricultural production, regulate climate, and keep water clean. His work with dark earths in the Brazilian Amazon led to the discovery of the importance of biochar in soil fertility and nutrient recycling from excreta. For more than 24 years, Lehmann has held a faculty position at Cornell University, where his research initially focused on carbon and nutrient cycles and went on to challenge the existence of soil humus and delve into the role of functional complexity in soil management.

“It was breathtaking,” says Johannes Lehmann, looking at a photo from his time in Brazil’s Amazon Basin. Lehmann is crouched in a pit, the surface well above his head as he takes samples of the rich soil around him, careful not to disturb the large shards of Pre-Columbian Indigenous ceramics jutting out from all sides. It was the tail-end of a three-year stint in the region that was wrapping up in 2000 and Lehmann—now a member of the National Academy of Sciences and a professor of soil biogeochemistry at Cornell University—had gone to the well-known biome to study multistrata agroforestry systems (1), how agroforestry changes soil properties, and how management can change the use of nutrients and water in soil between trees and crops.

Image credit: Allison Usavage (photographer).

However, it was his encounter with the Terras Pretas de Índio, as the fertile dark earth that surrounded him is known in Portuguese, that left him intrigued. “I thought there ought to be something to be learned from these soils that we can emulate or recreate,” he recalls.

Researchers had previously proposed that the soils, coveted by local farmers for their high nutrient and carbon content, were created by volcanic fallout or natural lake sediments. However, Lehmann’s research, along with that of other scientists, helped prove that the soils were made by Indigenous populations in the Amazon well before the arrival of Europeans. His work also revealed that the soils, which existed for thousands of years, held potential solutions to both environmental and agricultural issues. “It turned out that one of the most important things [the soils gave us] was biochar,” he says.

Soil Solution

Lehmann’s work is aimed at exploring and improving soil quality, which in turn helps secure food production, regulate climate, and keep water clean. His interest in the topic began during his time at the University of Bayreuth in Germany, where he first focused on geoecology, a program exploring the connections among natural sciences, sustainability, and the environment. The program also had a strong link to the study of sustainable agriculture and resource-poor farmers in the tropics, particularly in Sub-Saharan Africa.

During a 1991 internship with the federal government’s German Corporation for Technical Cooperation, where he mapped soils for a forestry and afforestation program in the Jebel Marra region of Sudan, Lehmann discovered that soil held answers to the issues he cared about. “The study of soil sat squarely in the middle of all these sustainability issues,” he said. “Whether we were talking about air, water, food, biodiversity, or forests, there was always a soil angle.”

A year later, he headed west to Togo, where he shifted his focus to agroforestry. There, he studied how leaves pruned from trees released nutrients that could help feed maize crops. For his Master’s degree in soil science, an area he would continue to work on during his PhD studies from 1994 to 1996 in Kenya, he examined the benefits of collecting surface runoff after a rainstorm for crop irrigation (2). The approach not only proved more practical for farmers but also more cost- and resource-efficient, replacing water collection in buckets, cisterns, and basins with a system that guided the water directly to fields and allowed it to infiltrate into soil. “We found that you need only one rainfall event to fill up the water holding capacity of the soil,” he says. “And then plants can reach moisture in the subsoil and grow with just that one event.”

Dark Earth Discoveries

With a PhD degree in hand, Lehmann headed to Brazil in 1997. For three and a half years, he worked at the Western Amazon office of Embrapa, the state-owned Brazilian Agricultural Research Corporation, located just outside the city of Manaus. Long before his arrival, the team there had installed agroforestry systems with a slew of timber and fruit trees native to the area—pupunha, cupuaçu, açaí, urucum, and Brazil nut among them. Already in full growth, the trees allowed scientists to explore a variety of research topics. Lehmann’s focus was on nutrient and carbon flows. He hoped to better understand how the different species of planted trees could improve degraded soils in the region, compared with the adjacent primary forest, as well as the nearby secondary forest, which was made up of spontaneous vegetation. While conducting those studies, he learned the importance of local dark earths—long shoveled onto trucks and sold by locals as high-value gardening soils in the city—and discovered how land use and management practices influence soil properties. “These soils were in an environment where they shouldn’t have had high carbon content,” he says. “These highly weathered soils shouldn’t have had high nutrient content, where crops grow really well. We started to study what makes these soils tick,” Lehmann adds. “And in the back of our minds, we were wondering, can we replicate that?”

