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

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].
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