Robert Oswald and Michael Ermel, PdD students with Ivonne Trebs, already speculated two years ago that bacteria in soil could also release HONO directly. Microbes that consume ammonia and are thus particularly comfortable in a neutral to slightly alkaline environment have already been known for a long time. However, microbiologists and geoscientists have always thought that they would only convert ammonia to nitrite which dissolves in the soil. The young researchers now assumed that the microbes also produce hydrogen nitrite, which they then released directly into the air.
In order to verify this assumption, the scientists investigated soil samples from 17 different ecosystems in the laboratory. Most of the samples were obtained from research groups all over the world: "We examined soil samples from a Rhine-Hessian field as well as samples from a stone desert in China and a eucalyptus forest in Australia", says Trebs. For each soil sample the researchers determined how much HONO and how much nitrogen monoxide (NO) were released.
Bacterial emissions of NO have been well known already. "These emissions are quantified rather well worldwide since NO is also an important trace gas and influences many chemical reactions, such as the formation of ozone in the atmosphere", explains Thomas Behrendt, who measured the NO emissions from the soils. "We have now discovered that neutral and alkaline soils can release as much HONO as NO, which allows deducing the direct HONO emissions from the known NO emissions of the soil bacteria."
The essential clues for direct bacterial HONO formation, however, came from a simple experiment: the researchers compared natural soil samples with sterilised samples. The HONO emissions from untreated soil samples were four times as high, compared with the sterile soils. "HONO is produced by bacteria which exist in the upper-most soil layer and oxidize ammonia", Michael Ermel explains. In order to confirm these findings, the chemist also produced a simple kind of artificial soil made from glass beads and added ammonia-oxidizing bacteria. This sample also released four times as much HONO than a sterile reference sample, to which the researchers added an aqueous nitrite solution.
According to the measurements of the various soil samples, the highest nitrous acid emissions were found in an arable soil from the vicinity of Mainz. "The emissions seem to be particularly high when the ground is alternately wet and dry from time to time", says Robert Oswald. The wet phases are needed for the bacteria to be particularly active. Under these conditions, they also produce nitrous acid that is partly retained in the wet soil and partly released into the atmosphere. Microbes in dry soils, on the other hand, release the gas directly into the air. (© Max-Planck Institute for Chemistry, AcademiaNet)