Geoscience Reference
In-Depth Information
In Sahel, the dry climatic strip lying between the Sahara desert and the Sudanian
savanna, and in the savanna itself, termites construct quasi-cylindrical mounds sev-
eral meters high with unique thermoregulation that prevents the complex structures
inside the mounds from overheating. With this type of air conditioning, termite
workers are able to cultivate fungal gardens and harvest suffi cient fungi to feed each
and every termite in the entire colony.
4.7
Soil as a Microgarden?
In spite of the fact that soil is a biological system with a high microbial diversity, the
space occupied by microorganisms is very small, since there are only few sites
appropriate for microbial life. When biological molecules such as DNA and
enzymes are adsorbed on the surface of particles due to their negative charge, they
are resistant to further microbial degradation. As a result, with those parts of the
pore space not being suitable for microbial life, microbes are not equally distributed
within all soil pores. In those portions of pore space providing free access to food
for microbes, we fi nd many thousand times more microbes than the average number
of microbes in one cubic centimeter of soil. A comparison of the distribution of
micro-life in soil pores to that of cultivated plants in a regular garden is not appro-
priate because we expect that plants grow across each and every square foot of the
garden, while we detect absolutely no microbes occupying micro- and nano-spaces
within soil pores. It is more appropriate to restrict the comparison to a big botanical
garden and a zoo where on an area of a couple hectares we encounter plants of sev-
eral climatic zones and animals from polar bears to leopards. This comparison is
more realistic because we are familiar with the different kinds of species potentially
living together in a garden and a zoo. Indeed, number of species in only 1 cm
3
any-
where within the garden is orders of magnitude higher than the number of animal
species in the entire zoo. Botanical gardens and zoos also have spaces without any
obvious plants and animals. They are paths and roads reserved for visitors and ser-
vice facilities for maintaining the garden and zoo. We emphasize here a certain
similarity to the irregular distribution of microorganisms in soil pores, while a great
biodiversity also exists in the soil as well as in the botanical garden and the zoo.
The soil pore space also has other specifi c features. It encloses fi ne roots and
provides the structural porosity for the rhizosphere. It is a nutrient-rich environment
that tends to partially curtail the impact of the abovementioned spatially variable
distribution of microbes. Root exudates offer food to all microbes within the entire
pore space and especially to those on the surface of fi ne roots. The numerous inter-
actions between a plant and microorganisms in its root zone are the reliance of the
microorganisms to utilize root exudates and to interact or compete with other types
of microorganisms. The interaction between plant and soil microorganisms may
range from mutual profi t up to parasitism.
