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Bioturbation with/without soil fauna
Bioturbation is the mixing of (plant) residues into soils and sediments by biotic activity. It is one of the fundamental processes in ecology, as it stimulates decomposition, creates habitats for other (micro)fauna and increases gas - and water flow through the soil. Soil fauna such as earthworms, potworms, collembola, mites and isopods are seen over a 15 weeks period compared removing the fauna but fungal spores survived.

We know more about the movement of celestial bodies than about the soil underfoot.” 
Leonardo Da Vinci, circa 1500’s  
(Loads more soil sayings)
Pine seed ectomycorrhiza
Note sprigtails flitting about keeping roots clean and accidentally moving fungal spores to roots. Thank you Gelinda De Deyn

Things haven't changed..whenever we 'find a new star or planet', i hear people saying: "Could it have life on it? Is there is a sign that there may be water, which is essential for life. What do you say Brian?". I am screaming: "If you want life, look right under your feet. There are 12 quadrillion specs of creatures. 12 Quadrillion - that is 12,000 trillion - surely a Brian Cox number? And they are living right under our feet. But most people could not name one of them, the governments ignore them, yet they are the last kingdom to be explored. They creatures are quite recognisable with bodies, legs and mouth parts. They have an incredible story to tell us when our soil - that we now call 'Earth', came about.  The how. where and why. Yet they are virtually ignored. 

"It is estimated that 5 - 10 tonnes of animal life can be found in one hectare of temperate grassland soil". From Global Soil Biodiversity Atlas.  Roughly half that will be the segmented worms we are familiar with, the other half - much more numerous, will be small arthropods and smaller unsegmented worms. Put into sheep that translates to 50-100 sheep - as small arthropods - under a hectare of ground.

Most of us have a good idea about worms - the gardeners' best friend. 
The most visible animals in the soil are the well known earthworms. Their importance has been well known ever since Charles Darwin spent years - or rather got his children to spend years, counting them and doing all sorts of things to them. HIs  Introduction to Earthworms.
When people realise there is more to the soil than just physics and chemicals, they often make the jump to thinking earthworms and fungi do the rest - ie break down plant debris and recycle carbon. (witness BBC programme 'Deep Down & Dirty: Science of Soil 2014 all clips). 

We can be forgiven for missing out the group know as the soil mesofauna. These are the animals just about visible like a speck of dust. You need a binocular microscope that multiplies about 2--40 times in order to see what they look like, Yet there are between 500 - 1000 for each earthworm and could be any of 50-100 species regularly found in most soils. 

There are three main groups  (need picture with all three in )
Springtails Are also called Collembolans and are quite 'primitive' arthropods; they keep plants healthy
Oribatid mites Are the denisons of the earth, dealing with the dead stuff. eating up leaves and other debris, by many minute mouthfuls. 
Nematode worms Are at first glance deadly dull, but when you find out out what they get up to, feeding on both plants but also on microbes and other animals...
Let's look at them in more detail...
They comprise 10-15 % of the mesoscopic soil animals and found near the surface and in leaf litter. They graze plant roots for plant debris and eat spores. The most characeteristic feature is the 'furcula' that is tucked underneath them.
The usual function ascribed to this organ - is escaping dry conditions or escaping predators (hence the British name 'Springtail' ) They

also have another sticky structure on the underside. It is this that gives them their latin name Collembola; this is ancient Greek kolla meaning glue and embolon meaning wedge or plug. The sticky plug  accidentally picks up fungal spores and carries them to the roots where the springtails are feeding. The spore then grows on the root and may form mychorhiza. 

Many collembolan species, mostly those living in deeper soil horizons, are parthenogenetic, which means they reproduce without sex. This favours reproduction of the same gene pool to the detriment of genetic diversity. Parthenogenesis is under the control of symbiotic bacteria of the genus Wolbachia, which live, reproduce and are carried in female reproductive organs and eggs of Collembola.[70] 
I saw live springtails in the root laboratories (now called 'Rhizolab') at East Malling Research (EMR) Station grazing roots -Photograh, taken during filming of BBC Gardener's World transmitted in Aug 2018.
Oribatid Mites
 Oribatid MiteOribatid mites constitute about 75% of all those speck sized creatures and colonise most soils, and are responsible for eating up all the leaf litter, so are very active mainly in the autumn. they may well be the most important Order of creatures on the planet. Without them plant debris would be washed to the sea, as it once was many millions of years ago. See Birth of the Earth for lots more  About 5% are predatory mesostigmatic mites,who eat nematodes and springtails.

