A new Cornell-led study shows that pyrogenic matter, also known as biochar, is chock-full of potential as a fertilizer because of its ability to soak up nitrogen from the air pollutant ammonia. And it does so in a surprising way: through a chemical reaction that forms covalent bonds far stronger than those found in electrostatic interactions.

“Balancing nitrogen management so that we provide enough nitrogen to our crops … without contributing to air and water pollution is a major challenge,” said Rachel Hestrin, Ph.D. ’18, first author on the paper, “Fire-Derived Organic Matter Retains Ammonia Through Covalent Bond Formation,” which published Feb. 8 in Nature Communications.

“As nitrogen-rich materials like animal manure break down, a lot of nitrogen can volatilize into the atmosphere as ammonia gas,” she said. “That represents a huge loss of a valuable nutrient from the agricultural system. It could also affect biodiversity if that ammonia is deposited in natural ecosystems and alters nutrient availability there.”

The project has its origins in Ethiopia, where researchers led by Johannes Lehmann, the Liberty Hyde Bailey Professor of Soil Science, have been working to improve smallholder agriculture by recycling farm and municipal resources to make fertilizers that are high in phosphorus or nitrogen.

“Fertilizers are a lot less available in East Africa, and there are more competing demands for organic resources like manure, straw and wood compared to in the U.S.,” said Hestrin, a doctoral student in Lehmann’s lab at the time and now a postdoctoral researcher at Lawrence Livermore National Laboratory.

“We saw an opportunity to convert underutilized sources of biomass or agricultural waste products into fertilizers,” she said, “but we knew that it would be important to use these resources as efficiently as possible.”

With support from the Atkinson Center for a Sustainable Future’s partnership with CARE USA, the team proposed to capture nitrogen that would otherwise be lost from compost by adding biochar – a solid, charcoal-like material formed by heating biomass in the absence of oxygen.

“Biochar production is a fundamental process in nature,” said Lehmann, senior author on the paper. “You have to consider it to understand natural nitrogen cycles and carbon cycles, and you can learn something important about how the natural world functions. You can also adapt it and modify it to manage some pressing sustainability issues.”

In order to better understand how biochar could retain nitrogen, Hestrin began to change the chemical composition of its surface, simulating how the biochar would change over time in the natural environment. Then she exposed the biochar to ammonia and measured how much nitrogen it retained after exposure.

She and Lehmann found that the biochar soaks the ammonia right up.

To gain additional insight into how nitrogen from the ammonia gas interacts with biochar under natural conditions, Hestrin took these samples to the Canadian Light Source at the University of Saskatchewan.

“Through analyses enabled by the particle accelerator, we found there were actually covalent bonds forming between the nitrogen from the ammonia and the carbon in the biochar,” Hestrin said. “Those covalent bonds are a lot stronger than the electrostatic interactions that we had assumed were responsible for nitrogen retention. The way that nitrogen is retained matters because it will affect whether the nitrogen will be readily available to plants or lost into the air or water.”

Lehmann considers biochar compost versatile enough to benefit a wide range of stakeholders. For farmers, it can help conserve nitrogen and improve productivity, as well as limit odor pollution from manure piles and lower the environmental impact farms can have on local water quality. It can help businesses recycle nutrients and make valuable fertilizer rather than leaving behind waste.

And for earth systems scientists and climate modelers, it is a potentially overlooked process by which nitrogen emitted through mineralization and fires is recaptured in terrestrial ecosystems, a natural sink.

“One way that agricultural nitrogen management is connected to climate change is through the conversion of fertilizer into nitrous oxide, a potent greenhouse gas,” Hestrin said. “Anything we can do to improve nitrogen delivery to plants and reduce the inefficiencies in nitrogen fertilizer application – whether it’s adding too much or adding it at a time that’s not optimized for plant demand – has the potential to reduce nitrous oxide emissions into the atmosphere.”

Other contributors were Dorisel Torres-Rojas, Ph.D. ’18, and researchers from Canadian Light Source, NMR Facility and Spectroscopy Lab, and the University of Adelaide, Australia.

David Nutt is managing editor of the Atkinson Center.