Soil Is Alive: The Living World Beneath Our Feet — SLICE EARTH — FULL DOC

[00:00:00] Speaker 1: Desert, swamp, moor, forest. Nature offers a dazzling array of landscapes. To understand where this wealth of diversity comes from, we must delve a few meters underground into what we commonly call the soil. We have explored the planet so thoroughly that we think we know everything that exists right below our feet. We know less than 10% of the organisms that populate the underground world and that, each in their own way, take an active part in creating the soil. We are just beginning to discover that plants form partnerships and create complex unions. Welcome to this subtle, surprising, and secret world. Let us begin our journey at the threshold, where the realms of air and earth meet, on the surface of the ground. Here in this border zone, nature recycles what it has produced. Leaves from trees and plant debris form what we call litter. In other words, the thin layer of organic detritus that covers the ground. Jean-Francois Pange is a researcher at the Museum of Natural History in Paris. Jean-Francois Pange : As an expert in soil life, when he walks through the forest, he sees things an ordinary hiker would barely notice. [00:02:18] Jean-Francois Pange: Jean-Francois Pange : For a leaf to decompose, it needs enough time [00:02:24] Speaker 3: for microorganisms to inhabit and use it, and for animals to transform it. [00:02:31] Jean-Francois Pange: Jean-Francois Pange : The plant debris of the litter is not consumed [00:02:48] Speaker 1: as it is by the creatures of the underworld. It must be refined, in other words, prepared, in order to make satisfactory food for underground life forms. The crucial element in the transformation of litter is a fungus which is all but invisible, except to Jean-Francois Pange's eyes. [00:03:29] Jean-Francois Pange: Jean-Francois Pange : Here is a beech leaf that has undergone the initial stages of transformation. Jean-Francois Pange : What do we see on it? Different colored zones, brown zones that are the same color as the leaves still on the tree and much lighter zones. Jean-Francois Pange : This is the result of certain fungi we call white rot, which can break down tannins. [00:03:48] Speaker 4: Jean-Francois Pange : This is important because tannins make the [00:03:52] Jean-Francois Pange: leaves inedible for animals. Jean-Francois Pange : The entire zone that's been colonized by white rot is being turned into a sort of lace. Jean-Francois Pange : It's the result of all those tiny creatures munching. [00:04:08] Speaker 1: Jean-Francois Pange : Once the fungus has done its work, the litter can then be eaten by most soil creatures. Jean-Francois Pange : No larger than a grain of rice, surface-dwelling springtails are partially responsible for changing leaves into lace. Jean-Francois Pange : Smaller yet, mites are the most numerous. They play their own part in the decomposition of leaves and other organic waste. Jean-Francois Pange : Wood lice and millipedes are veterans of recycling and have been attacking organic matter for millions of years. Jean-Francois Pange : By feeding on everything that has died, plants, fruit, animals, and insects, the creatures that decompose and recycle rid nature of its waste, they clean it. Jean-Francois Pange : Fly larvae, laid in the bodies of dead animals, hatch and devour the rotting flesh. Jean-Francois Pange : Ants carry organic detritus away. Jean-Francois Pange : And beetles take care of the extramurant that larger animals, both wild and domesticated, leave behind. Jean-Francois Pange : The remains of organic matter form humus, a highly fertile compost found just below the litter. Jean-Francois Pange : The layer of humus is about 30 centimeters thick. Jean-Francois Pange : This is where most of the soil's biological and chemical physical processes originate and develop. Jean-Francois Pange : How can life thrive in an environment with no light and little oxygen? [00:06:28] Jean-Francois Pange: Jean-Francois Pange : There are several thousand individuals in a clump of dirt about this size. Jean-Francois Pange : Like what I'm holding here in my hands. Jean-Francois Pange : Several hundred species coexist. Jean-Francois Pange : How is this possible on such a tiny surface? Jean-Francois Pange : There are actually two reasons. Jean-Francois Pange : On the one hand, there's a quantity of organic matter contained in the soil. Jean-Francois Pange : That's the fuel or food source for all these organisms. Jean-Francois Pange : Secondly, there's the fact the soil is a three-dimensional realm with both surface and depth. Jean-Francois Pange : In this infinitely fragmented world, there's a great many spaces that allow Jean-Francois Pange : Animals and microorganisms to live together without bothering each other or having to compete. [00:07:10] Speaker 1: Jean-Francois Pange : Pseudoscorpions, springtails, roundworms, mites. Jean-Francois Pange : A multitude of living creatures coexist in a fragmented world of irregular shapes. Jean-Francois Pange : The humus teems with living creatures that fill the space according to their size and mobility. Jean-Francois Pange : The tiniest cross paths harmoniously with the largest. Jean-Francois Pange : With each degree on the scale of observation, Jean-Francois Pange : We discover an ever greater abundance of life. Jean-Francois Pange : Right down to bacteria. Jean-Francois Pange : Among these minuscule creatures, Jean-Francois Pange : One giant reigns supreme. Jean-Francois Pange : The earthworm. Jean-Francois Pange : It aerates the humus by burrowing tunnels. Jean-Francois Pange : No sooner does the earthworm hatch from its egg, Jean-Francois Pange : Then it begins burrowing. Jean-Francois Pange : By adulthood, Jean-Francois Pange : Certain types of earthworms Jean-Francois Pange : Can bore up to one meter deep into the soil. [00:08:32] Speaker ?: Jean-Francois Pange : The earthworms can bore up to one meter deep into the soil. [00:08:35] Speaker 1: Jean-Francois Pange : How can a creature with such a soft body dig so easily and deeply? [00:08:40] Jean-Francois Pange: Jean-Francois Pange : Actually, the soft part is mainly water, liquid. Jean-Francois Pange : But the liquids divide it into segments. Jean-Francois Pange : And the earthworm is able to empty some segments and fill others. Jean-Francois Pange : When the segments fill, Jean-Francois Pange : It increases pressure. Jean-Francois Pange : And that's what makes it possible Jean-Francois Pange : For the worm to push the soil out of the way and move forward. Jean-Francois Pange : It's a truly original feature in terms of soil life. [00:09:06] Speaker 1: Jean-Francois Pange : The earthworm secretes a mucus that acts as a lubricant Jean-Francois Pange : And makes it easier for it to move through the soil. Jean-Francois Pange : Yet how can it move forward, Jean-Francois Pange : When its ringed body seems to have no way of gripping? Jean-Francois Pange : The worm's smooth appearance is just an illusion. Jean-Francois Pange : Along its sides, Jean-Francois Pange : Retractable bristles allow it to grasp the soil Jean-Francois Pange : And give it something to push off from. Jean-Francois Pange : Out of sight, Jean-Francois Pange : The earthworm works the soil relentlessly. [00:09:51] Speaker ?: Jean-Francois Pange : The earthworms [00:09:52] Speaker 1: Jean-Francois Pange : There are three types of earthworms. Jean-Francois Pange : Epijaics live on the surface in the litter. Jean-Francois Pange : Endojaics live below ground. Jean-Francois Pange : And feed on soil. Jean-Francois Pange : Aniseics pass through the different layers of soil Jean-Francois Pange : As they burrow vertically back and forth Jean-Francois Pange : Between the subsoil and the surface. Jean-Francois Pange : For Aniseics, everything happens at night. Jean-Francois Pange : Leaves are their favorite food. Jean-Francois Pange : They drag them underground to digest in peace. Jean-Francois Pange : By burying leaves, Jean-Francois Pange : They play a crucial role Jean-Francois Pange : In the distribution of organic matter Jean-Francois Pange : Throughout the various layers of soil. Jean-Francois Pange : Not only do earthworms enrich the soil, Jean-Francois Pange : They also till it just as effectively Jean-Francois Pange : As our most state-of-the-art farm equipment. Jean-Francois Pange : They aerate it. Jean-Francois Pange : They fertilize it. Jean-Francois Pange : In some, they give it life. Jean-Francois Pange : One hectare contains up to 250,000 earthworms. Jean-Francois Pange : Several hundred tons of earths Jean-Francois Pange : Pass through their digestive tract each year. Jean-Francois Pange : As it eats, Jean-Francois Pange : The earthworm takes in minerals from the soil Jean-Francois Pange : And combines them with the plants it ingests. Jean-Francois Pange : Worm excrement, called castings, Jean-Francois Pange : Is a potent blend of fertilizer. Jean-Francois Pange : On average, Jean-Francois Pange : Worm castings contain four times more nitrogen, Jean-Francois Pange : Seven times more phosphorus, Jean-Francois Pange : And three times more calcium Jean-Francois Pange : Than the surrounding soil. Jean-Francois Pange : Thus, through their digestive action, Jean-Francois Pange : Earthworms make it possible Jean-Francois Pange : For numerous plant species Jean-Francois Pange : To re-emerge in fallow land. [00:12:26] Jean-Francois Pange: Jean-Francois Pange : Earthworms eat seeds up on the surface Jean-Francois Pange : And blend them with the soil. Jean-Francois Pange : Then they bury them to make up Jean-Francois Pange : What we call the soil seed bank. [00:12:34] Speaker 3: Jean-Francois Pange : As long as the seeds are in the soil, [00:12:37] Jean-Francois Pange: Jean-Francois Pange : They can't sprout. [00:12:38] Speaker 3: Jean-Francois Pange : However, the earthworm [00:12:40] Jean-Francois Pange: Jean-Francois Pange : Also comes up out of the soil Jean-Francois Pange : And leaves its castings on the surface. [00:12:43] Speaker 3: Jean-Francois Pange : Once they're in the light, [00:12:45] Jean-Francois Pange: Jean-Francois Pange : The seeds inside can sprout. Jean-Francois Pange : That's why there are so Jean-Francois Pange : So many plants Jean-Francois Pange : On such a small surface [00:12:50] Speaker 1: Jean-Francois Pange : Long before man invented farming, Jean-Francois Pange : Sowing seeds Jean-Francois Pange : In hopes of a greater harvest, Jean-Francois Pange : Earthworms had already Jean-Francois Pange : Tilt and retilled every centimeter of soil. Jean-Francois Pange : They made it fertile and favorable Jean-Francois Pange : For plants to grow and thrive. Jean-Francois Pange : The humble tireless engineer Jean-Francois Pange : And technician that is the earthworm Jean-Francois Pange : Must stand up to pitiless