Keith Smith

Nitrous oxide, N2O, is present in earth's atmosphere at a trace level - its current 'mixing ratio' (i.e. the concentration in dry air) is of the order of 320 parts per billion (ppb). This mixing ratio has been increasing linearly over the last few decades (as can be seen in Plate 4.3) as a consequence of the introduction of N2O into the atmosphere at a rate greater than its rate of removal by natural processes.

N2O is environmentally important in two quite distinct respects. First, its capacity to absorb infrared radiation is about 300 times greater than that of carbon dioxide, CO2, and therefore, although its mixing ratio is a thousand times less than that of CO2, it contributes significantly to the greenhouse effect and thus to climate change. Second, when N2O reaches the stratosphere it contributes, along with some halogen-containing gases, to the loss of ozone that acts as a barrier to the penetration of ultraviolet radiation to Earth's surface, with consequences for human health.

A generation ago, the impact of N2O emissions on the ozone layer was the main environmental concern associated with this gas, but since then the increasing recognition that global warming is a major threat to life on Earth as we know it has led to a wide-ranging investigation of the factors that contribute to the warming, in particular the anthropogenic emissions of the long-lived greenhouse gases CO2, methane and N2O to the atmosphere. The desirability of explaining how and why N2O has become important in this context, the past, present and likely future trends in emissions, and how these emissions to the atmosphere might be mitigated, have been the motivation for producing this book.

N2O is a natural product of mainly microbial origin, as a result of the bio-geochemical processes occurring within the nitrogen (N) cycle. Emissions and natural destruction (mainly in the stratosphere) were broadly in balance until the advent of the industrial age, resulting in a fairly constant concentration in the atmosphere. However, emissions have been increasing, as a consequence of adding reactive forms of nitrogen into the biosphere beyond those natural additions from, principally, biological nitrogen fixation (by leguminous plants, plants with other forms of symbiotic association with microorganisms, and free-living N-fixing bacteria), and electrical discharges - lightning flashes - in the atmosphere. This introduction comes about chiefly through adding synthetic nitrogenous fertilizers and animal manures to agricultural land; creating new agricultural land from natural forests and grasslands, and thus liberating nitrogen from relatively inert forms in the soil; and releasing reactive nitrogen compounds into the atmosphere, which are subsequently deposited onto land and water. These compounds are predominantly NOx from industrial sources, power stations and vehicles, and ammonia from animal manures.

Chapter 2 by Elizabeth Baggs and Laurent Phillipot, and Chapter 3 by Hermann Bange and his co-authors, Alina Freing, Annette Kock and Carolin Löscher, describe and discuss the biochemical pathways within the nitrogen cycle that lead to the emission of N2O from terrestrial soils and marine environments, respectively, and thus provide the process understanding that underpins the remaining chapters.

The largest N2O source is now agriculture, driven mainly by the use, globally, of >80 million tonnes of N annually as synthetic nitrogen fertilizers, as well as biological nitrogen fixation by leguminous crops. Natural ecosystems also receive N compounds formed from the release into the atmosphere of NOx from fossil fuel combustion and biomass burning, and ammonia from livestock manure. Together, these inputs of reactive nitrogen compounds to the biosphere have virtually doubled the mainly natural inputs existing at the beginning of the industrial age, and this increase has been matched by a corresponding increase in N2O emissions. The relationship at the global scale between the magnitudes of reactive N inputs and the consequent N2O outputs, including the implications of agricultural expansion to provide crop-based biofuels, is reassessed in Chapter 4 by the present author in conjunction with Paul Crutzen, Arvin Mosier and Wilfried Winiwarter. It has been a pleasure to work with them on this chapter, much of the inspiration for which came from our earlier collaboration led by Paul, on the implications for global warming of the production of first-generation biofuels from agricultural crops (Crutzen et al, 2008).

The dominance of agriculture and land use as a source of N2O provides the justification for including three chapters focusing on this sector. In Chapter 5, Lex Bouwman, Elke Stehfest and Chris van Kassel cover the topic of emissions from arable land, ways of measuring emission factors, modelling and mitigation possibilities, while in Chapter 6, Cecile de Klein, Richard Eckard and Tony van der Weerden deal with analogous issues relating to N2O emissions from livestock-based agriculture. Chapter 7, by Franz Conen and Albrecht Neftel, reviews the complex subject of how changes in land use and management affect the scale of N2O emissions in different parts of the world.

A substantial proportion of the nitrogen applied to agricultural land in the form of synthetic fertilizers, animal manures and crop residues, and some of the N released from old soil organic matter by cultivation, is leached from land in drainage water into groundwater and into streams, rivers, estuaries and finally seas and oceans. Part of Chapter 8, by Reinhard Well and Klaus Butterbach-Bahl, deals with the problems of estimating the so-called indirect N2O emissions resulting from denitrification of this leached N. The remainder deals with emissions from natural and semi-natural land resulting from aerial deposition of reactive N compounds in the atmosphere; this deposition follows emission of NOx from industry and combustion sources, and ammonia from livestock farming, leading to short- medium- and long-range transport of these gases and their atmospheric reaction products, before deposition on the surface.

Combustion processes are responsible for direct emissions of N2O, not merely for emissions of gases such as NOx that can provide substrates for microbial N2O production. Two processes in the chemical industry are direct sources of N2O release to the atmosphere. The first of these is the production of nitric acid, and the second is the production of adipic acid, used chiefly in nylon manufacture. These non-biological sources, and the success of abatement measures employed to minimize them, are described by Peter Wiesen in Chapter 9.

Action is being taken to curb the industrial point-source emissions of N2O, but measures to limit or reduce agricultural emissions are inherently more difficult to devise. Thus as we enter an era in which measures are being explored to reduce fossil fuel use and/or capture or sequester the CO2 emissions from the fuel, it is likely that the relative importance of N2O in the 'Kyoto basket' of greenhouse gases will increase, because comparable mitigation measures for N2O are inherently more difficult, and because current and expected future expansion of the land area devoted to crops is likely to lead to an increase in N fertilizer use, and thus N2O emission, worldwide. These issues are examined briefly in Chapter 10.

I have already mentioned my co-authors in Chapter 4. May I take this opportunity also to thank all the other authors who have contributed to this book for their efforts. I am only too aware that all of them are very busy people, for whom the request to write a book chapter has come on top of a pile of other commitments; yet they have taken part in this project willingly, have acceded to editorial requests without complaint, and have helped to deliver what I hope readers will consider to be a valuable contribution to knowledge about one of the key gases that is affecting the environment.

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