Keywords

1.1 Introduction

The global climate is changing rapidly. This leads to the increasing occurrence of extreme weather events such as droughts and floods. The major cause of these events is the rising temperature in the Earth’s atmosphere, which is driven by increasing emissions of climate-relevant greenhouse gases (GHGs) that trap heat in the atmosphere. Major GHGs include carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) (Fig. 1.1).

Fig. 1.1
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Major greenhouse gas emissions and contributions by various sectors (IPCC 2014a, b)

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Carbon dioxide is the major GHG responsible for the increasing greenhouse effect of the atmosphere. Key natural sources of CO2 include ocean–atmosphere exchange, respiration of animals, soils (microbial respiration) and plants, and volcanic eruption. Major anthropogenic sources of CO2 include burning of fossil fuel (coal, natural gas, and oil), deforestation, and the cultivation of land that increases the decomposition of soil organic matter and crop and animal residues (Xu and Shang 2016).

Aside from CO2, CH4 is a major GHG, which is emitted by natural and anthropogenic processes. Natural sources of CH4 emission include wetlands, termite activities, and occan. Paddy fields used for rice production, livestock production systems (enteric emission from ruminants), landfills, and production and use of fossil fuels are the main anthropogenic sources of CH4. Furthermore, CH4 can be produced by anaerobic mineralization by methanogenic archaea in both natural and man-made systems. Also, plants have been shown to emit CH4.

The third major GHG is N2O (Zaman et al. 2012). Besides being a major GHG, N2O is a major ozone-depleting gas (Ravishankara et al. 2009). Oceans and soils under natural vegetation are non-anthropogenic sources of N2O. However, at a global scale, the emission of N2O is mostly caused by, or related to, anthropogenic agricultural and other land-use activities. The atmospheric concentration of N2O has increased by more than 20% from ~271 ppb to 331 ppb since the industrial era (ca. 1750) to 2018 (WMO 2019). Over the last decade, the rate of N2O increase was equal to 0.95 ppb yr−1 (IPCC 2013b; WMO 2019) with an increasing trend (Makowski 2019; Thompson et al. 2019). In 2006, the total anthropogenic source of N2O was 6.9 Tg N2O-N. Out of these direct emissions, agricultural sources dominated (4.1 Tg N2O-N), while indirect emissions accounted for 0.6 (with a range of 0.1–2.9) Tg N2O-N (IPCC 2013a). Such a large N2O emission is attributed to various factors including the intensification of agricultural and other human activities, increased use of synthetic fertiliser (119.4 million tonnes of N worldwide in 2019), inefficient use of irrigation water, deposition of animal excreta (urine and dung) from grazing animals, excessive and inefficient application of farm effluents and animal manure to croplands and pastures, and management practices that enhance soil organic N mineralisation and C decomposition. These activities affect the N cycle. The N cycle is rather complex, and it has even been disrupted due to increased N inputs and intensification of agriculture. Sources of increased N inputs, in particular reactive N, into the N cycle stem, for instance, from the Haber–Bosch process (Erisman et al. 2011), thereby transforming the N cycle into the so-called N cascade, which is characterised by the release of reactive N forms into the environment with various consequences (Sutton et al. 2011). There are still many uncertainties concerning the N cycle. For example, the role of individual factors controlling the occurrence and rate of the key N transformation processes, such as denitrification and nitrification, is uncertain (Butterbach-Bahl et al. 2013; Müller and Clough 2014; Smith 2017). Nitrification results in ammonium (NH4+) being converted to nitrate (NO3−) under aerobic conditions, while denitrification is the reduction of NO3− to N2 under anaerobic conditions. Nitrifier denitrification occupies the niche between nitrification and denitrification and occurs as oxygen concentrations approach an anaerobic status. Under these conditions, nitrifiers actually convert nitrite into N2O and N2 instead of nitrate (Wrage-Mönnig et al. 2018).

The three GHGs (CO2, CH4, and N2O), derived from various sectors, play a major role in regulating Earth’s temperature (Fig. 1.1). Without GHGs in the atmosphere, the average global soil surface temperature would be ~19 ℃, compared to the present values of 14 ℃ (Hossain 2019).

Recent data from the UN Intergovernmental Panel on Climate Change (IPCC) clearly show that anthropogenic emissions of GHGs are at the highest in history (IPCC 2014a). Since 1990, Earth’s average surface air temperature has increased by about 0.8 ℃, with much of the emission increases taking place since the mid-1970s (Fig. 1.2).