predators. Jean-Francois Pange : Above ground, Jean-Francois Pange : Earthworms are the main prey Jean-Francois Pange : For some breeds of birds. Jean-Francois Pange : And below ground, Jean-Francois Pange : They fall victim to the biggest Jean-Francois Pange : And strangest of all European insects. Jean-Francois Pange : The mole cricket, Jean-Francois Pange : Which can grow as long as 10 centimeters. Jean-Francois Pange : This creature, Jean-Francois Pange : Little known except to gardeners Jean-Francois Pange : Whose plots they wreak havoc in, Jean-Francois Pange : Lives almost exclusively underground. Jean-Francois Pange : It gets its name from its grasshopper-like body Jean-Francois Pange : And powerful front legs Jean-Francois Pange : That can clear earth as well as any digger. Jean-Francois Pange : The mole cricket can eat like a horse. Jean-Francois Pange : It devours everything in its path. Jean-Francois Pange : Plant roots, Jean-Francois Pange : And if it wasn't bad enough dealing Jean-Francois Pange : With their natural enemies, Jean-Francois Pange : Earthworms now have a previously unknown Jean-Francois Pange : And extremely unsettling predator to reckon with. Jean-Francois Pange : The platyhelminthi, Jean-Francois Pange : A flat worm from the other side of the globe Jean-Francois Pange : That keeps expanding its territory. Jean-Francois Pange : Jean-Loup Justine Jean-Francois Pange : From the Museum of Natural History in Paris Jean-Francois Pange : The platyhelminthi is one of the few scientists Jean-Francois Pange : Studying the spread of this unwelcome pest. [00:15:13] Speaker 5: Jean-Francois Pange : Over the past few years, Jean-Francois Pange : We've witnessed a new predator appear Jean-Francois Pange : In Europe and France, Jean-Francois Pange : Platyhelminthi, or flatworm, Jean-Francois Pange : Which lives on the ground. Jean-Francois Pange : It's mainly active at night, Jean-Francois Pange : So to see it during the daytime, Jean-Francois Pange : You have to look under rocks or pieces of wood. [00:15:27] Speaker 4: Jean-Francois Pange : The platyhelminthi [00:15:29] Speaker 1: Jean-Francois Pange : Jean-Lou Justine is worried. In some regions, the proliferation of these carnivorous flatworms is starting to have a devastating impact on local earthworm populations. Platyhelminthi are a branch of the animal kingdom with marine origins. Jean-Francois Pange : Some breeds emerged from the water Jean-Francois Pange : And became land dwelling Jean-Francois Pange : In specific regions of the earth. [00:16:11] Speaker 5: Jean-Francois Pange : Flatworms managed to adapt Jean-Francois Pange : To a land environment mainly in countries Jean-Francois Pange : Such as Australia, New Zealand, Jean-Francois Pange : In the south of South America, Jean-Francois Pange : In other words, several spots Jean-Francois Pange : In the south of the southern hemisphere. [00:16:23] Speaker 4: Jean-Francois Pange : Recently, these creatures [00:16:27] Speaker 5: Jean-Francois Pange : Have been turning up in Europe and France. Jean-Francois Pange : They didn't get here on their own. Jean-Francois Pange : Most of the time, Jean-Francois Pange : They've hitched along Jean-Francois Pange : With the transport of plants and earth Jean-Francois Pange : Like stowaways. [00:16:53] Speaker 1: Jean-Francois Pange : The invasive species Jean-Francois Pange : Is indigenous to South America. Jean-Francois Pange : More specifically, Jean-Francois Pange : Our new predator most likely Jean-Francois Pange : Comes from southern Brazil. Jean-Francois Pange : The world's only scientific study Jean-Francois Pange : Of the distribution Jean-Francois Pange : Of this type of flatworm Jean-Francois Pange : Was carried out in France. Jean-Francois Pange : The study has shown Jean-Francois Pange : That it's no coincidence Jean-Francois Pange : These predators Jean-Francois Pange : Have colonized the Atlantic coast. Jean-Francois Pange : Because they originate Jean-Francois Pange : In humid tropical regions, Jean-Francois Pange : They can't survive in dry climates. Jean-Francois Pange : Though this flatworm Jean-Francois Pange : Has only been recorded Jean-Francois Pange : In France for about 10 years, Jean-Francois Pange : It has already become Jean-Francois Pange : A threat to soil integrity. [00:17:35] Speaker 4: Jean-Francois Pange : It's not hard to understand [00:17:43] Speaker 5: Jean-Francois Pange : That if earthworms disappear Jean-Francois Pange : Or drop in number, Jean-Francois Pange : It will disrupt the soil Jean-Francois Pange : And soil ecology. [00:17:49] Speaker 4: Jean-Francois Pange : Here's a flatworm [00:18:03] Speaker 5: Jean-Francois Pange : Hiding under this rock Jean-Francois Pange : To escape the heat of the sun. Jean-Francois Pange : As you can see, Jean-Francois Pange : It's a few centimeters long. Jean-Francois Pange : We see it's Jean-Francois Pange : It's head moving here. Jean-Francois Pange : The other end is its tail. [00:18:15] Speaker 4: Jean-Francois Pange : You might think its mouth [00:18:18] Speaker 5: Jean-Francois Pange : Is up near its head. Jean-Francois Pange : But it's not. [00:18:21] Speaker 4: Jean-Francois Pange : The mouth of the [00:18:23] Speaker 5: Jean-Francois Pange : Flatworm is located [00:18:25] Speaker 4: Jean-Francois Pange : Under its body in the middle. [00:18:27] Speaker 5: Jean-Francois Pange : Here, this light colored part. [00:18:29] Speaker 4: