Nitrous Oxide and Climate Change

Keith Smith

• Publisher: Routledge
• Year: 2010
• eBook ISBN: 9781136541384
• Topic: Politics & International Relations
• Subtopic: Ecology
• Index: Politics & International Relations

Nitrous oxide, N2O, is the third most important (in global warming terms) of the greenhouse gases, after carbon dioxide and methane. As this book describes, although it only comprises 320 parts per billion of the earth's atmosphere, it has a so-called Global Warming Potential nearly 300 times greater than that of carbon dioxide. N2O emissions are difficult to estimate, because they are predominantly biogenic in origin. The N2O is formed in soils and oceans throughout the world, by the microbial processes of nitrification and denitrification, that utilise the reactive N compounds ammonium and nitrate, respectively. These forms of nitrogen are released during the natural biogeochemical nitrogen cycle, but are also released by human activity. In fact, the quantity of these compounds entering the biosphere has virtually doubled since the beginning of the industrial age, and this increase has been matched by a corresponding increase in N2O emissions. The largest source is now agriculture, driven mainly by the use of synthetic nitrogen fertilisers. The other major diffuse source derives from release of NOx into the atmosphere from fossil fuel combustion and biomass burning, as well as ammonia from livestock manure. Some N2O also comes directly from combustion, and from two processes in the chemical industry: the production of nitric acid, and the production of adipic acid, used in nylon manufacture.

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. 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 expansion of the land area devoted to crops, to feed the increasing global population and to accommodate the current development of biofuels, is likely to lead to an increase in N fertiliser use, and thus N2O emission, worldwide.

The aim of this book is to provide a synthesis of scientific information on the primary sources and sinks of nitrous oxide and an assessment of likely trends in atmospheric concentrations over the next century and the potential for mitigation measures.

1

Introduction

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 biogeochemical 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 NO_x 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 Loscher, 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 NO_x 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 NO_x 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 NO_x 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.

Reference

Crutzen, P. J., Mosier, A. R., Smith, K. A. and Winiwarter, W. (2008) ‘N₂O release from agro-biofuel production negates global warming reduction by replacing fossil fuels’, Atmospheric Chemistry and Physics, vol 8, pp389–395

2

Microbial Terrestrial Pathways to Nitrous Oxide

Elizabeth Baggs and Laurent Philippot

Introduction

Terrestrial systems are major sources of atmospheric N₂O, which accounts for about 6 per cent of the current greenhouse effect (IPCC, 2007). Production of N₂O in soil is predominantly biological, with bacteria possessing N₂O-genic enzymes, whilst the involvement of archaea or fungi is currently uncertain. Here we explore the biogeochemical pathways in which these microbes can produce and reduce N₂O, consider the approaches available for determining the predominant N₂O-producing process under certain conditions, highlight any current uncertainties in microbial sources of N₂O to direct future research, and examine how understanding the N₂O source can aid us in managing terrestrial systems to lower emissions of this greenhouse gas.

Why do we Need to know the Microbial Source of Nitrous Oxide?

The ultimate aim in determining the microbial source of N₂O is to better constrain the global N₂O budget and to inform mitigation strategies. This is essential for the formulation of appropriate and more targeted mitigation strategies, which at the time of writing are urgently required if industrialized countries are to reach the Kyoto and post-Kyoto targets for reduction in greenhouse gas emissions, for example in Europe a reduction of up to 80 per cent by 2050 compared to emissions in 1990. Soil is a complex heterogeneous matrix that renders attributing N₂O production to different processes a challenge, as different processes may occur simultaneously in different microsites of the same soil (Robertson and Tiedje, 1987). Simply relying on a net emission of N₂O from a system does not tell us the responses of the key component groups of the microbial community. However, it is essential to understand these responses, because when imposing strategies for mitigation of N₂O it is important that there is no detrimental effect on the long-term functioning of the ecosystem facilitated through changes in key microbial functional groups. To ensure this we should link source partitioning – i.e. determination of the microbial sources of N₂O – to the understanding of activity and ecology of the underpinning microbial community. This means that efforts to measure N₂O from different environments and under different management regimes should more regularly consider all N₂O processes, with the starting point being the identification of conditions under which particular processes predominate. Unless the controls on enzyme regulation associated with these processes, and associated product ratios (primarily N₂O and N₂) are determined, then it will not be possible to develop more targeted mitigation strategies. The different processes involved in N₂O production respond differently to environmental parameters or imposed management, and the enzymatic systems of each process are regulated differently. This means that the down-regulation of N₂O production in one process as a result of management practice or change in environmental conditions may well lead to the up-regulation of N₂O production in another. Thus appropriate management for one process, may not be appropriate for another, and may well need to be flexible depending on the system, the prevailing environmental conditions, and the management options available.

What is Producing Nitrous Oxide in Soil?

Soil microbial N₂O production occurs via nitrification (ammonia oxidation) and nitrate dissimilation (denitrification and nitrate ammonification) pathways (Figure 2.1). These processes rarely occur in isolation, with possible competition for substrates under near-limiting conditions, and the possibility of transfer of N₂O or intermediary products from one process to another depending on prevailing environmental conditions, microbial community structure and location within the soil matrix. This means that N₂O produced during several processes may form one pool before being reduced to N₂ during denitrification. This representation of biological pathways of N₂O production reflects fairly recent advances in understanding; until the 1980s N₂O emissions in terrestrial systems were solely attributed to denitrification. Whilst denitrification may often be the predominant source of N₂O, particularly following rainfall events or application of NO₃^–based fertilizers to moist soil, these are not always the prevailing conditions, and there is a growing body of evidence in support of ammonia oxidation contributing to N₂O fluxes.

The processes presented in Figure 2.1 are the main focus of this chapter, reflecting our current understanding of N₂O pathways in terrestrial systems. However, it is not beyond the realms of possibility that there are other as yet ‘undiscovered’ processes or interactions between cycles that facilitate N₂O production, and in their discovery we are currently constrained by our biogeochemical and microbiological terminology and technical ability. We already know that the representation of N₂O sources in Figure 2.1 is oversimplified, with ‘new’ interactions between processes, and an increasing range of microbial groups being able t...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright
• Contents
• 1. Introduction
• 2. Microbial Terrestrial Pathways to Nitrous Oxide
• 3. Marine Pathways to Nitrous Oxide
• 4. The Global Nitrous Oxide Budget: A Reassessment
• 5. Nitrous Oxide Emissions from the Nitrogen Cycle in Arable Agriculture: Estimation and Mitigation
• 6. Nitrous Oxide Emissions from the Nitrogen Cycle in Livestock Agriculture: Estimation and Mitigation
• 7. Nitrous Oxide Emissions from Land-Use and Land-Management Change
• 8. Indirect Emissions of Nitrous Oxide from Nitrogen Deposition and Leaching Of Agricultural Nitrogen
• 9. Abiotic Nitrous Oxide Sources: Chemical Industry and Mobile and Stationary Combustion Systems
• 10. Conclusions and Future Outlook
• Contributors
• Acronyms and abbreviations
• Index