Global Cycle Analysis of N2O Using Isotopomers
Sakae TOYODA, Tokyo Institute of Technology
Nitrous oxide (N2O) is one of the greenhouse gases in the troposphere (IPCC, 2007) and is the most important ozone-depleting gas in the stratosphere (Ravishankara et al., 2009). Its global average tropospheric concentration in 2010 is about 322 ppb, which is lower than that of carbon dioxide (CO2) by three orders of magnitude. However, it has about 300 times greater global warming potential than CO2 over a 100-year time scale, and is increasing at the rate of about 0.7 ppb yr-1. Sources of N2O include natural and agricultural soils, aqueous environment such as oceans, rivers, and lakes, industrial processes like fossil fuel combustion, biomass burning, animal and human wastes (IPCC, 2007). About 90% of these sources are related to microbiological processes such as nitrification and denitrification, which occur naturally and can be enhanced in soils and waters that are enriched in nitrogen species due to human activity. In spite of a number of studies based on concentration analysis, there is still great uncertainty about the estimated magnitudes of global N2O sources. This is mainly because microbial activity in soils or waters is sensitive to environmental factors and thus production of N2O is not occurring uniformly with respect to space and time. In addition, it is difficult to identify the microbial pathway that mainly contributes to the N2O production in each study site.
2. Stable isotopes as a tool to investigate global cycle of N2O
Although natural abundances of stable isotopes are almost constant (e.g., 14N and 15N respectively account for 99.64% and 0.36% of elemental nitrogen), they actually show a very small change during physical or chemical processes because of small difference in their bond energy or kinetic energy (isotope effect). Such a small variation is expressed by relative difference in isotope ratio (e.g., 15N/14N) between sample and reference material. In chemical reactions, isotope ratio (or isotopomer ratios, when molecular species are considered) are determined by isotope (isotopomer) ratios in precursors and isotope fractionation factor that is specific to each reaction. Therefore, isotope/isotopomer ratios provide qualitative information that complements the quantitative information obtained through mixing ratio analyses. We developed a mass spectrometric method to measure the isotopomer ratios of N2O with high sensitivity and high precision in order to resolve the complex N2O cycle and to provide a basis for reduction of anthropogenic N2O emission using isotopes as an alternative tool (Yoshida & Toyoda, 2000).
3. Isotopic observations of N2O in various environment
We have been conducting isotopic observations of N2O in the atmosphere, ocean, land waters, soils, and other sources. Monthly monitoring of surface air at Hateruma Island, Japan, showed that 15N/14N ratio in atmospheric N2O has been decreasing since 1999. In the western North Pacific, dissolved N2O showed a concentration maximum at about 500 m depth, and estimated 15N/14N ratio and 18O/16O ratio of N2O emitted from the ocean to the atmosphere are slightly larger than the values of atmospheric N2O. In the eastern tropical North Pacific, where biological productivity is high because of upwelling, the isotope ratios showed larger values suggesting that N2O is not only produced but also consumed (reduced) by denitrifying bacteria.
Nitrous oxide emitted from fertilized agricultural soils showed significantly low 15N/14N ratio reflecting the 15N/14N ratio of nitrogen fertilizer and microbiological isotope effect. From the characteristic isotope ratios in N2O dissolved in river water in Tokyo, we found that significant amount of N2O is emitted from wastewater treatment plant (WWTP). Detailed studies on N2O produced in WWTP together with laboratory studies on pure culture of nitrogen-metabolizing bacteria revealed that the N2O emission depends on type of biological treatments and that isotopomer ratios can be used to specify the bacterial process that mainly contributes to the N2O production.
4. Analysis of global budget
Based on above-mentioned observational data including those reported by other researchers, we analyze the global N2O budget using a mass balance of light and heavy isotopes between various sources, atmosphere, ocean, and removal processes. The isotope budget calculation with 15N- and 14N-containing N2O suggested that the isotopically light (i.e., 15N/14N ratio is low) sources such as agriculture and industry contribute to the increase of atmospheric N2O and that the contributing source might have become heavier in recent years probably because of qualitative change in anthropogenic sources.