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The soil biome is the community of plants and animals that occur naturally in soil, sharing common characteristics specific to it. The biomass of soil refers to the the total weight of these organisms in a given volume. The biodiversity is all the different kinds of life you'll find
Biomass
Biomass
There is clearly a lot of biomass in soil.
Let’s say the bones of the whale are equivalent to the mineral content of soil. How much does this weigh? (All that clay must be heavy - a rough estimate says mineral constituents equal 90-95% weight of soil living mass the rest..). Along with the dog’s worth of worms (macrofauna), there is another dog’s worth of small animals.
Tests carried out by CEH (Centre for Ecology & Hydrology) "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 C-13". This means that the small creatures making up a similar mass are doing more work. In terms of moving carbon around, breaking down plant structures into simpler substances that plants can use, and into energy and gases, they are as metabolic as the earthworms!
There is also a biomass consisting of plant bits both living and dead, and microorganisms like fungi and bacteria along with protists, as well as loads of nematodes - 0.02Gt of C = 10X that for birds.
Biomass is related to the live weight and volume of soil
Weight ( W) is a product of mass (M) and gravity (G). W = M * G.
Weight
Weight
There is an awful lot of soil and it is very heavy. Have you ever stuck a spade in the ground and tried to lift it. Or put a few spadefuls into a bag or bucket and tried to carry it a few hundred metres. Your arms feel they are dropping off. ou try moving a pile a metre high takes some shifting – as there will be several tonnes in that pile. No wonder there are so many mechanical diggers about. But why does it seem to weigh even more than a lump of rock? In fact, a cubic metre weighs in at about 1500kg, or 1.5 tonnes, for compacted soil. That is the weight of an average grey whale, hippopotamus, large car, or two dairy cows.
We know about worms and they weigh in around 20-25kg/m3 in normal soil. That is around the weight of a Labrador dog or lynx. Can you imagine how that much animal weight can be down there just beneath our feet in that whale crush. There is twice the weight of worms (0.2Gt of C) in the world than all domesticated animals. Yet there is the equivalent of dog down there in animal terms doing something – but what? And where is there any space to live for all this life, in that heavy lump of soil?
Recently somebody calculated the weight of springtails - one of the main small arthropod groups in virtually all soils. “Here, using a global dataset representing 2470 sites, we estimated the total soil springtail biomass at 27.5 megatons (=million metric tons) carbon, which is threefold higher than wild terrestrial vertebrates“. (Potapov 2023) That latter figure doesn’t include domestic dogs which come in at about 20 megatons. Picture all those dogs running around on the ground, while imagining more weight springing around just below the soil surface.
Mass and volume (V)) are related by density or D.
D = M/V.
Volume
Volume
You can just about believe that there could be the weight of a crushed car structure compacted into each cubic metre. It must be very heavy. If you have tried moving it you will know it is heavy. Have you ever tried digging and shifting soil? What sort of stuff can weigh so much in such a small volume? In a typical back garden of 10X10 metres, you could be looking at 100 crushed whales in terms of live weight . What is soil - animal, vegetable or mineral? Or all three?
Most of us we say that there is a fair bit of mineral – various bits of rocks, but it is harder to imagine how animal and vegetable that as heavy – as a crushed whale in a cubic metre. Yet, a lot is animal and vegetable, as there is more weight of life (biomass) underground than above ground. 80% of all the biomass on earth - including all the overground beasts and bugs, along with all the plants and trees - is below ground. But how much do we know about that underground biomass - four times as much as above? Can we picture what is making up that mass of biology moving round? On the one hand soil is very heavy so we think it must be dense, yet on the other there must be a lot of spaces for things to live and move. How can this be? That is one of the amazing properties we hope to unravel, as time goes on.
