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

2.3. Soil atmosphere composition 2 Theory
2.3 Soil atmosphere composition
2.3.1 Sources, sinks and profiles of O2 and CO2
The most important subsurface source of CO2 and sink of O2 is soil respiration, which summarizes
the production of CO2 caused by root respiration and microbiological processes. How
important each process is in a given soil is hard to estimate and varies with many parameters.
Usually the fraction of CO2 production caused by root respiration is given as ranging from 0
– 50 % [Amundson and Davidson, 1990; Scheffer and Schachtschabel, 2010]. Tang et al. [2005]
give an annual mean of 44 % and a growing season average of 56 % in a forest soil.
In the presence of O2, CO2 is produced by aerobic bacteria gaining energy from the decomposition
of glucose:
C6H12O6 + 6 O2 −→ 6 CO2 + 6 H2O + 2800 kJ/mol (2.26)
This reaction produces equimolar amounts of gas and therefore does not influence the partial
pressure equilibrium of the soil atmosphere directly as the sum of O2 and CO2 should still be
20.9 Vol%. The produced CO2 has a much higher solubility in water than O2, leading to a
reduction of CO2 partial pressure. In acidic soils (pH < 5) this removal of CO2 from the gaseous
phase is generally described by the following equilibrium reaction [Yamaguchi et al., 1967]:
CO2(g) + H2O ⇋ H2CO3 ⇋ HCO − 3 + H+ ⇋ CO 2−
3
+ 2 H+
(2.27)
Oxygen depletion by microbial activity can therefore lead to a deficit in partial pressure, which
is the basis for the proposed OD model presented in Section 2.1.3.
When O2 is not available (close to the water table or under waterlogged conditions), anaerobic
instead of aerobic bacteria flourish, leading to different carbohydrate decomposition processes.
In the absence of O2 the oxidation of glucose
C6H12O6 −→ 2 CH3 CO COOH + 4 H + + 4 e −
(2.28)
requires a matching reduction reaction to be provided by the microorganism, leading to various
possible reactions [Rowell, 1997] such as denitrification or the production of methane:
2 NO − 3 + 12 H+ + 10 e − −→ N2 + 6 H2O (2.29)
CO2 + 8 H + + 8 e − −→ CH4 + 2 H2O (2.30)
Another anaerobic process is given by Scheffer and Schachtschabel [2010] as
C6H12O6 −→ 3 CO2 + 3 CH4 + 188 kJ/mol (2.31)
Anaerobic processes are mostly non-equimolar regarding the gas phase, leading to an increase of
gas concentration and therefore a partial pressure gradient. In the case of waterlogged marshes
this can cause significant flow and emissions of CH4 from the soil.
In general, microbial activity reduces the amount of O2 in the soil and increases the amount of
CO2, CH4 and of some nitrogenous gases. The main source for O2 is the atmosphere, resulting in
28