The soils, which were modified by Amazonian Indigenous populations between 600 and 8,700 years ago, likely for agricultural use, not only had high carbon, phosphorus, and calcium contents but also carried the pottery sherds and charred biomass, organic material that would initially be referred to as charcoal or black carbon and, later, as pyrogenic organic matter and biochar. Lehmann knew he couldn’t make an exact copy of Terras Pretas de Índio—“That’s a cultural phenomenon developed over thousands of years,” he says—but he hoped to mimic some of their attributes. “We learn something from bygone civilizations that now inspire us to treat our soils more responsibly,” he says.

The discovery opened the door to a slew of questions. “Can we emulate some essential properties of these soils by adding charcoal to other soils?” he says. To answer the question, he examined the nature of vegetation growth, crop growth, and nutrient leaching in the biochar-rich replica soil (3) and tested whether the addition of charcoal led to an increase in the soil’s carbon stocks.

Unconventional Idea

While working in Brazil, Lehmann was offered a faculty position at Cornell University, where he moved in 2021. There, he focused on carbon and nutrient cycles, not only in the dark earth he had already studied but also in other soils naturally heavy in charcoal and in biochar-supplemented soils. He soon discovered that soils in the American Midwest were high in biochar-type materials, potentially explaining why the region, known as the “Breadbasket of America,” has such fertile soils and abundant agricultural production. Lehmann went on to study natural charcoal deposits in the United States, eventually extending his work geographically. Previously, Lehmann had worked in western Kenya, adding biochar to farms and examining how biochar changed productivity and nutrient and carbon cycles (4).

At Cornell, Lehmann received numerous awards and accolades, including the Alexander von Humboldt Research Prize, the Soil Science Society of America’s International Research Award, and the Marion L. and Chrystie M. Jackson Soil Science Award, among others. He has also published a succession of papers that have advanced the field of soil organic matter, questioning the concept of soil humus (5) and delving into functional complexity (6) and soil carbon cycles (7).

Before long, he explored an unconventional idea that he was convinced could vastly improve the lives of farmers, given the right support. “I started thinking about biochar as a vehicle for nutrient recycling, specifically from waste,” he said. “Most of the nutrients are in human feces and urine, which are typically not used.” Concerns related to human health, regulatory hurdles, and the psychological taboo of working with human waste had hampered research on the topic. Lehmann recognized that biochar could help address the unequal distribution of mined resources for fertilizers, such as phosphates, and the supply-chain risks raised by conflicts in countries exporting the product, such as Ukraine and Russia. “If the countries in Sub-Saharan Africa would recycle all their human excreta, they could easily—by a long shot—be nutrient self-sufficient and wean themselves off of the global fertilizer market,” he says (8).

In his Inaugural Article (9), Lehmann explores the role of biochar in the circular bionutrient economy and how it could help transform nutrients from residues, specifically excreta. He suggests that “leveraging biochar to close the nutrient circle requires public–private partnerships in forms of a community of practice and green alliances.” Such partnerships, he says, are required to incentivize private investment in a marketable product.

When Lehmann is not engaged in research, he is helping students develop their own research agendas at his lab at Cornell. His students address questions such as, “What happens to soil in space?” and “Under what conditions can biochar systems mitigate environmental issues in the dairy industry and be economically viable?” Lehmann says teaching is energizing. He often takes inspiration from artists, whom he invites to workshops to help his students think differently and question the scientific process. He also likes to work alongside artists and has recently started collaborating on a graphic novel about the origin of soil with political cartoonist Pedro Molina from Nicaragua. “I would like to think like an artist in doing science. A graphic novelist looks at the world in a different way, and the narratives are strung together very differently than the way we write scientific papers,” Lehmann says. Artistic endeavors, he adds, can generate new ideas to move science forward.