Geoscience Reference
In-Depth Information
pressure of 1 bar (or 1 atm). After enclosing the top of the second barrel, we use an
air pump to maintain a pressure of 10 bars on its water-fi lled surface. Next, we open
the taps on the bottom of each of the barrels at the same time. Everybody knows
from experience that the water fl ows faster from the second barrel that will be empty
much earlier than the fi rst barrel. A similar mechanism acts in diffusion.
Now we consider a more exact explanation. Let us imagine that 10,000 mole-
cules of CO
2
are in the topsoil immediately within the surface of a fi eld soil. Above
the soil surface there are only 100 molecules of CO
2
in the air. Owing to the thermal
oscillation of the CO
2
molecules, there is a high probability that 80 molecules shall
penetrate from the soil pores into the atmosphere above soil surface, while at the
same time, there is an equal probability that only two molecules shall penetrate
from the outside atmosphere into the soil pores. These simultaneous penetrations
exemplify a very simplifi ed diffusion of CO
2
in both directions - out from the soil
to the atmosphere and from the atmosphere into the soil. The net result is the escape
of 78 molecules of CO
2
from soil to atmosphere. In the nineteenth century and fi rst
half of the twentieth century, this diffusion process involving CO
2
was frequently
called soil respiration. It was not an appropriate term inasmuch as respiration is a
mechanically active process, while diffusion through the soil is a passive process. In
our simple example had the CO
2
concentration in soil pores been higher, the net fl ux
of CO
2
from the soil would have also been higher. From a practical consideration,
we know that whenever the decomposition of organic matter is more intensive than
the release of CO
2
into the atmosphere, the concentration of CO
2
increases within
the soil profi le. On the other hand, when the CO
2
production rate equals the CO
2
diffusion rate, we observe a so-called quasi equilibrium. The term quasi is used
since the rates of production and rates of diffusion change instantaneously - each at
values differing from the other.
The decomposition rate of organic matter depends upon the amount of water
within a soil, e.g., after a rain, an initially dry soil becomes wet and subsequently
releases more CO
2
. The rate of decomposition also increases whenever the soil tem-
perature gets higher. And because the production of CO
2
decreases when the tem-
perature falls down, the sequestration of CO
2
from the soil is smaller at night than
during the day.
Diffusion is also infl uenced by the content of water in soil. The higher the soil
water content (“soil moisture”), the smaller is the space through which CO
2
mole-
cules must travel during diffusion. The conditions for diffusion are relatively favor-
able if air fi lls at least 30 % of the soil pores. When the fraction of soil pores fi lled
by air is only 10 % or less with water fi lling 90 % or even more of the pores, the free
connection of soil air stops with air existing in soil only as isolated bubbles. The
concentration of CO
2
increases inside those bubbles at the expense of oxygen, i.e.,
the concentration of O
2
decreases. If the soil is fully or nearly fully saturated by
water, i.e., if the soil is waterlogged for a long time, the absence of oxygen leads to
a reduction of oxygenation and to the dominance of chemical reduction processes.
After extremely long times of soil waterlogging, oxygenation stops completely.
Because such conditions are unfavorable for a great majority of cultivated plants,
we shall later on describe their consequences in more detail. Now, just for a brief
