Geoscience Reference
In-Depth Information
ing remains of plants and to the formation of humus (see Sect.
5.3
later on).
Actinomycetes are therefore an important crucial link in the carbon cycle. Increasing
temperature in soils intensifi es the rate of decomposition. Thus, climatic warming
has a positive infl uence upon their processes and has a direct bearing on the release
of higher plant nutrients and the increase of soil productivity, provided organic
materials are adequate to supply all of the plant nutrient requirements. The warming
results in increased organic matter decomposition, faster release of plant nutrients,
and greater amounts of vegetative life, all of which accelerate releases of CO
2
into
the atmosphere where it is quickly utilized by photosynthesis to increase the genera-
tion of plant bodies which, in turn, provide increasing amounts of plant remains to
eventually decay. Climate warming therefore accelerates the carbon cycle.
And vice versa, climate cooling leads to the decrease of actinomycetes' activity
and to slower decomposition rates of plant remains. This is why low levels of agro-
nomic productivity characterize tundra soils of cold regions of Earth, even though
they are relatively rich in organic materials.
Many of the aerobic actinomycetes produce a specifi c odor of moistened soils,
e.g., after a rain. Experienced farmers recognize different levels of soil fertility
according to this odor when they wet a handful of soil. After stirring the soil in their
palm or between their fi ngers, the occurrence of strong pleasant odor means that the
potential fertility of the soil is high. The soil scientist's explanation is simple. The
procedure and knowledge gained by the farmer is due to the strong immediate
response of an abundance of actinomycetes decomposing decayed organic matter
(most likely plant residues) and releasing plant nutrients with the fi nal consequence
of the production of a pleasant odor. Everything looks simple if we and the farmer
have a lot of experience.
4.3
Soil Fungi
Soil fungi are members of a large group of
eukaryotic
organisms and are the most
numerous constituent in the phytoedaphon. They are scientifi cally classifi ed as a
kingdom among kingdoms of animals, plants, etc., in the domain of Eukaryota.
Their size is about ten times greater than the size of soil bacteria. They contain more
than one cell and individual fungi are usually linked into a long chain of micro-
molds called mycelium consisting of a mass of branching, thread-like
hyphae
of few
micrometers thickness. Their numbers in topsoil are very high, in 1 g of soil there is
as many as one million fungi. On 1 m
2
of topsoil we fi nd about one million fungi.
They are most widely present in the acidic conditions of peat and forest soils where
they may be even more numerous than bacteria.
Because fungi contain no chlorophyll, they must gain energy for their life pro-
cesses from the chemical breaking of bonds in organic compounds. They use there-
fore chemical sources of energy rather than that of direct sunlight. This is why they
have to participate in the decomposition of fresh organic matter like lignin and cel-
lulose. Hence, decomposing straw very effi ciently, some species of fungi produce
