Burned organic matter captures ammonia

Burned organic matter—so-called pyrogenic organic matter—retains ammonia by forming covalent bonds with the molecule at normal temperatures and pressures, according to a new study by former Cornell University graduate student Rachel Hestrin and coworkers (Nat. Commun. 2019, DOI: 10.1038/s41467-019-08401-z). The findings could have implications for the global nitrogen cycle and provide a new source of fertilizers.

“We did not know that ammonia gas would adsorb to the extent we reported,” says Johannes Lehmann of Cornell University, who led the research. Under normal temperatures and pressures, the pyrogenic material, obtained by burning maple wood chips, retained more nitrogen by mass than is found in plants or most animal manures, Lehmann says.

When the researchers artificially weathered the pyrogenic matter by oxidizing it, the amount of trapped ammonia increased sixfold. More than half of the nitrogen remained in the material via covalent bonds. Spectroscopic analysis revealed that ~10% of the covalent bonds involved nitrogen-containing heterocyclic compounds.

“We weren’t at all prepared for the extent of the interactions,” Lehmann says. “We were actually banking on this being just a transformation of ammonia gas into ammonium salts on the surfaces.” The researchers don’t yet know the mechanism by which the heterocycles formed, Lehmann says.

The material could have practical uses for converting ammonia pollution into fertilizers. For example, ammonia-polluted effluents from farmlands or wastewater treatment plants could be passed through beds of biochar—burned plant material—before being discharged, says Michael Bird, a geochemist at James Cook University who studies biochar and was not involved in this study. “You end up discharging cleaner water and with an agricultural product with carbon sequestration potential, soil improvement potential, and now a built-in source of fertilizer.” Previous work by others has shown that plants can take up nitrogen in biochar.

Lehmann sees the material as a way to clean up agricultural wastes. “I’m thinking of dairy manure lagoons or poultry barn floors,” he says. Poultry barns “pretty much stink of ammonia.”

In addition, Lehmann says, the results suggest that pyrogenic organic matter may play a previously unappreciated role in the global nitrogen cycle by serving as a nitrogen sink. “Since nobody knew that it can happen, nobody even looked to see whether it’s happening,” Lehmann says.