Jean-Francois Pange : A pharynx comes [00:18:31] Speaker 5: Jean-Francois Pange : Out of its mouth Jean-Francois Pange : And envelops the earthworm Jean-Francois Pange : And digests it from the outside. [00:18:36] Speaker 4: Jean-Francois Pange : When hunting, [00:18:42] Speaker 1: Jean-Francois Pange : The flatworm glides along Jean-Francois Pange : On a slimy mucus trail Jean-Francois Pange : It excretes from its abdomen. Jean-Francois Pange : To capture its prey, Jean-Francois Pange : It presses close Jean-Francois Pange : And wraps itself around it, Jean-Francois Pange : Immobilizing it Jean-Francois Pange : And slowly devouring it Jean-Francois Pange : With its abdominal mouth. Jean-Francois Pange : The predator's victims Jean-Francois Pange : Are entirely dissolved Jean-Francois Pange : By its gastric juices. [00:19:24] Speaker 5: Jean-Francois Pange : These flatworms Jean-Francois Pange : Are hermaphrodites, Jean-Francois Pange : Which means each individual Jean-Francois Pange : Is both male and female. Jean-Francois Pange : It takes two hermaphrodites Jean-Francois Pange : To make babies. Jean-Francois Pange : Flatworms Jean-Francois Pange : Flatworms can reproduce Jean-Francois Pange : Very quickly, Jean-Francois Pange : At about the age of two weeks. [00:19:41] Speaker 4: Jean-Francois Pange : At that point, [00:19:42] Speaker 5: Jean-Francois Pange : It makes an egg coon Jean-Francois Pange : That contains Jean-Francois Pange : Between five and ten Jean-Francois Pange : Little fully formed flatworms. Jean-Francois Pange : So, we can see Jean-Francois Pange : How quickly Jean-Francois Pange : They can invade an area. Jean-Francois Pange : After just a couple of weeks, Jean-Francois Pange : One flatworm has already Jean-Francois Pange : Really produced Jean-Francois Pange : About ten offspring. [00:19:58] Speaker 1: Jean-Francois Pange : There is no effective Jean-Francois Pange : Way to fight against Jean-Francois Pange : This flatworm. Jean-Francois Pange : It is too recent Jean-Francois Pange : In France to have Jean-Francois Pange : Any predators. Jean-Francois Pange : With its rapid Jean-Francois Pange : Rate of reproduction Jean-Francois Pange : And the fact there's Jean-Francois Pange : Currently nothing Jean-Francois Pange : To keep it in check, Jean-Francois Pange : The flatworm population Jean-Francois Pange : Is expanding faster Jean-Francois Pange : And faster. Jean-Francois Pange : In some regions, Jean-Francois Pange : Flatworms have decimated Jean-Francois Pange : The earthworm population Jean-Francois Pange : To such an extent Jean-Francois Pange : That the soil has deteriorated. [00:20:51] Speaker 4: Jean-Francois Pange : This soil is flooded. [00:20:54] Speaker 5: Jean-Francois Pange : That's what happens Jean-Francois Pange : When the earth is impermeable Jean-Francois Pange : And won't let rainwater through. Jean-Francois Pange : The role of the earthworm Jean-Francois Pange : Is to make holes in the soil Jean-Francois Pange : To allow rainwater to penetrate. Jean-Francois Pange : In Great Britain, Jean-Francois Pange : Another type of flatworm Jean-Francois Pange : Has invaded the British Isles. Jean-Francois Pange : It has diminished Jean-Francois Pange : The earthworm population, Jean-Francois Pange : There are fewer earthworms, Jean-Francois Pange : And the soil is less permeable. Jean-Francois Pange : And there are more Jean-Francois Pange : And more flooded zones of this sort. [00:21:22] Speaker 1: Jean-Francois Pange : How far will these flatworms go? Jean-Francois Pange : Will we see entire Jean-Francois Pange : Regions made sterile Jean-Francois Pange : Because these intruders Jean-Francois Pange : Have exterminated Jean-Francois Pange : The local worm population. Jean-Francois Pange : The earthworm's high fertility rate Jean-Francois Pange : May allow it to survive Jean-Francois Pange : The flatworm's appetite. Jean-Francois Pange : Depending on the species, Jean-Francois Pange : An earthworm lays between Jean-Francois Pange : Three and one hundred cocoons per year. Jean-Francois Pange : Each cocoon contains about 10 eggs. Jean-Francois Pange : Consequently, Jean-Francois Pange : An adult earthworm can Jean-Francois Pange : Produce between 30 and 1000 offspring each year. Jean-Francois Pange : The earthworms Jean-Francois Pange : Unfortunately, not all soils Jean-Francois Pange : Are favorable to the propagation Jean-Francois Pange : Of our exceptional ally, the earthworm. Jean-Francois Pange : Each soil has specific properties Jean-Francois Pange : That have been created and fostered Jean-Francois Pange : Throughout its formation and evolution. Jean-Francois Pange : Soil is either rich or sterile, Jean-Francois Pange : Depending on whether or not Jean-Francois Pange : It favors the development of the fauna Jean-Francois Pange : That live and proliferate in its depths. Jean-Francois Pange : The landscapes below ground Jean-Francois Pange : Are just as varied as those up above. Jean-Francois Pange : Within the space of a kilometer, Jean-Francois Pange : They tell completely different stories Jean-Francois Pange : To this soil expert, Jean-Francois Pange : Pedologist Bernard Jabiul. [00:22:59] Jean-Francois Pange: Jean-Francois Pange : With a given topography, given climate, Jean-Francois Pange : Given life conditions, Jean-Francois Pange : Given period of evolution, Jean-Francois Pange : All those things together mean Jean-Francois Pange : That soil is going to vary greatly Jean-Francois Pange : From one spot to the next. Jean-Francois Pange : Sometimes there's an evolution Jean-Francois Pange : Within the space of about 10 meters. Jean-Francois Pange : Sometimes we see very visible evolutions Jean-Francois Pange : Within the space of several hundred kilometers. Jean-Francois Pange : But on the global scale, Jean-Francois Pange : We find a great diversity of soils, Jean-Francois Pange : And therefore, a great diversity Jean-Francois Pange : Of properties belonging to them. [00:23:38] Speaker 1: Jean-Francois Pange : Bernard Jabiul leaves his study pit Jean-Francois Pange : In the forest, Jean-Francois Pange : To walk 800 meters Jean-Francois Pange : Over to one of his colleagues' pits Jean-Francois Pange : In a field of grain. [00:23:52] Jean-Francois Pange: Jean-Francois Pange : Look at you, Joel. Jean-Francois Pange : Your pits really deep. Jean-Francois Pange : Less than a kilometer away up there, Jean-Francois Pange : I was in the forest and there were only 50 centimeters. Jean-Francois Pange : Why is it so thick here? [00:24:03] Speaker 5: Jean-Francois Pange : Earlier, you were uphill on the slope. Jean-Francois Pange : So the soil wasn't very thick and it was on limestone. Jean-Francois Pange : Here we're downhill with very thick Jean-Francois Pange : Soil that's over a meter and a half deep. Jean-Francois Pange : Why? Jean-Francois Pange : Because there was erosion upon the slope Jean-Francois Pange : With an accumulation of deposits Jean-Francois Pange : At the bottom of the valley. Jean-Francois Pange : This soil has strong agricultural potential Jean-Francois Pange : And good depth. Jean-Francois Pange : Which means large water reserves Jean-Francois Pange : And large mineral reserves. Jean-Francois Pange : Because there are no obstacles, Jean-Francois Pange : Roots can easily grow Jean-Francois Pange : To depths of one or one and a half meters. Jean-Francois Pange : It's also healthy soil Jean-Francois Pange : With strong biological activity. Jean-Francois Pange : And with earthworms Jean-Francois Pange : That travel back and forth Jean-Francois Pange : From the depths to the surface. [00:24:48] Speaker 1: Jean-Francois Pange : The soil's depth Jean-Francois Pange : And structure determine Jean-Francois Pange : Its capacity to store water. Jean-Francois Pange : For the animal life, Jean-Francois Pange : A heavy rain Jean-Francois Pange : Is the equivalent of a tsunami. Jean-Francois Pange : The water dissolves Jean-Francois Pange : Organic matter on the surface Jean-Francois Pange : And washes it below Jean-Francois Pange : As it trickles down the tunnels Jean-Francois Pange : Dugged by earthworms. Jean-Francois Pange : Once it reaches Jean-Francois Pange : The impermeable layers, Jean-Francois Pange : Like certain clays, Jean-Francois Pange : Water accumulates Jean-Francois Pange : And forms underground reservoirs. [00:25:38] Speaker ?: Jean-Francois Pange : The land of the river Jean-Francois Pange : The land of the river Jean-Francois Pange : The land of the river Jean-Francois Pange : The land of the river Jean-Francois Pange : The land of the river Jean-Francois Pange : The land of the river Jean-Francois Pange : The land of the river Jean-Francois Pange : The land of the river [00:25:46] Speaker 1: Jean-Francois Pange : The land of the river Jean-Francois Pange : The land of the river Jean-Francois Pange : The land of the river Jean-Francois Pange : The land of the river Jean-Francois Pange : The land of the river traveled about 20 kilometers away to visit a third pit, dug in an area with few trees. The soil here is very different from that in the previous pits. [00:26:10] Jean-Francois Pange: - The soil here resembles the cultivated soil we saw. The quality is very loose, it's easy to push into, and yet at the same time, it's very deep. - But it's sandy soil. - Right, it's sedimentary. The sand was deposited by the sea several tens of millions of years ago, and that's what the soil is currently developing in. Sand has two major drawbacks. The first one is acidity. In other words, the absence of nutritional elements. What's more, this acidity is hostile to the activity of earthworms. They can't live here, so none of the organic matter provided by the vegetation gets mixed into the soil. It simply accumulates up on the surface. The second major drawback is that the water drains very quickly. The soil can't store water, it doesn't have any reserves. The soil is very dry in the summer. In conclusion, there are two major constraints for vegetation. Acidity and dryness. [00:27:23] Speaker 1: Nevertheless, these impoverished, dry, acidic soils in which no earthworm can survive are not completely sterile. Rustic plants adapted to harsh conditions, grow on the surface. In the litter, we find springtails, roundworms and other habitual decomposers. Luckily, there's bacteria in the dirt, with no earthworms around. This is what imparts a bit of fertility to the soil. Bacteria are composed of microorganisms that are invisible to the naked eye, but they replicate so quickly and multiply at such a pace that they wind up saturating the soil, be it fertile or poor. Because of their density, bacteria are also an essential element when it comes to soil dynamics. Lionel Rangard and his team from the National Institute of Agricultural Research, the INRA, take samples of bacteria contained in different