Grazed pastures are recognized as a dominant source of nitrous oxide (N2O), a highly potent greenhouse gas. Studies have examined soil physical controls on N2O emissions, including soil moisture status. Limited attempts to link N2O emissions with soil‐diffusivity (Dp/Do), using repacked soil cores, have shown peak N2O emissions to align with a relatively narrow window of Dp/Do, despite a relatively wide range in water‐filled pore space (WFPS), across a range of soil bulk densities. Such detailed studies have not been performed with intact soil cores. We investigated the effects of soil‐water characteristic (SWC) and Dp/Do on N2O emissions from intact soil samples, retrieved at three depths (0–5, 5–10, 10–15 cm) from three perennial pasture sites that received a KNO3 solution (1800 mg, N mL−1). We observed distinct fingerprints of SWC and Dp/Do, which showed clear effects of soil structure on diffusion‐controlled gas emissions. Depth‐wise variation in soil moisture diminished as the soil was subjected to higher matric potential (> ∼ ‐100 kPa). Variation in Dp/Do, was more pronounced in the dry soil (> ∼ ‐1000 kPa), being largely constrained by soil moisture in wet soil (∼ ‐100 kPa) with little depth‐wise variation. Measured N2O fluxes peaked within narrow ranges of WFPS and Dp/Do, 0.90–0.95 and 0.005–0.01, respectively. The value of Dp/Do can be determined using parametric models and presents a pasture management (e.g., irrigation, soil physical disturbance such as pasture renovation and animal treading)) tool to minimize N2O emissions: soil Dp/Do should be maintained above a range of 0.005–0.01 to minimize N2O emissions. Core Ideas Peak N2O fluxes from intact soil cores occurred when diffusivity ranged from 0.005–0.01 This peak N2O flux diffusivity range was 0.005–0.01 regardless of soil or depth (0–15 cm) The diffusivity range for peak N2O flux equalled that observed in repacked soil cores Diffusivity values are readily determined and add to the suite of soil management tools

A new technique is presented for the rapid, high-resolution identification and quantification of multiple trace gases above soils, at concentrations down to 0.01 microL L(-1) (10 ppb). The technique, selected ion flow tube mass spectrometry (SIFT-MS), utilizes chemical ionization reagent ions that react with trace gases but not with the major air components (N2, O2, Ar, CO2). This allows the real-time measurement of multiple trace gases without the need for preconcentration, trapping, or chromatographic separation. The technique is demonstrated by monitoring the emission of ammonia and nitric oxide, and the search for volatile organics, above containerized soil samples treated with synthetic cattle urine. In this model system, NH3 emissions peaked after 24 h at 2000 nmol m(-2) s(-1) and integrated to approximately 7% of the urea N applied, while NO emissions peaked about 25 d after urine addition at approximately 140 nmol m(-2) s(-1) and integrated to approximately 10% of the applied urea N. The monitoring of organics along with NH3 and NO was demonstrated in soils treated with synthetic urine, pyridine, and dimethylamine. No emission of volatile nitrogen organics from the urine treatments was observed at levels >0.01% of the applied nitrogen. The SIFT method allows the simultaneous in situ measurement of multiple gas components with a high spatial resolution of < 10 cm and time resolution <20 s. These capabilities allow, for example, identification of emission hotspots, and measurement of localized and rapid variations above agricultural and contaminated soils, as well as integrated emissions over longer periods.

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GHGemissions are usually the result of several simultaneous processes. Furthermore, some gases such as N2 are very difficult to quantify and require special techniques. Therefore, in this chapter, the focus is on stable isotopemethods. Both natural abundance techniques and enrichment techniques are used. Especially in the last decade, a number of methodological advances have been made. Thus, this chapter provides an overview and description of a number of current state-of-the-art techniques, especially techniques using the stable isotope15N. Basic principles and recent advances of the 15N gasflux method are presented to quantify N2 fluxes, but also the latest isotopologue and isotopomermethods to identify pathways for N2O production. The second part of the chapter is devoted to 15N tracing techniques, the theoretical background and recent methodological advances. A range of different methods is presented from analytical to numerical tools to identify and quantify pathway-specific N2O emissions. While this chapter is chiefly concerned with gaseous N emissions, a lot of the techniques can also be applied to other gases such as methane (CH4), as outlined in Sect. 10.1007/978-3-030-55396-8_5#Sec12.

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Several approaches exist for measuring greenhouse gases (GHGs), mainly CO2, N2O, and CH4, from soil surfaces. The principle methods that are used to measure GHG from agricultural sites are chamber-based techniques. Both open and closed chamber techniques are in use; however, the majority of field applications use closed chambers. The advantages and disadvantages of different chamber techniques and the principal steps of operation are described. An important part of determining the quality of the flux measurements is the storage and the transportation of the gas samples from the field to the laboratory where the analyses are carried out. Traditionally, analyses of GHGs are carried out via gas chromatographs (GCs). In recent years, optical analysers are becoming increasingly available; these are user-friendly machines and they provide a cost-effective alternative to GCs. Another technique which is still under development, but provides a potentially superior method, is Raman spectroscopy. Not only the GHGs, but also N2, can potentially be analysed if the precision of these techniques is increased in future development. An important part of this chapter deals with the analyses of the gas concentrations, the calculation of fluxes, and the required safety measures. Since non-upland agricultural lands (i.e. flooded paddy soils) are steadily increasing, a section is devoted to the specificities of GHG measurements in these ecosystems. Specialised techniques are also required for GHG measurements in aquatic systems (i.e. rivers), which are often affected by the transfer of nutrients from agricultural fields and therefore are an important indirect source of emission of GHGs. A simple, robust, and more precise methodof ammonia (NH3) emission measurement is also described.

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