Soil crusts emit nitrogen oxides and nitrous acid
For some time, researchers have been puzzled by the large amounts of reactive nitrogen compounds observed after rainfall in arid regions without being able to identify a specific source. A study recently published in the scientific journal "Proceedings of the National Academy of Sciences of the United States of America" (PNAS) has now shed some light on the matter. Bettina Weber and her colleagues from the Max Planck Institute for Chemistry in Mainz, the Max Planck Institute for Biogeochemistry in Jena, and the Research Center for Biodiversity and Climate in Frankfurt (BiK-F) were able to demonstrate that biological soil crusts from arid regions release nitrogen monoxide (NO) and nitrous acid (HONO) when moistened. These two reactive nitrogen compounds play a key role in the production of ozone and OH radicals, which control the oxidation and self-cleaning power of the atmosphere.
For several years now, cryptogamic layers, which consist, among other things, of soil crusts, have been causing a stir in Earth system and climate research. In 2012, a research team led by the Max Planck Institute for Chemistry demonstrated that soil crusts are responsible for approximately half of the biological nitrogen fixation on the Earth's surface. Bettina Weber and her colleagues followed up on the question of what happens to the large amounts of fixed nitrogen in the wider material cycle. They have now succeeded in finding an initial answer and uncovering a previously unknown release process for reactive nitrogen compounds.
"Our studies have shown that biological soil crusts in arid regions release NO and HONO, with the amount corresponding to approximately 20 percent of the global amount of nitrogen oxide released by soils," explains Bettina Weber, group leader in the Department of Multiphase Chemistry at the MPI for Chemistry, and emphasizes: "While the release of nitrogen monoxide has already been shown in other studies, we have now been able to demonstrate for the first time that nitrous acid is also formed and released by biological soil crusts."
Previously, it was assumed that the release of nitric oxide was due to abiotic processes. However, Bettina Weber's team has now clearly demonstrated that the organisms contained in the soil crusts are responsible for the release.
Kleine Kraftwerke in kargen Gegenden
"Biological soil crusts are like small power plants," explains Bettina Weber enthusiastically. "These layers, just a few millimeters thick, contain a concentration of organisms consisting of producers, consumers, and decomposers, making them one of the smallest ecosystems in the world. Many processes important to the Earth system occur here in a very small space." As the study shows, biological soil crusts in arid regions apparently play a key role in the release of atmospherically reactive nitrogen compounds. "Precipitation plays an important role here, as moisture is what enables metabolic processes in biological soil crusts," explains Hang Su, also a group leader in the Department of Multiphase Chemistry at the MPIC, who participated in the data analysis as a modeler.
In addition, a research team with participation from the Max Planck Institute for Chemistry recently demonstrated that cryptogamic layers also release nitrous oxide and small amounts of methane into the atmosphere. "Until now, cryptogamic layers have not been included in global climate models. Given the numerous new findings about their strong influence on biogeochemical cycling processes, they can no longer be ignored," summarizes Hang Su.
Biological soil crusts cover approximately one-ninth of the Earth's surface. Since climate change is expected to continue to alter the occurrence of soil crusts, as well as the distribution and frequency of precipitation, the extensive soil vegetation should be further investigated and the results incorporated into computer models of global material cycles.