A gram of garden soil can contain around one million
fungi, such as
yeasts and
moulds, and around 700 km fungal
hyphae can live in 1 g of soil. Fungi have no
chlorophyll, and are not able to
photosynthesise. They cannot use atmospheric carbon dioxide as a source of carbon, therefore they are
chemo-heterotrophic, meaning that, like
animals, they require a chemical source of energy rather than being able to use light as an energy source, as well as organic substrates to get carbon for growth and development. Given these requirements and the development of a dense hyphal network (
mycelium) they actively participate to the degradation of freshly deposited organic remains and their transformation in
humus (humification) and
carbon dioxide (
mineralization). Many fungi are
parasitic, often causing
disease to their living host plant, although some have beneficial relationships with living plants, as illustrated below. In terms of soil and humus creation, the most important fungi tend to be
saprotrophic; that is, they live on dead or decaying organic matter, thus breaking it down and converting it to mineral forms (e.g.
nitrate,
ammonium,
phosphate) that are available to the higher plants. A succession of fungi species will colonise the dead matter, beginning with those that use
sugars and
starches, which are succeeded by those that are able to break down
cellulose and
lignins. Fungi spread underground by sending long thin threads known as
mycelium throughout the soil; these threads can be observed throughout many soils and
compost heaps. From the mycelia the fungi is able to throw up its fruiting bodies, the visible part above the soil (e.g.,
mushrooms,
toadstools, and
puffballs), which may contain millions of
spores. When the
fruiting body bursts, these spores are dispersed through the air to settle in fresh environments, and are able to lie
dormant for up to years until the right conditions for their activation arise or the right food is made available. Fungal spores are dispersed by wind, water, but also by a variety of fungal-feeding animals, from small invertebrates (e.g. springtails) to big mammals (e.g. wild boars), helping them to colonize new, sometimes remote environments, hence the cosmopolitan distribution of many fungal species.
Mycorrhizae Those fungi that are able to live
symbiotically with living plants, creating a relationship that is beneficial to both, are known as
mycorrhizae (from
myco meaning
fungus and
rhiza meaning
root). In mycorrhizae plant roots are invaded by the
mycelia of the mycorrhizal fungus, which lives partly in the soil and partly in the root, and may either penetrate the root
cortex without entering its cells (forming the
Hartig net) and cover the root as a sheath (
ectomycorrhizae) or be present in cortical cells in the form of arbuscules (
arbuscular mycorrhizae). The mycorrhizal fungus obtains the
carbohydrates that it requires from the root, in return providing the plant with nutrients, including nitrogen and phosphorus, and with moisture. Later the plant roots will also absorb the mycelium into its own tissues. In some cases mycorrhizae could provide their host, either directly or indirectly, with nutrients issued from the degradation of more complex soil organic matter (
humus). Mycorrhizae can also benefit nutrients (other than
sugar carbon) and moisture from the host, and exchange nutrients (including carbon) and moisture between plants through common
mycorrhizal networks.
Chemical signalling between plants through common
mycorrhizal networks, although a beautiful concept, is still a matter of conjecture, more research being needed. Beneficial mycorrhizal associations (either
ectomycorrhizae or
arbuscular mycorrhizae) are to be found in many of our edible and flowering crops, to the exception of
Brassicaceae (e.g.
cabbage,
turnip) as well as in the majority of
tree species, especially in
forests and
woodlands, with
Ericaceae (e.g.
bracken,
bilberry) harbouring a special type, called
ericoid mycorrhizae. Tree mycorrhizae create a fine underground mesh that extends greatly beyond the limits of the tree's roots, greatly increasing their feeding range and actually causing neighbouring trees to become physically interconnected. The benefits of mycorrhizal relations to their plant partners are not limited to nutrients, but can be essential for plant reproduction. In situations where little light is able to reach the
forest floor, a young
seedling cannot obtain sufficient light to
photosynthesise for itself and will not grow properly, causing a deficit of regeneration. But, if the ground is underlain by a mycorrhizal mat, then the developing seedling will throw down roots that can link with the fungal threads and through them obtain the nutrients it needs.
David Attenborough points out the plant, fungi, animal relationship that creates a "three way harmonious trio" to be found in forest
ecosystems, wherein the plant/fungi symbiosis is enhanced by animals such as the wild boar, deer, mice, or flying squirrel, which feed upon the fungi's fruiting bodies, including truffles, and cause their further spread. A greater understanding of the complex relationships that pervade natural systems is one of the major justifications of the
organic gardener, in refraining from the use of artificial chemicals and the damage these might cause. Recent research has shown that
arbuscular mycorrhizal fungi produce
glomalin, a protein that binds soil particles and stores both carbon and nitrogen. These glomalin-related soil proteins are an important part of
soil organic matter. ==Invertebrates==