Deserts, swamps, moors
forests
nature offers an astonishing diversity of landscapes
to understand the origin of this wealth, we must dive a few feet underground
in what we call the soil
we explored our planet so much, we think we know all about what is under our feet
We know less than 10% of all organisms that live underground
each one of whom participate, in its way, in the creation of the soil
Thus, we are just starting to realize that plants form alliances
they are bound by complex unions
welcome to this subtile, surprising, and secret world
let's begin the journey at the crossroad between air and earth
on soil's surface
at this frontier, nature recycles what it has produced
tree leaves and organic wastes make up the "litter"
it is a thin layer of organic trash that covers the ground
J-F Ponge is a researcher for Paris' natural history museum
when he walks in a forest, he notices things which a random hiker wouldn't pay attention to
for a leaf to be decomposed, it takes time
time to be used and inhabited by micro-organisms, and transformed by animals
it can take between 1 to 10 years or more
depending on the soil's wealth, and on the climate
the litter's organic waste can't be consumed as such by underground organisms
they must be prepared, transformed into an edible food for the underground fauna
the essential actor of this transformation is a mushroom, almost invisible
but not for J-F Ponge's eye
this is a beech's leaf that has undergone the first stages of its transformation
you can see parts of different colors, brown parts, same color as the tree
and much whiter parts, results of the work of mushrooms we call "the white rot"
its property is to degrade tannin
it's very important because tannin is what makes the leaf unedible to animals
this area that was colonized by the white rot is turning into a little lace
created by the grazing of little animals
after the mushroom has done its job, the litter can be eaten away by most of its fauna
no bigger than a grain of rice, the springtail is partly responsible for the creation of this lace
smaller yet, the acarids are the most numerous specie
they take part in the leaves, and other waste's decomposition
woodlice and millipedes are recycling's veterans
they have dealt with organic matter for millions of years
feeding on all that is dead, vegetals, fruits, animals, insects...
the recycling fauna rids nature of its trash. It cleans it
the larvae which flies lay in cadavers are born, and feed on the putrefied flesh
ants carry vegetal wastes
and the beetles are in charge of the faeces of bigger animals, domestic or wild
the organic residus make up the humus
a very fertile compost located just beneath the litter
the humus layer is about 12 inches thick
this is where most of the soil's biological and chemical's life is born and evolves
how can life be so abundant in a place so dark, with so little oxygen
In a small piece of soil like this one, we'll find thousands of animals
several hundreds species
living together, on this tiny space
it's possible for 2 reasons: the amount of organic matter
which you find in the soil. It's the fuel and food for all these organisms
2nd, the fact that the soil is a 3 dimensional world
the surface, and the depths. And in this very fragmented world...
there is a lot of space, allowing animals and micro-organisms to co-exist without competing
pseudo-scorpios, sprintails, nematodes, acarids...
so many living things co-exist in this fragmented universe, with irregular shape
the humus' fauna occupies the space depending on their size and mobility
the smallest peacefully meets the biggest
at each scale of observation, one finds a life always more abundant
all the way to bacterias
among these tiny creatures, lives a giant, with the most important task
the earthworm
he airs the humus by digging galleries
from his escape from his egg, he starts digging
once adults, some earthworms can dig several feet into the ground
how can something with such a sluggish body dig so easily and so deep?