While the species vary and the proportion of soil mesofauna and nematodes, the same groups appear throughout the world.

There are several sorts  of these unsegmented worms in the soil. Some are parasites on plant roots, and considered pests by farmers (eg Potato cyst nematode). Other, free living, nematodes decompose organic matter, other feed on bacteria and fungal spores. They are really just a bit of skin covering a gut. Whereas the massive earthworms are segmented and are hermaphroditic - ie carry both sets of sex organs, nematodes  have the usual male and female forms. 
SlugkillerMany people will know about nematodes which are purchased as slug killers. These enter the slug in a hole behind the head, and then start to multiply inside the slug. This is just like parasitic wasps do with caterpillars. Yet this led Charles Darwin to say: "I cannot persuade myself that a beneficent and omnipotent God would have designedly created parasitic wasps with the express intention of their feeding within the living bodies of Caterpillars".  Somehow nematodes killing slugs - the same as the parasitic wasps, doesn't bring out such ethical soul searching. ("Nobody likes us because we are slugs")
Dung BeetleThere are relatively few insects living in the soil - considering how many have colonised above ground. Ants and termites are surface to ground dwellers and there are several beetle and fly larvae that grow up in the soil. Perhaps the most important for soil life are dung beetlesThey can rapidly bury dung deep in the subsoil, opening up the subsoil and allowing roots to follow the channels deep into the soil profile.

Or it could those creatures with ..legs and a hard carapace that run round under stones and pots. They are herbivores eating the plant debris on the surface and pooing on the soil surface composted materials. For more on  Soil Animals & Global Warming

Which of the above do you
Small soil animal made a vital contribution to the evolution of soil, allowing the development od terrestrail life several hundred millions of years ago See Birth of the Earth . n the process they helped plants evolve into tree forms, by keeping them healthy and then dealing with their dead debris/ See 'How Plants Grew Up
The three main groups (mites springtails and nematodes) of soil animals are an incredible piece of the jigsaw of life in the soil, where they do all the running about keeping the rest of the web connected.  They make, along with other elements in the soil food web, a major contribution to Soil Health, increasingly seen as important in maintaining the soil in a condition that is sustainable for many years to come.

To give you an idea how important they are, a test carried out by CEH (Centre for Ecology & Hydrology) with NERC funding "found that although earthworms made up the majority of the soil animal biomass, other smaller but abundant organisms, such as collembolans and mites, used more of the 13C". This means that the Mesofauna are doing more in terms of moving carbon around, breaking down plant structures into simpler substances that plants can use, and into energy and gases, than the earthworms!

CEH also found "far from depending on decaying organic matter, soil creatures were rapidly using organic matter containing ‘pulse’ 13C which means that the soilfauna are feeding on plant extracts directly and not relying on breakdown products, like we tend to think they are there for." This fits with what I have seen - collembolans grazing roots and oribatids eating up leaves


The International Panel on Climate Change recognises the importance of soil in the carbon calculations, as soil holds more carbon than both the air and vegetation together. They generally believe the capacity to absorb carbon is limited because microbes will feed on it, and as the earth warms up will release more carbon dioxide and methane.   However the role of soil animals is poorly understood. One key paper shows how soil creatures can keep down microbes, thus saving the world! Check out Soil animals and Global Warming

CEH state "smaller but abundant organisms, such as collembolans and mites, used more carbon". This means that whereas most gardeners and environmentalists value worms, we should also be valuing the small soil flecks. We feel that earthworms do most of the work in the soil, and certainly in terms of heavy moving, but in terms of actually turning carbon round, we owe more to the small creatures - the soil mesofauna. Worms do the hard work, while the mesofauna all the moving around. So let's look a bit closer at them..

Let's take a closer look, from Top Down, Going Down and Deeper Down.