Biodiversity
Biodiversity
Nearly 2/3 of the world's biodiversity lives in the soil (Anthony et al 2023)" Soil is likely home to 59% of life including everything from microbes to mammals, making it the singular most biodiverse habitat on Earth. Our enumeration can enable stakeholders to more quantitatively advocate for soils in the face of the biodiversity crisis. "
“We therefore estimate an approximately two times greater soil biodiversity than previous estimates, and we include representatives from the simplest (microbial) to most complex (mammals) organisms. Enchytraeidae have the greatest percentage of species in soil (98.6%Soil), followed by fungi (90%) and plants 85%”
Many of us have an idea that "one gram of soil can contain several billion bacteria from thousands of different species". But there is a lot more to biodiversity than microbes in the soil. These are billions of small animals, with heads, eyes and legs crawling around in soil. There are an estimated 14 quadrillion (14,000,000,000,000,000) of them in UK soils. Soils are moving all the time. But the creatures are hard to see, and even harder to identify. What are they all doing? Most of us will not have heard of them, let alone be able to identify any. Yet they are some of the most important creatures – not on earth – but in earth. The evolutionary biologist E.O. Wilson who believed we need an international body like we have for climate change for biodiversity, and said that the most important species we should consider were oribatids. He said most people will never have heard of them, but they are vital here in the soil.
Other fabulous figures of soil life include over 10 million nematodes can live beneath a square meter of soil along with 500-200,000 insects. Fungi will occupy 1-3/tenths the area round roots. A teaspoon of healthy soil can contain between 100 million and 1 billion microbes, one in ten of which are called archaea and resemble bacteria but are more stable and can resist extreme conditions. For this amount of life to exist means this soil is not like a mixture ground out by a food processor, but a multitude of stable structures, with life running round in it and looking after it too.
There is twice as much soil life under trees (forest) as there is under grass (pasture), which is twice that under ploughed (arable) land. See Countryside Survey 2007 Chapter 8 for details.
Video of presentation by Frank Ashwood re Biodiversity of soil invertebrates
There is twice as much life in soil under trees compared with grass (pasture), itself twice that under ploughed (arable) land.
Number of organisms belonging to a given size group times their linear size cubed varies very little.
Biomass Equivalence
Biomass Equivalence
There are not many laws in biology – like there are in chemistry and physics – as the objects keep moving about in unpredictable ways. However there is one called the law of biomass equivalence. M.S. Ghilarov, in his paper of 1944, on soil creatures, set out his Biomass Equivalence Rule: "The biomass of soil organisms of different natural body-size groups is approximately of the same order of magnitude: the product of the number of organisms belonging to a given size group times their linear size cubed varies very little." Ghilarov checked out "natural" body-size groups of soil organisms from bacteria, millipedes, potworms, mites and springtails, nematodes and insects. They each occupy approximately equal size intervals on a logarithmic scale (where the numbers 10, 100, 1000, 10000, and 100000 would be equally spaced). These equal logarithmic intervals of body-size contain approximately equal biomasses for soil (and also) sea organisms. While claims to this law were first made about sea-dwelling creatures, it is now acknowledged that the discovery of this law – applicable among all ecosystems - belongs to Ghilarov. We will meet him again in this story of soil evolution. His insights involve soil in the origin of insects.
Biomass of soil animals
Biomass of soil animals
All of the great five kingdoms. all are represented in soils – Monera, Protista, fungi, plants and animals. A meta-analysis of several studies of soil taxa showed soil fauna on the basis of biomass of mites, collembolans, enchytraeids, nematodes, and earthworms throughout the world under seven biomes, including desert, tundra, temperate grassland, temperate deciduous and coniferous forest, and tropical forest (Wu et al 2011 ). This was at a ‘class’ level of taxonomy – ie groups rather than phyla, but it did not find any group that was truly cosmopolitan – ie everywhere. Very few groups (called tazon) were found in more than 3-4 locations. But, taxonomists love to argue about ‘taxa’ - who is going to be in their group.
This study found 20 phyla of soil animals sampled from 11 locations covering a range of biomes and latitudes around the world. The locations were dominated by microarthropods or dominated by nematodes. Arthropods dominate primarily forests with lower soil pH, root biomass, mean annual temperature, low soil inorganic N and higher C:N, litter and moisture. Nematode-dominated locations were mostly grasslands. There is no grass at this period. But lots of arthropods and nematodes. However, at a much more detailed level, no studies have examined the distribution of small soil animals at the species level at a global geographic scale.
Some kingdoms came and stayed. These soils have existed in various but similar ways for over 300 million years. For that the creatures must have withstood some pretty big attacks, and found ways to adapt. These creatures have been around for hundreds of millions of years, surviving a few ‘extinctions’ along the way.
How on earth did all this lot get here? Few have asked that question. Here we try to answer it and explain what this vast biomass is doing
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