Reduction of Climate-affecting Nitrous Oxide Has Been Underestimated so far

Soils can effectively release reactive nitrogen in non-hazardous form back into the atmosphere. This is the result of a status report on the production of nitrous oxide, which was drawn up with the support of researchers from the KIT Institute of Meteorology and Climate Research. As outlined in the journal “Philosophical Transactions of the Royal Society B: Biological Sciences”, reduction of climate-affecting nitrous oxide to non-hazardous molecular dinitrogen is of much higher relevance than assumed so far. This finding shows the way towards future reduction of nitrous oxide emissions.

 

Nitrous oxide (N₂O) is a greenhouse gas changing the climate and damaging the ozone layer. The greenhouse effect of a certain amount of nitrous oxide in the atmosphere exceeds that of the same amount of carbon dioxide (CO₂) by a factor of 300. Since the prohibition of chlorofluoroscarbons (CFC), N₂O has been damaging the stratospheric ozone layer more strongly than any other substance. Mankind has more than doubled the global cycle of reactive nitrogen by the production and use of mineral fertilizers since the beginning of industrialization. This has not only led to a gradual enrichment of nutrients in ecosystems which reduces biodiversity, but also to an enhanced nitrogen conversion rate in soils and water bodies.

 

Increased decomposition of nitrate by special microorganisms, so-called microbial denitrification, results in an increased formation of nitrous oxide in the soil. It is emitted into the atmosphere. Compared to pre-industrial times, the concentration of N₂O in the atmosphere has increased by about 20%. However, denitrification does not only produce N₂O, but can also reduce it to molecular dinitrogen (N₂). “Hence, the volume of nitrous oxide released from soils into the atmosphere corresponds to the balance of N₂O production and reduction to N2. Molecular dinitrogen is completely harmless in ecological respects,” says Dr. Michael Dannenmann from the Atmospheric Environmental Research Division of the KIT Institute of Meteorology and Climate Research (IMK-IFU).

 

Recently, a team of scientists around the KIT climate researchers Professor Klaus Butterbach-Bahl, Dr. Ralf Kiese, and Dr. Michael Dannenmann from IMK-IFU was invited by the British Royal Society to publish a status report on N₂O production and the processes responsible for it. In the journal “Philosophical Transactions of the Royal Society B: Biological Sciences”, the scientists list eight different microbiological and chemical processes of N₂O formation. At the same time, eight processes are known to reduce N₂O to N₂.

 

So far, microbial reduction of N₂O to N₂ has hardly been understood. In contrast to N₂O, measurements of the new formation of N₂ are technically challenging, as the atmosphere consists of 78% N₂. The most frequently used method to measure N₂-N₂O emission ratios has been the so-called acetylene inhibition method that bypasses the problem of N₂ measurement by inhibiting N₂O reduction: N₂ production is measured indirectly as increased N₂O production due to the addition of acetylene. This method revealed that per kilogram of emitted N2O, about the same amount of N₂O was reduced to N₂ by denitrification. Hence, it did not enter the atmosphere as a greenhouse gas and ozone destroyer, but as a non-hazardous gas.

 

In fact, N₂O consumption by conversion into N₂ is much higher, as is outlined by the IMK-IFU researchers in their publication. “The acetylene inhibition method leads to a systematic underestimation of the reduction of N₂O to N₂,” Dr. Michael Dannenmann explains. This is demonstrated by the use of modern methods for the direct measurement of N₂ production, such as the helium incubation method developed by KIT. Studies based on such complex direct and, hence, more reliable N2 measurements reveal that the amount reduced to N₂ exceeds the amount of emitted N₂O by a factor of four on the average. Consequently, N₂O reduction to N₂ is of much higher relevance to the deactivation of reactive nitrogen than assumed so far. It may therefore be concluded that soils release non-hazardous reactive nitrogen much more effectively into the atmosphere.

 

The results of the researchers do not mean that less N₂O is emitted. Increased relevance of N₂O reduction to N₂, however, shows a way towards future reduction of N₂O emissions: “If we succeed in better understanding the control of N₂O reduction, we will be able to increasingly convert the climate- and ozone-damaging nitrous oxide into a harmless gas by an adapted management of agricultural ecosystems for instance,” Michael Dannenmann says. According to the KIT scientists, it is still required to better understand how microbial diversity in the soil influences the reduction of N₂O to N₂. First promising findings have been obtained with the help of new molecular biology methods.

 

Klaus Butterbach-Bahl, Elizabeth Baggs, Michael Dannenmann, Ralf Kiese, Sophie Zechmeister-Boltenstern: Nitrous oxide emissions from soils – how well do we understand the processes and their controls? Philosophical Transactions of the Royal Society B: Biological Sciences; 368; July 5, 2013. DOI 10.1098/rstb.2013.0122.

 

The KIT Climate and Environment Center develops strategies and technologies to secure the natural bases of life. For this purpose, 660 employees of 32 institutes produce fundamental and application-oriented knowledge relating to climate and environmental change. It is not only aimed at eliminating the causes of environmental problems, but increasingly at adapting to changed conditions.

 

Being “The Research University in the Helmholtz Association”, KIT creates and imparts knowledge for the society and the environment. It is the objective to make significant contributions to the global challenges in the fields of energy, mobility, and information. For this, about 10,000 employees cooperate in a broad range of disciplines in natural sciences, engineering sciences, economics, and the humanities and social sciences. KIT prepares its 22,800 students for responsible tasks in society, industry, and science by offering research-based study programs. Innovation efforts at KIT build a bridge between important scientific findings and their application for the benefit of society, economic prosperity, and the preservation of our natural basis of life. KIT is one of the German universities of excellence.

or, 13.06.2013