Soil emissions of important greenhouse gas nitrous oxide (N2O) due to the use of nitrogen fertilisers are inaccurate due to significant diurnal variation.
From the University of East Finland
Nitrous oxide, carbon dioxide and methane are the most important greenhouse gases. Nitrous oxide also participates in the destruction of stratospheric ozone. To mitigate global warming, we have to control nitrous oxide emissions. A recent study by the University of Eastern Finland, the University of Helsinki and the Natural Resources Institute Finland provides new knowledge on nitrous oxide emissions and shows that there can be significant inaccuracies in the traditional emission measurements.
A major part of the carbon dioxide entering the atmosphere originates from the use of fossil fuels but microbial activities in our environment, especially in soils, are largely responsible for nitrous oxide emissions. Microbes are able to transform soil nitrogen to nitrous oxide. Microbial nitrous oxide production is enhanced by an increase in the availability of nitrogen in soil. Therefore, nitrogen fertilized agricultural soils are the most important sources of nitrous oxide.
Measuring nitrous oxide emissions from soils is demanding because the emissions have large spatial and temporal variation. Traditionally various chamber techniques have been used to measure these emissions. For the purpose, chambers with a diameter of about 50 cm are set on the soil surface and emissions are estimated from the gas accumulated in the chambers within a short measurement period (30 -60 min). Computer controlled chambers can also be used to measure emissions, e.g., for every hour. However, it is possible to use only a limited number of chambers at a site, such as an agricultural field. This implies that the spatial variation in nitrous oxide emissions can not be accurately determined causing inaccuracies in the emission calculations. Chambers can also cause bias in emissions because environmental conditions within chambers differ from those of natural conditions. New technologies are now available to the scientists to overcome the problems associated with chambers. The eddy covariance method uses accurate laser spectrometry for estimating nitrous oxide emissions and allows continuous measurements within an area of several hundred metres. With this method, temporal and spatial variations in emissions are averaged over the entire area. Because no chambers are needed, the measurement system does not change the environmental conditions and associated bias in the emissions is avoided.
Eddy covariance technique
Researchers from the University of Eastern Finland, the University of Helsinki and the Natural Resources Institute Finland applied the eddy covariance technique combined with the most-modern laser technology in the market to measure nitrous oxide emissions from a field where a bioenergy crop was cultivated (Maaninka, Eastern Finland). In the early summer, the nitrogen availability in the soil was high after the nitrogen fertilization. Nitrous oxide emissions were high during this time. The emissions, however, had significant diurnal variation. The emissions were higher during daytime than during night time. The researchers explained these results by the variation in soil temperature and moisture. Later in the growing season when the effect of nitrogen fertilization diminished, the diurnal variation in the emissions changed surprisingly. Then the emissions were higher during night time. Excluding the diurnal variation in nitrous oxide emissions causes inaccuracies in the annual emission estimates.
These results published in a highly ranked scientific journal, Scientific Reports (Nature Publishing Group), have international significance. The results support the development of reliable measuring methods for nitrous oxide emissions and improve our understanding of the nitrous oxide emission mechanisms and their controlling factors. Competition for soil nitrogen between plants and microbes has a crucial role for the nitrous production in the soil. When soil nitrogen availability is low, nitrous oxide emissions are higher during night- time than during daytime because plants do not consume soil nitrogen at night and more nitrogen is available for microbes and their nitrous oxide production. Stable isotope experiments with labelled nitrogen fertilizer additions confirmed the higher night time emissions observed by the eddy covariance technique.
The research shows how advances in measuring technology support the generation of new knowledge needed to obtain reliable emission estimates and to better understand the mechanisms behind greenhouse gas production in the soil. The understanding of the controlling factors behind the emissions allows the use of cultivation methods with low greenhouse gas emissions. The developing bioeconomy requires such cultivation practices for biomass production. This research was made possible by combining the knowhow and technological facilities of three leading Finnish organisations in greenhouse emission.
Nitrous oxide (N2O) is an important greenhouse gas produced in soil and aquatic ecosystems. Its warming potential is 296 times higher than that of CO2. Most N2O emission measurements made so far are limited in temporal and spatial resolution causing uncertainties in the global N2O budget. Recent advances in laser spectroscopic techniques provide an excellent tool for area-integrated, direct and continuous field measurements of N2O fluxes using the eddy covariance method. By employing this technique on an agricultural site with four laser-based analysers, we show here that N2O exchange exhibits contrasting diurnal behaviour depending upon soil nitrogen availability. When soil N was high due to fertilizer application, N2O emissions were higher during daytime than during the night. However, when soil N became limited, emissions were higher during the night than during the day. These reverse diurnal patterns supported by isotopic analyses may indicate a dominant role of plants on microbial processes associated with N2O exchange. This study highlights the potential of new technologies in improving estimates of global N2O sources.
Narasinha J. Shurpali, Üllar Rannik, Simo Jokinen, Saara Lind, Christina Biasi, Ivan Mammarella, Olli Peltola, Mari Pihlatie, Niina Hyvönen, Mari Räty, Sami Haapanala, Mark Zahniser, Perttu Virkajärvi, Timo Vesala & Pertti J. Martikainen; Neglecting diurnal variations leads to uncertainties in terrestrial nitrous oxide emissions; Scientific Reports 6, 25739: doi 10.1038/25739 (2016)
University of East Finland news release.