First, animals such as Arthropods, Crustaceans and Annelids eat organic matter (M.O.) on the surface. This feeding action tears the M.O apart, increasing the surface of contact accessible to other decomposing organisms (such as Protozoa). The populations of M.O fragmenters and decomposers are controlled by predators, in consequence, there is a true food chain in soils.
At the end of the chain, bacteria and fungi also feed on small molecules. In addition to eating protozoa, bacteria and fungi, nematodes play an important role in the soil food chain because they regulate the populations of their prey. Also, because the concentration of elements in bacteria and fungi is much higher than what nematodes need for their metabolism, when they eat bacteria an fungi, they ingest too much nutrients for them and N, P, S, K, Fe… in surplus are released in soluble form by nematodes in their feces.
When a plant needs nutrients, through its roots, it will secrete a definite exudate that will attract a species of bacteria or fungi that will accumulate on the root surface. In turn, protozoa, nematodes and micro-arthropods, attracted by the accumulation of bacteria and fungi, will reach the root surface and devour them. Because pluricellular nematodes are less concentrated in elements than fungi and unicellular bacteria, they excrete what they do not need from their prey. The elements contained in the nematode wastes are in a soluble form and, therefore, in the form of available nitrogen for plants in ammonium (NH4+), phosphorus in phosphate (PO4-), sulphur in sulphate (SO4-), iron (Fe), calcium (Ca), potassium (K), magnesium (Mg), sodium (Na)… in chelated minerals (mineral added to an amino acid). It is these soluble elements that can be passively or actively transported if they are in the periphery of the small roots.
Some of the micro-fauna consists of earthworms. The epigeal earthworms eat bacteria, fungi, protozoa and nematodes in the surface layer of the soil.
Feces of earthworms are extremely rich in nutrients. When anecic earthworms descend in the deepth of soil, they encounter the clay that comes from the alteration of rocks by acids secreted by tree roots but especially by fungi (clay = negatively charged silicates). When anecic worms return to the surface each night to feed on organic matter decomposed into humus by the microfauna, they bring up the clay and bring down the humus (the humus is also negatively charged), thus mixing the whole. The earthworms have a gland located on segment 14 to 16 of their body, called Morren's gland, which secretes Ca2 + which allows the creation of the clay-humic complex (CAH). Ca2 + binds the clay particle to one of its two positive charges and the humus particle to its second positive charge.
Aware of the importance of microorganisms for soil fertility and the nutritional value of vegetables, at La Evidencia, we want to feed them adequately by covering the soil with a layer of pruning residues and animal excrement (mulching).
Plants also established mutualistic symbioses (beneficiary for each of the individuals involved in the symbiosis relationship) with soil organisms:
1. Mycorrhizae:
Mycorrhizae is an interaction between the cells of the root of a plant and a hyphae of a fungus.
As you can see from the photo of a plant cell, the separation between the cytosol (inner part of the cell) and the outer world consists of two layers: the plasma membrane (rather flexible) and the cell wall (rather rigid).
In the ecto-mycorrhizae, hyphae are in contact with cell walls, while in the endo-mycorrhizae, hyphae penetrate cell walls to be in direct contact with the invaginated plasma membrane (arbuscularmyccorhyza). Both types of mycorrhizae allow a large area of exchange between the two organisms.
Current science recognizes that the first plants appeared on earth between 400 and 450 million years ago, thanks to the symbiosis of endo-mycorrhizae that supplied water and minerals to this first plant without roots. Today, all the plants on the planet are engaged in a symbiosis of endo-mycorrhizae, with the exception of 15% of them: 10% are the first species to grow during the process of colonization of the bare soil (pioneer) and plants growing on very rich soils; 5% are plants that evolved to another type of symbiosis called ecto-mycorhiza (which would have appeared probably 50 million years ago).