In fact, the soft part is mainly water, it's liquid
but this liquid is divided into segments
and the worm can empty segments and fill others
and as it fills segments, pressure mounts, and he can spread the soil open
and move forward
it's a unique ability
the earthworm secretes a mucus, used as a lubricant
it allows him to move more easily, sliding along
how can he move forward, as his body doesn't seem to have grips
the smooth appearance of the worm is an illusion
on his sides are retractable hooks that allow him to have a better grip
invisible, the worm is constantly at work
there are 3 species of worms
the epigeals live on the surface, in the litter
the endogeals live in the depths, and feed on soil
the anecic move back and forth through different layers
from the depths to the surface
for the anecics, everything happens at night
leaves are their favorite food. They take them underground to digest them in peace
as they bury the leaves, they play a critical part
they distribute organic matter through different layers of the soil
not only do they make the soil richer, they also plow it
with the same efficiency as our modern farming machines
They air it, fertilize it, allow it to live
there are up to 250,000 worms per acre
several hundred tons of soil go through their digestive tubes each year
as it digests, the worm mixes minerals of the soil with the vegetals it fed on
his excrements, called castings, are a powerful fertilizer
castings contain 4 times as much nitrogen, 7 times more phosphorus, and 3 times as much calcium as regular soil
thus, through their digestion, worms allow the return of many vegetal species to wastelands
worms feed on seeds, on the surface, which they mix with soil and then bury
this makes the soil's "seed band"
when these seeds are buried they can't sprout
but worms also come up, and form these castings on the surface
once in the light, the seeds inside can sprout
this is why, in a wasteland, you have this many plants on such small aread
each worm is a restless worker
from his birth, all he does benefits the soil
and luckily, he reproduces often
depending on the specie, a worm can lay from 3 to 100 cocoons a year
each cocoon contains about 10 eggs
thus, each year, an adult worm gives birth to 30 to 1000 youngsters
unfortunately, not all soils are good hosts for worms
all soils have unique properties which have evolved throughout their formation
a soil is rich or sterile depending on its ability to host fauna in its shallowest layers
the underground landscapes are as varied as those we see
at 1 mile distance, 2 different soils can tell Dr Jabiol 2 very different stories
different topographies, climates, life conditions, and evolutionary spans
the mix of all these factors mean that from one place to another, soils will change a lot
sometimes an evolution on just a few feet
sometimes evolutions on several hundreds of miles
and on earth's scale, a great diversity of soil, and of their properties
Dr Jabiol has left a study pit in the forest to join his colleague's pit
2,600 feet away, in a cereal field
- hey Joel, this pit is really deep
- less than a mile away, in the forest, I had under 20 inches, why is it so deep here?
- you were higher up, on a limestone soil
- we are lower here, with a soil about 60 inches deep
- it's because of the materials' erosion on the slope
- Here is the accumulation of deposits at the bottom of the slope
- it's a soil with great farming potential
- very deep, so a good water stock, good mineral stock
- and no stones, so the roots can easily go down to 40, 60 feet
- and a great biological activity with many worms that can go back and forth to the surface
from the soil's depth and structure depend its capacity to stock water
to the fauna's scale, a heavy rain is like a tsunami
water disolves the surface's organic matter, and bury it underground, through the worms' galleries
once it reaches waterproof layers, like clay, it accumulates into water reserves
Dr Jabiol and Michelin have moved 12 miles further
they reached a 3rd pit, in a scarcely wooded area
it reveals a soil very different from the last 2
- here a soil that is quite similar to a farming soil
- with a soil that is both loose and deep
- yes but this is sand
- yes, sedimentary sand
- this sand was left here by the sea millions of years ago
the soil is currently developing inside of it
- but sand has 2 great faults
- first, its acidity, meaning its lack of mineral and nutritive elements
- an acidity which prevents the presence of worms
- they can't live here, so there's no mix of organic matter
- the organic matter remains on the surface
- 2nd problem, the water moves through very fast
- it doesn't accumulate into reserves, it's a very dry soil in summer
- so you have 2 constraints on the flora
- the constraint of acidity, and that of dryness
these poor, dry, and acid soils in which worms cannot live
are nevertheless hosting life
on the surface, rustic plants are growing, adapted to severe conditions
in the litter live springtails, nematodes, and other usual recyclers
fortunately, soils also host bacterias, and when the worms can't survive, they create fertility
bacterias are micro-organisms, invisible by the naked eye
but they multiply so fast that they end up saturating the soil
whether it is poor or highly fertile
through their density, bacterias are also an essential actor in the soil's dynamic
a team from the national agronomic research institute
takes samples from different soils
in order to create a conservatory of these micro-organisms
- when we want to study the soil's micro-organisms, we look at the first 8 inches
- it's in this superficial layer that is concentrated the greatest abundance and diversity of micro-organisms
- in just 1 gram of this soil we have over 1 billion bacterias
- and in this billion, several million species
- so the greatest diversity is in this first layer
- and if you look at a field area like this one
- there is more living mass underground than there is above ground
- even with all the plants, even if you added 10 cows or 50 sheep
it is therefore not an overstatement to say that when conditions are good...
