Science - September 9, 2010

Wageningen researcher solves laughing gas riddle

The production of laughing gas in the soil is a complex process. PhD researcher Dorien Kool has found some new pieces of the puzzle and put them in place.

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Laughing gas (N 2 0) is a key greenhouse gas which is largely produced in soils. Bacteria and fungi in soil convert ammonium (NH 4 + ) and nitrate (NO 3 - ) into laughing gas. Until now it was assumed that there were two main ways of doing this: nitrification and denitrification. It turns out there is more to it than that.
Dorien Kool has shown there is a third route along which organisms responsible for nitrification also use an alternative ‘denitrification-like’ way of forming laughing gas. This possibility has been known for some time from laboratory tests on micro-organisms. What Kool has demonstrated for the first time is that this actually happens in many soils and on a significant scale.

New method
For demonstrating the origin of laughing gas in soils, labelled Nitrogen is commonly used: the N-15 isotope. But by this method you cannot distinguish this third route from the two traditional processes. Kool therefore developed a new method making use of isotopes of nitrogen (N-15) and of oxygen (O-18). A series of elegant experiences enabled Kool to map out the various production routes for laughing gas. By doing so she has largely solved the puzzle of laughing gas production in soils.

Exchange
What is more, Kool discovered that during the formation of laughing gas, intermediate products continually exchange oxygen atoms with water in the environment. This discovery is of great practical significance for the use of the O-18 isotope. The amount of O-18 in the end product tells you something about the route by which the laughing gas was formed. But this method does not work so well if the isotope continually changes places with unlabelled oxygen. Kool found a way of quantifying that exchange.
‘You can now include the oxygen exchange in your calculations, at any rate’, Kool says, in explaining the significance of her discovery. ‘That does not change anything for the total production of laughing gas, but we now understand better where it comes from. And that in turn gives us a better grasp of which measures do or do not affect the production of laughing gas.’

Prizes
Kool has already won three prizes for her work. In 2009, she won the publication award of the PE&RC research school, as well as the prize for the best presentation in the field of biogeochemistry during the international meeting of the AGU. In October she will receive the Isotopes Award 2010 from the German Habfast foundation. Kool will defend her thesis, On the origin of nitrous oxide and its oxygen on 17 September.

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