Diploma thesis in Physics submitted by Florian Freundt born in ...

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2.3. Soil atmosphere composition 2 Theory where K0 is a reaction specific constant, E the reaction specific activation energy and R the universal gas constant. However, due to denaturation of essential proteins at high temperatures, this approximation is only valid over a limited temperature interval. Winkler et al. [1996] found that respiration rates of CO2 from soils generally followed the exponential predictions of the Arrhenius equation between soil temperatures of 4 – 38 ◦ C, see also Figure 2.10. The Q10 value describes the increase of respiration rate per 10 ◦ C and was found to be between 1.7 – 1.9, decreasing linearly with increasing temperature. This means that for every 10 ◦ C increase in soil temperature within the 4 – 38 ◦ C range, the soil respiration would nearly double, if not limited by other parameters. This is the reason temperature dependency is the primary controlling factor on soil respiration, introducing an annual variability. Temperature also influences the diurnal cycle of CO2 production, however under certain conditions (high soil water content, production larger than transport) this influence can be quite complicated, resulting in a hysteresis between production rate and temperature [Riveros-Iregui et al., 2007]. Soil water content becomes the controlling factor of CO2 production and concentrations when soil temperatures are ideal during spring and summer [Buyanovsky and Wagner, 1983; Yuste et al., 2003] as high water availability enhances the biological activity and inhibits diffusive transport. The lack of water reduces microbial activity, the more prolonged a drought, the greater the adverse effect on size and functionality of microbial life within the soil [Schimel et al., 1999]. Both field [Liu et al., 2002; Tang et al., 2005] as well as laboratory [Orchard and Cook, 1983] experiments showed a rapid and strong response of CO2 production after the rewetting of dry soils. Smaller, but still notable modulations of soil respiration are caused by soil type, nutrient availability and vegetation above ground [Buyanovsky and Wagner, 1983] to name a few. 2.3.3 Molecular nitrogen and nitrogenous gases Molecular nitrogen (N2) is a major component of soil air, as it is also the most abundant gas in atmospheric air. Within the soil regime, N2 is influenced by a complex system of processes both consuming as well as producing N2 and nitrogenous gases. Under aerobic conditions nitrification takes place, producing nitric oxide NO and nitrous oxide N2O. Under anaerobic conditions, N2 an N2O are produced by denitrification [Nieder and Benbi, 2008]. While rarely measured in studies of soil atmosphere composition, Magnusson [1994] found N2 concentrations to fluctuate between 78 – 95 % in water saturated soils, where CO2 is removed from the soil atmosphere by dissolution in water. Based on its high abundance and the existence of a subsurface source additional to the atmospheric reservoir it shares with the noble gases, N2 was speculated to be able to restore equilibrium conditions when O2 depletion and CO2 dissolution lead to a partial pressure deficit in the soil atmosphere by Schneider [2010]. 32
2 Theory 2.3. Soil atmosphere composition 2.3.4 Radon Even though it is a nobel gas, radon is irrelevant to noble gas paleotemperature studies as all of its isotopes are radioactive and therefore not conserved in ground water. 222 Rn is produced within the soil as a member of the radioactive decay chain of 238 U, 220 Rn within the 232 Th decay chain. The source strength for radon isotopes is dependent on the uranium and thorium concentrations of the given soil and will therefore fluctuate depending on the soil constituent’s chemical composition and their spatial allocation. Since atmospheric radon concentrations are negligible [Porcelli et al., 2002], 222 Rn and 220 Rn are used as tracers to mark the soil atmosphere origin of sampled air, as is being done in this study. 33
Page 1: Faculty of Physics and Astronomy He
Page 5: Measuring annual variation of soil
Page 8 and 9: CONTENTS CONTENTS 3.1 Setup of the
Page 11 and 12: Chapter 1 Introduction Understandin
Page 13: 1 Introduction Molins and Mayer [20
Page 16 and 17: 2.1. Noble gas temperatures 2 Theor
Page 24 and 25: 2.2. Physical processes and propert
Page 26 and 27: 2.2. Physical processes and propert
Page 28 and 29: 2.3. Soil atmosphere composition 2
Page 30 and 31: 2.3. Soil atmosphere composition 2
Page 35 and 36: Chapter 3 Sampling sites and method
Page 37 and 38: 3 Sampling sites and methods 3.1. S
Page 43 and 44: Chapter 4 Measuring methods 4.1 Mas
Page 45 and 46: 4 Measuring methods 4.1. Mass spect
Page 47: 4 Measuring methods 4.2. On site me
Page 50 and 51: 5.2. Temperature profiles 5 Results
Page 52 and 53: 5.3. O2 and CO2 profiles 5 Results
Page 54 and 55: 5.4. Borehole sealing 5 Results 5.4
Page 56 and 57: 5.4. Borehole sealing 5 Results Fig
Page 58 and 59: 5.5. Noble gases 5 Results 8 days,
Page 60 and 61: 5.5. Noble gases 5 Results 5.5.3 So
Page 62 and 63: 5.5. Noble gases 5 Results 5.5.4 So
Page 64 and 65: 6 Discussion This rather extreme am
Page 66 and 67: )! )! *+,-./-01234*-56-20301.7*38*9
Page 68 and 69: 6 Discussion Table 6.1: Synthetic d
Page 71: Chapter 7 Summary The results of th
Page 74 and 75: 8 Outlook Considering the manifold
Page 76 and 77: A.1. Gas sample size A Calculations
Page 78 and 79: A.3. Fractionation-Values A Calcula
Page 80 and 81: Table B.3: Noble gas volume mixing
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Table B.5: Calculated values of rel
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Table B.8: Precipitation record of
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Table B.10: Complete dataset of nob
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Appendix C Additional plots and fig
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C Additional plots and figures Figu
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C Additional plots and figures �
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C Additional plots and figures O 2
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C Additional plots and figures 4 He
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C Additional plots and figures 20 N
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C Additional plots and figures 10 1
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Appendix D Datasheets 107
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D Datasheets LogTag TREX-8 Technica
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Bibliography [Aeschbach-Hertig 1994
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BIBLIOGRAPHY BIBLIOGRAPHY [Friedric
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BIBLIOGRAPHY BIBLIOGRAPHY [Riveros-
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Acknowledegment At the end of this
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