Geoscience Reference
In-Depth Information
Karl Popper, regarded as one of the greatest philosophers of science during the
twentieth century, said that a theory or hypothesis cannot be proven true if found as
false in only one instance or situation. Such a hypothesis has to be modifi ed or even
rejected. A single falsifi cation annihilates an existing hypothesis even if held or sup-
ported previously by the majority of scientists. Hence, we are confi dent that a lot
still has to be done to quantify to what extent the atmospheric CO
2
concentration
contributes to climate change. Moreover, until confronted with more convincing
scientifi cally accepted evidence, we steadfastly believe that atmospheric CO
2
has
not played a dominant role in global warming during the previous 200 years.
Let us now continue without saying that studies about CO
2
concentration in soil
pores and its transport from soil to the atmosphere are useless. Quite opposite, the
CO
2
concentration and fl ux through soil to the outer atmosphere is a good indication
on humifi cation.
The majority of biological reactions in soil rely on the presence of oxygen, e.g.,
the ever-present decomposition of organic matter and the production of soil humus
moves up by diffusion through the soil pores into the atmosphere, the conditions for
diffusion in soil are less favorable than those in the atmosphere. Consequently,
because the sequestration rate of CO
2
from the soil is smaller than its production
rate, the CO
2
concentration within soil root zone is higher than in the atmosphere.
This substantially increased CO
2
concentration increases the concentration gradi-
ent - the driving force for diffusion. After reaching this increased diffusion rate, a
new balance is established between the production and release of CO
2
from the soil
into the atmosphere. If the rate of CO
2
production subsequently increases again,
diffusion also increases owing to the increase of CO
2
concentration in soil air and
vice versa. The rate of organic matter decomposition depends on many factors - the
amount of decayed parts of plants, temperature, humidity and soil water content,
degree of pore connectivity, and atmospheric pressure. Inasmuch as these factors
are not constant in time, we expect continual fl uctuations of CO
2
concentration in
soil pores and observe persistent changes of the rates of CO
2
release from the soil.
A very rough indication on the CO
2
concentration in the air of soil pores is about ten
or more times higher than that in the outside atmosphere above the soil surface.
Furthermore, because the value of CO
2
concentration fl uctuates drastically through-
out the soil profi le, the CO
2
concentration at some depths of subsoil could reach
even 30 times higher than its value in the atmosphere. The global CO
2
concentration
in the atmosphere is now close to 390 ppm, while its value in the pores of the soil
humus horizon fl uctuates in a broad range of about 4,000 ppm. The symbol ppm
means the Latin
pars per million
, part of one million in volumetric units. The value
390 ppm CO
2
is 0.039 % volumetrically or 39 ml of CO
2
in 1 l of air.
The ten times greater concentration of carbon dioxide in the soil compared with
that in the air above the soil surface is the essential ingredient to overcome obstacles
preventing diffusion, namely, those caused by dead-end and chaotic soil pores as
well as the relatively small volume of air through which gaseous transport must
occur within fi eld soils. Just for our imagination, let us consider two 100-l barrels
fi lled with water. We leave the water level in the fi rst barrel free to the atmospheric