soils to make up a sort of conservatory for microorganisms. [00:28:57] Speaker 3: When you study microorganisms in soil, you generally look at the first 20 centimeters, which correspond more or less to the height of the auger, since the superficial layer of soil actually contains the most and greatest diversity of microbial organisms. For example, in one gram of soil, which is a tiny amount, you have over a billion bacteria, and within this billion, there are millions of different kinds. The reservoir of microbial diversity in the soil is found in this upper layer. To give you another example, if we look on the scale of a plot of land like this, sub-prairie, there is more living microbial mass under the ground than there is living mass above it, even if we include all the plant clover and add in livestock, such as 10 cows per hectare or 50 sheep. [00:29:43] Speaker 1: So it's no exaggeration to say that when conditions are favourable, there's as much life deep down in the soil as up on the surface. The sample taken by Lionel Ranjard's team is examined under an electronic microscope. It reveals how bacteria are organized within their habitat. Thinner than a hair's breadth, the bacteria lodge in the tiny instances of bits of earth. And these spaces are so restricted that their predators, the bulky protozoa, are too big to get in. Sheltered away, the bacteria shift into a slow-motion lifestyle that allows them to conserve energy. Earthworms, if there are any present in the soil, can shake them from their torpor as they pass by. The mucus secreted by the worms and the oxygen they convey through their tunnels stimulate the bacteria. Active once more, the bacteria transform organic matter into the mineral salts that are indispensable for plant growth. [00:31:12] Speaker 3: Bacteria are able to break down and mineralize the organic matter that finds its way into the soil to transform it into mineral matter nitrogen, phosphorus, sulfur, etc. This mineral matter is what provides nutrition for the plants. So bacteria are deeply implicated in soil fertility. [00:31:33] Speaker 1: To assess the impact of bacteria on soil fertility, the INRA has built a completely automated experimental platform. plant development is measured under highly controlled conditions. Lionel Ronjar's team grow flats of alfalfa in pots of soil where the number and diversity of bacteria are controlled and measured. After six weeks of cultivation, [00:32:08] Speaker ?: we see the alfalfa in normal soil has grown well, [00:32:10] Speaker 6: but its growth is much slower in the soil [00:32:11] Jean-Francois Pange: where the diversity of bacteria has been reduced. What about these two pots [00:32:23] Speaker 3: What about these two pots where the alfalfa seems more developed than in the two preceding pots? [00:32:26] Jean-Francois Pange: It's exactly the same experiment, except here the plants have been sprayed with fertilizer. So we see increased growth linked to that. And despite the addition of fertilizer, we continue to see a major difference in growth with a plant that grows less when there are fewer kinds of bacteria. [00:32:44] Speaker 1: The experiment shows that bacteria have a crucial influence on soil fertility. The more bacteria soil contains, the more fertile it is. Whether or not fertilizer is added. Soil fertility differs according to the type of bacteria present. [00:33:13] Speaker 3: Each soil contains a sort of genetic image of its microbial diversity and abundance, an image of its bacterial diversity. Why? Because bacterial diversity depends on the type of soil, its chemical physics, climate, as well as its historical use, whether it was agricultural, forest or urban. Each soil has a unique image in terms of microbiology. [00:33:36] Speaker 1: In a way, the microorganisms present in a given soil give it its identity. Currently, the INRA team has genetically identified and analyzed over 10,000 samples. These samples are kept on file. Each will be used as reference to understand the evolution of soil over time. We're a long way from a complete inventory of the interactions between bacteria and plants. For example, we've discovered that partnerships between plants and bacteria are forged at the root level. [00:34:20] Speaker 3: We have three experimental systems that let us observe the root development of three different plants: clover, pea, and rapeseed. We see root architectures or developments that differ greatly from plant to plant, depending on density, as well as their ability to grow deep. If we look more closely at the roots of the pea and clover, which are legumes, we see little bulbs on the roots called nodules. These nodules contain thousands of bacteria of the same strain, rhizobia. The plant raises the bacteria by nourishing them with sap, and in exchange, the bacteria fix nitrogen from the atmosphere to make it easy to assimilate directly by the plant. We have a symbiotic relationship where each partner maintains a beneficial report with the other, which increases the biological fertility of the soil for plant development. [00:35:11] Speaker 1: I give you what you don't have, and you bring me what I lack. Bacteria and roots enjoy a give-and-take relationship where each feeds the other for their own benefit. Let's continue our journey, delving even deeper underground as we follow along the roots. We know roots serve to anchor plants in the soil, and that the underground network they