Fig. 1.2
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Recent anthropogenic emissions of GHGs (IPCC 2014a; WMO 2019)

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The global warming potential (GWP) of a GHG relates to the amount of heat trapped by a certain mass of a gas to the amount of heat trapped by a similar mass of CO2 calculated over a 100-year time horizon (IPCC 2016). For example, the GWP of N2O is 265–298, which means if the same masses of N2O and CO2 were emitted into the atmosphere, N2O would trap 265–298 times more heat than CO2 over a 100-year time period (Table 1.1) (IPCC 2016).

Table 1.1 Global warming potential (GWP) and atmospheric lifespan of various GHGs (IPCC 2016)
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The Kyoto Protocol was negotiated by parties to the United Nations Framework Convention on Climate Change (UNFCCC) in an effort to stabilise the continued increase in atmospheric GHG concentrations. The Kyoto protocol outlines GHG reduction targets for participating countries. The signatories to the protocol must develop and report on their annual national inventory of anthropogenic GHG emissions. Guidelines on how to construct inventories were prescribed by the IPCC (2014b). Country-specific emission data can be considered but that requires accurate inventory data based on precise GHG measurements.

1.2 Impact of Ammonia on GHG Emissions

Besides direct sources, GHGs also come from different indirect sources such as upstream and downstream in agricultural systems (Plate 1.1).

Plate 1.1
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Schematic representation of direct (from the cropping system) and indirect (upstream and downstream) GHG emissions from crop production. As an example, ammonia emitted from a cropping system will be deposited and potentially oxidised to nitrate, which can further be denitrified, thus enhancing the risk for N2O emission (Aguilera Fernández 2016)

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Ammonia (NH3) itself has no direct greenhouse effect. It is a gas with a relatively short residence time in the atmosphere (2–10 days) compared to some GHGs, such as CO2 (3–4 years), CH4 (12 years), and N2O (114 years). However, after NH3 is emitted to the atmosphere and reacts with acids, it forms salts. These salts then return to The Earth’s surface and act as a N source source for N2O emissions, similar to a fertiliser-N application. When the soil is submitted to conditions near the optimum for urease activity (e.g. pH close to neutrality, soil moisture near field capacity, temperature >30 ℃), the N losses through NH3 volatilisation from urea-based fertilisers applied on soil surface can be as high as 50% (Martins et al. 2017; Rochette et al. 2013). Therefore, the measurement of NH3 emission is important to estimate indirect N2O emissions derived from soil amendments, such as urea-based fertilisers, green manures, animal excreta, or ammonium-based fertilisers in alkaline soils. A default emission factor defined by IPCC, known as EF4, can be applied for the estimation of indirect N2O emissions derived from volatilisation of NH3 and other nitrogen oxides (NOx) (IPCC 2006). The mean value of EF4, considering the N volatilisation and consequent re-deposition, is 0.01 kg N2O-N per kg N volatilised as NH3 with an uncertainty ranging from 0.002 to 0.05 (IPCC 2006). Therefore, management options that reduce NH3 volatilisation from soils are considered mitigation practices because they reduce indirect N2O emissions (IPCC 2014a; Lam et al. 2017).

1.3 Aim of the Book

In 1992, the International Atomic Energy Agency (IAEA) published a “Manual on measurement of CH4 and N2O emissions from agriculture” (IAEA 1992). Since the publication of the manual, progress has been made in analytical techniques. The progress includes advances in automation technologies as well as the theoretical understanding of how soil microbial processes affect CH4 and N2O emissions and the factors influencing those microbial processes.

Hence, the aim of this book is to provide an updated account of the state-of-the-art techniques to measure direct GHG emissions (Plate 1.1), as a necessary step to propose and assess any mitigation strategy. The focus is on CH4 and N2O emissions. Additionally, information on techniques to measure indirect GHG sources is provided. Indirect GHG sources in this book include volatilised ammonia (NH3). NH3 is a reactive N gas highly affecting the environment through eutrophication and acidification of natural ecosystems as well as human health due to the promotion of particulate matter formation (Sanz Cobena et al. 2014). Moreover, in line with the 1992 IAEA Tecdoc document, the hands-on approach is also followed here so that researchers, who want to use the techniques described in this book, can easily apply them to their own work.