Less climate-damaging laughing gas with Comammox bacteria
Jülich, 26 April 2019 - The Earth's nitrogen cycle is massively influenced by man, in particular by fertilization in industrial agriculture. The ecological consequences include the release of nitrous oxide (N2O, laughing gas), which is a greenhouse gas almost 300 times stronger than carbon dioxide and which contributes to the destruction of the ozone layer. Laughing gas is produced by microorganisms that carry out key processes in the nitrogen cycle and are stimulated by the addition of fertilizers. An international team of researchers, including Prof. Nicolas Brüggemann and Holger Wissel from the Institute for Agrosphere Research at Forschungszentrum Jülich, has now discovered that the recently discovered Comammox bacteria release much less laughing gas than other microbes – and could therefore be of great interest for more environmentally friendly agriculture. The study has now been published in the journal "Nature Communications".
Modern agriculture would not function without nitrogenous fertilizers. However, intensive fertilization leads to many problems: a large part of the nitrogen is not absorbed by the plants at all, but is washed out of the soil and ends up in groundwater, rivers, lakes and oceans. There, the excess nitrogen causes a reduction in drinking water quality and eutrophication (the "overturning") of water bodies, with dramatic consequences such as the death of many aquatic organisms. In addition, N2O is released as a by-product during the conversion of nitrogen from fertilizers by microorganisms. The N2O escapes into the atmosphere and contributes significantly to ozone depletion and global warming.
The most common nitrogen compounds in artificial fertilizers and liquid manure are ammonium and urea, which in turn is converted into ammonium in the soil. In the nitrogen cycle, ammonium is first converted by microorganisms into toxic nitrite and then into the somewhat less harmful nitrate. In this two-stage process, the so-called nitrification, some N2O is always produced. The amount of nitrous oxide increases dramatically when the microbes literally run out of air, i.e. when there is a lack of oxygen, many of these "nitrifiers" release large amounts of the dangerous greenhouse gas. This occurs not only in fertilized soil, especially after precipitation, but also in wastewater treatment plants, where nitrification plays a central role in biological wastewater treatment. Even these facilities, so important for our clean water, contribute to global nitrous oxide emissions.
The international research team from Vienna, Edmonton and Jülich has now investigated the nitrous oxide production of a Comammox bacterium for the first time. Comammox bacteria ("complete ammonia oxidizers") convert ammonium into nitrate alone, whereas other microbes are dependent on the division of labor. The scientists found out that the Comammox bacteria not only release much less laughing gas than most other nitrifiers that have been investigated so far. They do not produce the small amounts themselves, as they lack the necessary enzymes. The nitrous oxide is only produced by chemical reactions with hydroxylamine, a substance that Comammox bacteria release into their environment. Nicolas Brüggemann and his team first described this mechanism a few years ago and showed that it is an essential, if not the decisive, process of N2O formation during nitrification.
The low nitrous oxide release by Comammox could be of interest for agriculture and wastewater treatment. It may be possible to reduce N2O emissions in soils and wastewater treatment plants if the growth of Comammox bacteria can be promoted instead of other nitrifying bacteria. However, this requires better research into the recently discovered type of bacteria, such as the conditions they need for optimal growth.
The researchers involved agree that Comammox alone cannot solve the nitrogen problem. Their targeted use could, however, make valuable contributions. Another possibility for N2O reduction could be the manipulation of soil properties, for example an increase in the pH value or the humus content of the soil. This would minimize the chemical conversion of hydroxylamine excreted by soil bacteria into nitrous oxide.
“Low yield and abiotic origin of N2O formed by the complete nitrifier Nitrospira inopinata”: K. Dimitri Kits, Man-Young Jung, Julia Vierheilig, Petra Pjevac, Christopher J. Sedlacek, Shurong Liu, Craig Herbold, Lisa Y. Stein, Andreas Richter, Holger Wissel, Nicolas Brüggemann, Michael Wagner, Holger Daims; Nature Communications 10, 1836, DOI: 10.1038/s41467-019-09790-x
Prof. Dr. Nicolas Brüggemann
Institute of Bio- and Geosciences, Agrosphere (IBG-3)