create stabilizes the land. The more they develop, the more they expand in diameter, and the more they compact the soil. Roots also draw the nutrients that the plant needs from the soil. These exchanges either acidify the soil, or on the contrary, alkalize it. Roots, therefore, have a direct influence, not on the nature of the soil, but on its quality. Plant roots can reach dimensions we hardly imagine. Some trees that have adapted to drought, such as the eucalyptus, arouse the interest of researchers. Christophe Jourdan, from the Agricultural Research Center for International Development, the CIRADE, has turned a Brazilian plantation of eucalyptus into an open-air laboratory. He's interested in the specificity of roots, depending on the layer of soil they grow in. [00:37:17] Speaker 5: Last century, botanists would describe the upper parts of plants and just slightly the roots. It's always been the neglected zone in plant studies. It's true that more recent studies have attempted to go further in root physiology and mortality, in particular deep down. We now expect to find a different way of functioning and different microorganisms. [00:37:44] Speaker 1: Roots have different shapes and different diameters, and not all of them have the same function. [00:37:54] Speaker 5: Most people think of roots as big roots that are directly attached to trees, like this one we see in the ground. Actually, these roots bear smaller roots, medium-sized roots, and at the end of those are fine roots. These are the roots in direct contact with the soil that absorb water and mineral elements. We're particularly interested in these fine roots, how they function, and their lifespan at different levels and different depths in the soil. [00:38:36] Speaker 1: Christophe Jourdan mainly studies the life cycle of surface roots. He places a scanner about 50 centimeters below ground. [00:38:56] Speaker 5: We scan the roots about once every two weeks. We take images from an entire year and compile them to get a picture and estimate of growth rate, which here is about one or two centimeters per day, and to calculate the lifespan of the roots, which here is about eight to 10 months. When they die, these roots help produce root litter [00:39:19] Speaker 4: that gets injected into the ground at different depths. [00:39:26] Speaker 1: Through their fine roots, which die and are constantly renewed, trees bring organic matter into the soil, just like earthworms that carry the leaves and the roots they feed on underground. Just how deep do the roots of eucalyptus trees, which are exposed to periods of drought, grow? Christophe Jourdan had a pit dug 20 meters deep, opening the doors to a little-explored world. [00:40:11] Speaker 5: We're about 15 meters underground here. There are lots of roots branched out behind the glass and even beside it. If we had the tree's height of 25 meters to the 15 meters below ground, that means these roots are 40 meters away from the crown of the trees. We were completely surprised by this. We did not expect to find roots this deep. [00:40:33] Speaker 1: Deep roots are subject to strong pressure. So, surely, they are different from surface roots. Yet nothing about them looks special in any way. Although there is nothing visibly different on the outside, perhaps their internal structure will reveal distinctions. After selecting the best root samples, Christophe Jourdan makes very thin cross-sections of them. He then compares them with similar cross-sections of surface roots. [00:41:22] Speaker 5: Here in this cross-section, we see the internal structure of the root sample collected out in the field. We see how the vessels that transport water take up a large amount of space. Most of the main part of the root is filled with these large vessels. When we compare this to a surface root, the vessels there are much smaller, for the same diameter. What's more, the deep root does not have a perfectly cylindrical shape. We see how it's constrained by the ground and by the pressure exerted by it, the root had to deform in order to grow. These roots with special structure allow trees to seek water in great depths. This is especially useful in periods of drought and opens up very interesting perspectives for us, notably in terms of climate change. [00:42:11] Speaker 1: One of the strategies developed by certain plants or tree species to increase their chances of survival consists of sinking roots, specialized in water transport as deep as possible. Over the course of evolution, a rather unsuspected ally has grown indispensable to the root systems of the majority of plants. [00:42:43] Speaker ?: Soil dynamics [00:42:48] Speaker 1: Marc-André Seulose, professor at the Paris Museum of Natural History, studies the role of fungi in soil dynamics. Mushrooms appear scattered, fragile, and ephemeral, but they're just the tip of the iceberg when it comes to the secret rain of fungi, whose importance is largely unrecognized. [00:43:13] Speaker 3: This part is what we commonly call a mushroom, but for biologists it's just one part of the fungus. It's a reproductive organ that's actually connected to mycelium, which is much hardier and spreads across a much larger area inside the soil, or in dead wood. This means that even if we don't see this part, called a fruitification, mycelium is still present and that is what actively contributes to the fungus's nutrition and to the biology and function of the