the earth is as alive underneath as on the surface
the samples are studied through an electronic microscope
it reveals how bacterias are organized in their habitat
Thinner than a hair, bacterias are lodged in the cracks between 2 patches of soil
these spaces are so narrow, that the protozoas, their predators, are too big to access them
in this shelter, the bacterias enter a slower life, and save their energy
when a worm comes by, they can escape this torpor
the mucus that it secretes and the provision of oxygen through the galleries stimulate the bacterias
once they are active again, they transform organic matter into minerals
which are needed for the growth of vegetals
- they can degrade and mineralize the organic matter within the soil
- changing it into nitrogen, phosphorus, or sulfur
- and these minerals are greatly involved in the growth of plants, as it provides a great fertility
to study the importance of bacteria for the soil's fertility, the NIMR has created an experimental platform
fully automated
the development of plants is measured in drastically controlled conditions
the NIMR team is growing alfalfa plants
in pots filled with a soil whose bacteria's density and diversity is monitored
- after 6 weeks, we can see that the pot with a regular soil has a good growth
- and in this pot with little bacteria diversity, there is little growth
- and these 2? It seems they have a better growth?
- it's the same experiment, but here we added fertilizer
- so the growth was accelerated by it
- and in spite of the fertilizer, a plant will develop less with fewer bacterias
the experiment shows that bacterias are crucial for a soil's fertility
with or without fertilizer, a soil's fertility is determined by them
scientists note another important correlation
the soil's fertility changes with the type of bacteria
- in its abundance and diversity of bacteria each soil has a type of DNA
- because this diversity depends on the soil's nature
- on its chemistry, on the climate, but also on the soil's history
- whether it was farmed, or a forest, or inside a city
- so in micro-biological terms, each soil is unique
the micro-organisms of a soil are its identity card
to this day, the NIMR has studied and identified over 10,000 samples
these samples are archived. They will be references to study the evolution of the soil with time
we are far from having an exhaustive inventory of interactions between bacterias and plants
recently, it was found that plants, at their roots, form partnerships with bacterias
- we have 3 experimental systems where we observe the root development of 3 samples
- a clover, a peas-plant, and rapeseed
- we have root architectures that change a lot between each plant, with different densities and capacities to colonize
- if we look more closely at the pea and the clover
- you can see these small balls on the roots, called nodules
- in these nodules are thousands of bacterias of the same specie
- the plant helps these bacterias grow, through its sap
- and in return the bacterias will fix the atmosphere's nitrogen for the plant to assimilate more easily
- so this is a symbiotic relationship, each one benefiting from the other
- and a relationship which increases the soil's fertility and the plants' growth
I give you what you don't have, and you bring me what I lack
between roots and bacterias, it's give and take
they feed each other for greater results
let's dive a little deeper underground, along the roots
we know that roots are used to stabilize plants in the ground
that the root networks stabilize terrains
the deeper they dive, the thicker they grow
the more they make the soil compact
in the soil, the roots get the nutrients necessary for the plant
these exchanges make the soil more acidic or alkalin
they have a direct influence on the soil's quality
plants' roots can reach unimaginable proportions
some trees, adapted to drought, like the eucalyptus, are intriguing to scientists
Dr Jourdan from the CIRAD, has converted a Brazilian eucalyptus plantation into an open sky research lab
he's focusing on the root's specificity depending on the soil's layer in which it grows
- last centuries botanists described mainly the upper parts of the trees
- rarely the roots
- more recent studies are trying to know more about their physiology
- their mortality...
- and as we study deeper we expect to find different dynamics and micro-organisms
the roots have different shapes, diameters, and they have different roles
the architecture of a tree's root network is complex
- most people are aware of the big roots, those attached to the trees
- like this one here
- but these roots carry smaller ones
- and attached to these middle roots we find tiny roots
- these are directly connected to the soil, and they supply water and minerals
- we are mostly interested in the tiny roots, their functioning, their lifespans
- in different layers of the ground, various depths
Dr Jourdan is studying the life cycle of surface roots
He places a scanner, 20 inches below ground
- here we scan roots every 15 days
- over a year, it allows us to see a dynamic of development
- to estimate growth rates, which are about 0.4 to 0.8 inches a day
- and to calculate their lifespans
- here, about 8 to 10 months
- at some point, these roots take part in creating litter
- which is injected into the soil at different depths
through the thinnest roots, constantly dying and regenerating...
trees add organic matter to the soil
just like the worms who bury the leaves they eat
how deep do the roots of eucalyptus go, in spite of periodic droughts?
