Life in the Soil - Soil: Reflections on the Basis of our Existence - page 61

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Filament lifelines. Fungal mycelia from the roots of a spruce penetrate through the ground. Many
boletes form white mycelia similar to those in the photo, but the yellow mycelium belongs to a
fungus that never forms any fruit bodies. A pine needle is visible for comparison in the upper part
of the image and a spruce root appears to the lower right
The king bolete in the bottom photo forms mycorrhizae with both deciduous and coniferous
trees. It is recognisable by the white reticulations on its stem. Intact specimens of this mushroom
are among the most prized by pickers

organisms, of which over sixty thousand have been described. The true numbers
are even bigger. One of the pioneers of nematode research wrote a century ago that
one could remove everything except nematodes from the ecosystem and be unable
to notice any immediate impact: mountains, valleys and flora would look much the
same. So numerous are nematodes that the numbers are almost mind-boggling. A
single square metre of soil can hold up to ten million of them.
After my lecture to the Royal Academy of Sciences I visited the adjoining
Bergius Botanic Garden. Strolling round in the hothouses, I wondered if my mes-
sage had got through. A single spadeful of earth from a Swedish forest contains
more species of microbe than all the plant and animal species found in the world's
tropical rainforests.

The Soil's Web of Life

A question that soil scientists have long struggled with is how all these life forms
interact. Who eats what? In a forest or savannah grassland it is relatively easy to
see which animals are herbivores and which are carnivores. Plants serve as food
for gazelles on the savannah or leaf-eating larvae in the forest. In turn, these ani-
mals become food for predators like lions or insectivorous birds. Sometimes there
is an extra link in the chain: small birds are food for birds of prey, for example.
But what does the food chain look like below ground?
One starting point is the organic exudates secreted by plant roots. In part, these
secretions serve as a lubricant for the roots as they grow through the soil and as food
for mycorrhizal fungi. In many plants, the soil adjacent to the roots hosts a rich flora
of root bacteria that thrive on this ready source of carbohydrate. Root bacteria can
reproduce one hundred times faster than bacteria living a millimetre or so away from
the root surface. The carbohydrate glut also attracts bacteria-eating organisms like
amoebae, protozoa and nematodes, whose presence in turn benefits plants because
nitrogen in the bacteria is released during the feeding process. Some bacteria secrete
substances that stimulate root formation, a beneficial role that tends to result in pred-
atory organisms ignoring rather than eating them. Aphids can multiply when soil
organisms facilitate plant growth, and hence plants serve as a link between the fauna
above and below ground. Scientific studies indicate that the interaction between
these different worlds is far more complex than previously thought.
Another starting point for the soil food chain is the organic matter present
in the soil, originating from dead leaves, roots and insects and other organisms.