soil. [00:43:54] Speaker 1: The mycelium of some fungi create a network of filaments in the soil until they come in contact with roots. Over the course of evolution, the fungi root union has basically become the norm because 90% of plant species are not autonomous. they need fungi to extract the phosphate and nitrogen they need from the environment they grow in. The details of the symbiotic relationship between fungi and roots can only be glimpsed through a microscope. [00:44:32] Speaker ?: they need fungi to grow in the soil. They need to grow in the soil. They need to grow in the soil. They need to grow in the soil. They need to grow in the soil. They need to grow in the soil. [00:44:54] Speaker 3: We see a lovely root with lots of lateral ramification that's been colonized by various fungi. Black fungi, white fungi. The fungi form like a sock around the root branches. The root tends to be extremely ramified here, which increases the contact between fungus and roots. In the end, the fungus is just as present in the environment. Here, the filaments are a little damaged, but it also explores the soil to find nutritive salts, mineral salts and water for the plant. [00:45:26] Speaker 1: The symbiosis between fungi and roots is called mycorrhiza. When mycelium establishes contact with a root, it weaves a sheath of filaments around it. Then penetrates the root and creates a network that molds the shape of the cells. There's another type of mycorrhiza that is more common than the preceding one. In this kind, mycelium penetrates directly inside the root cells. The plant increases its surface exploration of the soil a thousand-fold. And as for the fungus, it feeds on the sugars produced by the plant. The other fungi adopt a different strategy. Their mycelium seeks sustenance on the surface of the ground. A hearty and rigid food that they alone are able to digest. [00:46:51] Speaker 3: Here's a piece of wood that is in the process of becoming incorporated into the soil. And when we open it up, we see that the wood inside is completely decomposed. All that's left is this soft white substance, which was eaten, digested by the fungi. This white substance is called lignin, a component that represents almost half of this wood's mass and gives it its rigidity. Here, fungi play a decisive role because they are the only organisms able to digest lignin. No other organisms can do that. They are the only ones that can recycle it. What they leave behind is rich in cellulose that in turn will be eaten by other fungi, other animals or bacteria. We can clearly see the fungi present here that form mycelium clustered into white threads. [00:47:44] Speaker 1: Most fungi that seek food on the surface go for litter, which is much more tender than wood. The mycelium of these fungi is folded over onto itself into thousands of fine strands. So much so that each gram of litter contains several kilometers of filaments. This multiple ramification spreads over a large area, allowing the fungi to reach most of the dead leaves that fall off trees and plants. We've seen this fungus before, white rot. By digesting tannins, it makes leaves edible for the tiny animals living in the litter. Thus, the soil is generated by a great cycle that involves a multitude of elements. Over time, the joint action of animals, bacteria, roots and fungi mark the soil with a succession of layers that can be seen when it's dug into. These strata allow podologists to travel back in time to the origin of the soil. In other words, the bedrock. [00:49:36] Speaker 6: This is a nice pit here. [00:49:42] Jean-Francois Pange: We're going to freshen it up a bit and it will reveal, deep down, way at the bottom, something very hard. This rock is the departure point of what we call pedogenesis, the formation of soil. It was formed through the contact of this rock with the biosphere. We see a part of it here, vegetation and roots, which act on the rock, as well as all the hidden life of the soil. Everything we don't see, animals, micro-animals, bacteria, all that is also going to create chemical reactions that gradually transform the rock over thousands and thousands of years, tens of thousands of years, hundreds of thousands of years. [00:50:29] Speaker 1: Born out of a long process that blends rock with organic detritus, soil, the thin skin of our planet, is fragile. We are just beginning to understand its extraordinary complexity. The interactions between plants and animal species are proving to be much greater than we'd imagined. We have only just begun to take stock of the relationships and exchanges that connect the elements that produce and create the soil. We still have a lot to discover to understand Mother Earth. Who makes us what we are? [00:51:36] Speaker ?: We still have a lot to discover Mother Earth. We still have a lot to discover Mother Earth. We still have a lot to discover Mother Earth. We still have a lot to discover Mother Earth. We still have a lot to discover Mother Earth. We still have a lot to discover Mother Earth. We still have a lot to discover Mother Earth. We still have a lot to discover Mother Earth. We still have a lot to discover Mother Earth. We still have a lot to discover Mother Earth. We still have a lot to discover Mother Earth. We still have a lot to discover Mother Earth. We still have a lot to discover Mother Earth. We still have a lot to discover Mother Earth. We still have a lot to discover Mother Earth.