Geology Reference
In-Depth Information
turned to the atmosphere every year by the activities of wood and leaf-decaying fungi,
and mycorrhizas make it possible for plants to fix massive amounts of carbon from the
atmosphere into plant biomass every year. But fungi are also vitally important in run-
ning the earth's long term carbon cycle ('the great chalk journey' that we encountered
in a previous chapter)—the background regulator of the earth's temperature.
This takes us back to our story of the love affairs between carbon princes in the air
and calcium princesses in silicate rocks such as granite and basalt rocks. As you will
remember, the roots of land plants help to slowly weather away these rocks by phys-
ically crushing and by chemically dissolving them, which greatly increases the number
of chemical marriages between carbon dioxide molecules in the air and calcium ions in
the rocks. Once bonded together, carbon and calcium end up in chalk (calcium carbon-
ate) at the bottom of the ocean—excellent long-term storage for carbon atoms that once
helped to warm the air and hence the surface of the earth. Subtle and unobtrusive they
may be, but it seems to me that for hundreds of millions of years humble mycorrhizas
have played a major role in the long term carbon cycle by helping land plants to aug-
ment their rock-weathering powers in the face of an ever-brightening sun. I suggest that
mycorrhizas have done this by helping to increase plant biodiversity over the last 400
million years.
The argument hinges on the fact that mycorrhizal mycelia connect not just members
of the same plant species, but also members of
different
species. Various mycologists
have demonstrated this in the field, including Simard and colleagues, who used radio-
active tracers to show that mycorrhizal mycelia transferred sugars from a birch tree in
the light to an artificially shaded Douglas fir sapling. There is good evidence that these
inter-species mycorrhizal connections increase the uptake of soil nutrients by the plant
community as a whole, which reduces competition amongst the plants and allows many
species to grow together. Over geological time, this effect must have promoted the evol-
ution of new species by giving plants the freedom to explore new morphologies and eco-
logical spaces. The resulting increase in plant species diversity could have increased the
structural complexity of plant roots and mycelia in contact with underlying rocks and
hence the effectiveness of rock weathering.
Decomposer fungi also help to promote rock weathering. By digesting dead plant ma-
terial, including wood and leaves, they help to make soil, a multi-species super-organism
that enhances rock weathering when the multitude of organisms living within it exhale
carbon dioxide onto fragments of rock moistened by rain percolating easily through its
porous structure. Fungal mycelia increase the porosity of the soil by holding it together
with their hyphae and by exuding sugar-like binding molecules. Soils, thus stabilised,
can store large amounts of carbon in organic matter over long periods of time, thereby
helping to keep the earth cool.
