29 October 2020 by Camilla Brodam

Nitrous oxide, or laughing gas, is a greenhouse gas that is up to 300 times more powerful than carbon dioxide (CO₂) when it comes to global warming. Globally, nitrous oxide accounts for 7% of the total greenhouse gas emissions (https://bit.ly/3l8bYFZ), and for Denmark the proportion is up to 10% (http://dce2.au.dk/pub/SR318.pdf).

"Crop production within agriculture leads to nitrous oxide emissions when microbes in the soil convert nitrogen in synthetic fertilisers and livestock manure to nitrate, and part of the nitrate is converted to nitrous oxide," explains Professor Søren O. Petersen from the Department of Agroecology at Aarhus University.

Reducing nitrous oxide emissions benefit not only to the climate

Lower emissions of nitrous oxide benefit the climate, but for agriculture, lower emissions also mean better potential for utilisation of the nitrogen applied, and this in turn can reduce the climate footprint of crops. It is important to be able to measure the emission of nitrous oxide from the soil accurately, because accurate measurements are a precondition for finding sustainable solutions for greenhouse gas reduction.

Headed by Principal Scientist Cecile de Klein of New Zealand's AgResearch, a group of researchers specialising in nitrous oxide emissions, including Søren O. Petersen, have created a collection of articles. The articles are now published in the Journal of Environmental Quality as "Global Research Alliance N₂O Chamber Metholody Guidelines". They describe a set of recommendations for "best practice" for measuring nitrous oxide emissions with so-called Non-Steady State (NSS) chambers.

The "chamber method"

It has taken two years to prepare the articles with state-of-the-art recommendations, with topics spanning from information on chamber design to sample collection, flux calculations, statistical considerations, and modelling methods.

"The use of NSS chambers is widespread all over the world, but there is no common standard for how to use the method," says Søren O. Petersen. “Globally, there is a strong focus on climate change, and so there is a widespread need to be able to determine nitrous oxide emissions from arable land. We simply need to better understand the mechanisms behind nitrous oxide emissions if we are to be able to change agricultural practices to limit them. Studies based on NSS chambers can help to understand the emissions, also in countries that depend on cheap analytical methods,” says Søren O. Petersen.

The method is neither advanced nor expensive. In principle, a box is placed on the ground, the opening facing down, and the accumulation of nitrous oxide is monitored over time.

“This is of course a very simplified description, and in practice there is great variation in how chambers are designed and used, and how the emission of nitrous oxide is calculated and included in different models. This creates problems with understanding and comparing results,” explains Søren O. Petersen.

The method is well suited for comparisons of fertiliser types or cultivation methods, because the measurements can be carried out within a small area, for example in field plots, where uniform soil conditions can be assumed. However, even at this scale the emission of nitrous oxide is affected by soil conditions and climate, and measurements of both nitrous oxide emissions and other variables are needed to document the distribution of fluxes. Factors such as time of day, soil temperature, and spatial variation in the distribution of fertiliser and plants can affect emissions. The design and use of the chamber, including the number and distribution in the field, help to determine how well the different sources of variation are described. 

“It is a labor-intensive measurement method, and with many individual chambers on an experimental area, you will have to measure over a longer time period. New recommendations in the article on flux calculation provide greater flexibility for the planning of field studies,” says Søren O. Petersen. "Asger Roer Pedersen, statistician at Aarhus University, has also contributed with new functions to a calculation method developed at Aarhus University, HMR, which has become widespread over the past 10 years."

A method for the world

NSS chambers are economically accessible and used in most of the world, an important reason why the researchers focused on this measurement method. The hope is that the articles in the Journal of Environmental Quality, which are in open access, can help to distribute the recommendations and thus promote the quality and comparability of measurement results globally.

“People around the world face the same challenges in terms of food production and climate impact. The new guidelines can be a fantastic resource for researchers who have not previously measured nitrous oxide emissions, and can help ensure that both new and experienced users adopt the method in the same and most appropriate way,” explains Søren O. Petersen. "It will help to standardise our results so that we can ultimately have an even better understanding of nitrous oxide emissions on a global scale."

FACTS
Authors Global Research Alliance N2O chamber methodology guidelines consist of eight articles with a total of more than 50 authors. In addition, there is an introductory article by Cecile A. M. de Klein, Mike J. Harvey, Tim J. Clough, Søren O. Petersen, David R. Chadwick and Rodney T. Venterea, who also served as editorial committee.
Funding The Government of New Zealand and the Global Research Alliance on Agricultural Greenhouse Gases (GRA) supported the publication process. In Denmark, which is a member of GRA, this occurred through the policy contract of the Department of Agroecology with the Ministry of Environment and Food of Denmark.
Read more The articles are published in the Journal of Environmental Quality (Wiley Periodicals LLC). All articles are open access under the terms of the Creative Commons Attribution License, which allows use, distribution and reproduction in any medium, provided that the original work is properly cited. The articles can be found here.
Contact Professor Søren O. Petersen, Department of Agroecology, Aarhus University. E-mail: sop@agro.au.dk. Phone: 28124304