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Soil Food Web

Soil Mesofauna, the springtails, mites and nematodes, relate with the other features of the Soil Food Web, as followsSoil Food Web (USDA)USDA Soil Food Web


Earthworms mating
Earthworms 
They do a lot of soil moving, they are the big earth mixers and movers. There are:
surface active non-burrowing worms which consume decaying plant residues on the soil surface,
burrower in the mineral horizons of the soil but feed at the surface usually at night and
deep horizon worms inhabit the deep organo-mineral horizons constructing branching burrows More on worm biodiversity
While earthworms do the digging, the mesofauna  (much smaller but not microscopic) do all the running around. The worms create the spaces for these creatures to move. These soil animals  the mesofauna relate with all these other soil organisms.....
Fungi
Fungi and humus probably contribute most to the texture of soil. They mix with mineral elements (sand silt & clay, each different sizes) to produce a 'colloid' that can dry up, expand, and also aggregate, giving soil that crumbly feel when you rub it in your hands. The texture of the sand, gravel and silt, together with the bacteria and fungi and organic debris explain the soil properties. 
Fungi and roots can form a very close relationship or mycorrhiza; the relationship is symbiotic, which benefits both by providing food for each, and sometimes essential (eg citrus). There are two main types of fungi: endotrophic, where the fungus invades the hosts’ roots (e.g., orchids), and ectotrophic, in which the fungus forms a mantle around the smaller roots (e.g., pines). 
Soil Food Web
BioTorch describe how "mycorrhizal fungi are highly influenced by exudates coming from the plant roots, in return the mycorrhizal fungal hyphae attached to the roots reach out into the soil and also release secretions of exudate like secondary metabolites which can then have tremendous effects on the other microbes within the mycorrhizosphere, such as attracting or repelling them...
Mycorrhizal fungi create mycelium that extends from the root out into the rest of the soil composition, and it is said that up to 30% of the total carbon mass (mostly due to Glomalin) within healthy soil all around the world is directly related to the networks of different fungal species" More on Mycorrhiza.  
An example of how soil animals interact with these fungi is that  some little creatures accidentally pick up fungal spores and take them to the roots. These springtails have a sticky organ on their underside so the fungal spores stick to them. It is this sort of interrelationship that reflects on this complexity of life beneath our feet, yet so vital to all life on the planet. For more about 'the bees of the soil'.

Glomerales fungus

The recent discovery of glomalin, a 'superglue' produced by fungi, is opening up insights into how the soil feels and works. It is strange how soil can be dry as a bone, seemingly dead, yet come back to life soon after adding water. Some thing this has something to do with glomalin.
The fungus Glomus moves down the growing roots and forms new hyphae. Older hyphae left higher up the roots slough off their protective glycoprotein, called 'glomalin', into the soil. Springtails eat this stuff. It also attaches to particles of minerals, like sand, silt, and clay, and organic matter, forming clumps. (More on Glomalin). 
Fungi tend to use more complex compounds, such as fibrous plant residues, wood and soil humus. 

Predaceous fungi snaring nematode
Some fungi are predaceous and get their nutrients from trapping and digesting animals like nematodes. First world video of nematode caught by fungi. They use sticky exopolysaccharide - a strong mycoglue produced by a nematophagous fungus to trap and immobolize soil nematodes.

There are more than 200 species belonging to the phyla Ascomycota, Mucoromycotina, and Basidiomycota. They live in the soil and trap nematodes using webs of hyphae, while others attack amoebae or collembola.
Numbers 100um apart



Bacteria
Myxococcus bacteria
Bacteria are found in worm guts to help mix the mineral particles with the newly arrived organic matter.Bacteria, both in earthworms and in the soil, tend to use simple organic compounds, such as root exudates or plant residue. The Myxococcus in the picture feeds on other bacteris. The most famous soil bacteria are those that 'fix' nitrogen from the air, like this Rhizobium
Rhizobium bacteria
They turn nitrogen gases into a feed stuff (nitrates) for plant nutrient.

Plant Decomposition
When the plants die or the leaves drop off, the micro and mesofauna work together  to break the plant structures down. 
First the leaves leach out chemicals, then there is mineralisation, following by the small soil creatures breaking down the mainplant structures. Springtails have a powerful jumping organ. (see in operation) that can also be used to prise the epidermis of the leaf from the underlying parts. 
Soil Organic Matter
Soil organic matter is many different kinds of compounds, made of roughly equal parts humus and 'active organic matter'. Humus is a chemical entity, consisting of such compounds as humic acid - now available as a commercial product to add carbon to your soil. The 'active organic matter' is not consistent but includes all sorts of bits of debris. This is where the soil animals are at work.

The small soil animals eat the fungi, dead springtails and other debris, remnants from the decomposing process. By transforming bits of plant and animal debris and digesting them into 'poo' chemicals, they provide nutrients for plant roots to absorb and for other creatures to live on. .

Oribatid mites are everywhere in the soil and have been there for 100s of millions of years, breaking down the plant debris into smaller particles, that the fungi and bacteria can get at. Oribatids burrow away, increasing the surface area for bacteria and fungi to digest the plant into chemical compounds.
Nematodes are another common small soil animal (or 'soil mesofauna'). There are an incredible variety of nematodes, which function at several food 'levels' of the soil food web. Some feed on the first level - plants and algae, others graze on the second level - on bacteria and fungi, and some feed on other nematodes - higher food (trophic) levels.

The absorption of the decomposing mass into the soil and mixing with minerals and clay, occurs in both the soil and earthworm guts. There is repeated absorption of the clayey humus and organic matter by earthworms and enchytraeid worms, The burrowing and digging animals loosen up the clayey material into 'crumbs', so that eventually the classic soil state - 'friable and crumbly' is reached.

Lets look in more detail as we start from the top and go down the soil..

Quick Question : What is that "Smell of the Soil"? - Click to find out


More, at 'The Decomposers'
Fabulous photos of soil animals at 'Chaos of Delight'.
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