Dr Jourdan dug a 65 ft deep well
it opens the door to a little known world
- Here we are 50 ft below the surface
- and we find a great number of roots behind the window
- so 50 ft, if you add the tree's 82 ft height,
- it means the roots are 130 ft distant from the tree top
- it has surprised us a lot
the deepest roots undergo strong pressure
they are probably different from shallow roots
but nothing in their appearance indicates any difference
if no difference is visible on the outside, it may be on the inside
after selecting the best samples, dr Jourdan slices through them
he then compares them to shallow roots that were also sliced
- on this slice of roots you can see the internal structure
- you can see that the vessels that carry the water
- make up the biggest part of the root
- and when you compare it with a shallow root
- these vessels are much smaller for the same diameter
- then, the deep root has an imperfect shape
- we can tell it is constrained by the soil's pressure
- in order to grow, it has to get disformed
- these roots allow the trees to get water from much deeper, which is very useful
- especially in dry periods, and it opens new perspectives to us, in light of climate changes
one of the strategies that some plants have developed to increase their chances of survival
was to send some specific roots as deep as possible
throughout evolution, an unlikely ally has become necessary to the root networks of most plants
Dr Selosse, from Paris natural history museum, studies the role of mushrooms in soil dynamics
they seem to us scarce, fragile, and short-lived
but this is only the visible part of a secret reign, whose importance is largely unknown
- this part here, is what is commonly known as the mushroom
- but for the biologists that's just one part, a reproductive organ
- which is connected to the mycelium, a long-lived and much wider network
- inside the soil or inside dead wood for instance
- meaning that even when you don't see this fruit part
- the mycelium is still there, and it feeds the mushrooms
- and they contribute to the soil's biology and its dynamics
some mushrooms' mycelium spreads a filament network through the soil
until it gets in touch with roots
throughout evolution, the union between mushrooms and roots became the norm
since 90% of vegetal species aren't autonomous
they need a mushroom to extract from the soil the phosphate and nitrogen they need
the detail of this symbiosis between mushroom and root is only visible through a microscope
- we see a beautiful root with many lateral growths that are colonized by mushrooms
- different mushrooms, black, white, and they create a sort of sock around the root
- at this level, the root will start expanding more, which increases the connection with the mushroom
- the mushroom is also very present in that area
- meanwhile exploring the soil, looking for water, minerals, for it and for the plant
the symbiotic union between root and mushroom is called the mycorrhiza
when the mycellium connects with a root,
it weaves a sock of filaments around it
it then penetrates the root and builds a network that fits the position of the cells
there exist another form of mycorrhiza, more common than the last one
here, the mycelium penetrates even the root's cells
the plant decuplates its surface for exploring the soil
and the mushroom feeds on the sugar produced by the root
other mushrooms have yet another strategy
their mycelium looks for food on the soil's surface
a rigid and consistent food, which only they can digest
- here you have a piece of wood which is slowly integrating the soil
- and when you take it apart, you can see that the wood is completely decomposed
- all that's left is this white and sluggish component
- and what was eaten and digested by mushrooms is just that
- a composite which is half of the mass of the wood and makes it still hard
- mushrooms are the only organism able to digest this wood
- only they can use it and leave behind cellulose
- which will be eaten by other animals or bacterias
- and these mushroom are very visible here, integrated into a white mycelium
most mushrooms looking for food on the surface are dealing with the litter
much less tough than wood
these mushrooms' mycelium is bent over itself, into millions of tiny filaments
each gram of litter can contain several miles of filaments
these vast and diverse ramifications...
make most leaves and wastes available to the mushroom
you know these, these are the "white rot". As they digest tannin,
they make leaves edible for the fauna living in the litter
thus, the soil is built around a huge cycle, necessiting the intervention of many actors
the joint action of the fauna, the bacterias, the roots and mushrooms
marks the soil with a succession of layers that can be found while digging
these layers allow these pedologists to travel back in time
all the way to the soil's origin, the mother rock
- well, here is a nice pit, nicely dug
- we're going to clear out the corners
- it will allow us to see into the depths, especially here, something hard
- this rock here, this is the starting point of pedogenesis, the soil's formation
- which was formed in contact with this rock and the biosphere
- the roots, the vegetals, that also impacted the rock
- and all this hidden life in the soil
- what we don't see, the microbs, the micro animals...
- all this creates chemical reactions
- which little by little, through thousands of years, will transform this rock
- tens of thousands of years, hundreds of thousands of years...
but from this long process, involving rocks and organic wastes
the soil, this thin skin of out planet, is made fragile
we are only just starting to acknowledge its complexity
interactions between plants and animals are proving much more complex than we had thought
the inventory of interactions and links that unite the actors that produce the soil
is just coming into sight
there is much more to discover before we can understand our planet
which makes us who we are