In simple terms, ecto-mycorrhizae form on trees in temperate zones, while endo-mycorrhizae form on trees in tropical zones and all the world’s plants. The symbiosis works as follows: there is a flow of sugars from photosynthesis from the plant to the fungal hyphae while a flow of water and minerals (mainly nitrogen, phosphorus and potassium), passes from the hyphae to the plant. Let’s see what the advantages are for each of the two symbiotes:
Advantages for the fungus:
-) Being non-photynthetic, fungi cannot produce sugars without which living things cannot survive. Up to 40% of sugar production from photosynthesis goes to fungus
-) Thanks to the sugars received, fungi can spread in the soil by searching for other resources (minerals and water), colonizing new areas and being able to reproduce.
-) Once in the plant root, hyphae are protected from predators (nematodes, arthropods or mammals).
Advantage for the plants:
-) On average, there are 1 km of hyphae per metre of roots. It is estimated that hyphae production costs to the plant 100 times less sugar than producing the same length of thin roots. So there is an undeniable gain in energy for the plant. -) Mycorrhized plants have access to water and minerals absorbed in a large volume of soil by fungi. -) The tips of young roots are covered with hyphae, which physically protects them from mechanical abrasion, pathogens and soil predators. -) Many studies have shown that mycorrhized plants are more resistant to pathogens and predators that would attack their roots, stems or leaves. This could be explained by the fact that mycorhization, constituting an invasion of the root is controlled and simply tolerated by the plant’s immune system. Thus, mycorhization activates the immune system and the mechanisms of defense are already activated, they would be much more reactive to another aggression.
A fungus is able to establish mycorrhizae with several plants at once (several individuals of the same species but also several individuals of different species). Similarly, a plant (even a root) engages in mycorrhizae with several species of fungi simultaneously. Of course, this capacity is necessary because each fungus and plant cannot bring the same resources to their partners depending on the species and their location in time (age: young vs old) and in space. With this ability to connect with many partners, we could quickly imagine an underground network linking each individual plant in an ecosystem. Some studies have shown that certain nutrients such as nitrogen and phosphorus, but also carbon, can be transported from one plant to another (even from different species) via this hyphae network. Although the mechanism is still not understood, scientists have speculated that fungi could regulate the amount of nutrients and the health status of each individual with whom they are in symbiosis, that for his own benefit.
At La Evidencia, we will do our best to: - Feed the mycorrhyzes all year round by planting a cover crop, it means: plant a crop that will grow every season covering the soil. When plants grow on the ground, the soil is filled with mycorrhizae, forming a network between them and the already established plants. By "connecting" to this network, a germinating plant will easily obtain all the nutrients it needs for healthy growth. - Keep fungi alive by avoiding the use of chemicals such as pesticides, fungicides or herbicides and tillage. - Promote the diversity of mycorrhizal fungi in the soil by planting different plants. Plant diversity enhances the diversity of micro-organismes in soil such as fungi. In addition, fungi could redistribute nutrients collected from one type of plant to another (for example: nitrogen from Fabaceae species, read below). Soil structure is disturbed by tillage, hyphae are fragmented and separated from roots. In this disturbed soil, only fungal species with a high ability to recover quickly in disturbed habitat will be favoured. Repeated tillage will, therefore, lead to lower fungal diversity. In consequence, we will not plow our soil. - Maintain mycorrhizae, which are essential to the proper functioning of the plant’s immune system and useful in terms of nutrient exchange. I guess if I use fertilizers that provide nitrogen, phosphorus, and potassium to plants, they will no longer engage in mycorrhizae because they will no longer need to be in symbiosis with fungi in order to find nutrients. A non-mycorrhizal plant will be unable to absorb the amount of nutrients it needs to be productive and will then become dependent on fertilizer. In addition, without the benefits of mycorrhizae on the reactivity of its immune system, non-mycorrhizae plants will become dependent on pesticides. Pesticide use further reduces the biodiversity of mycorrhizal fungi in soils. In total, an infernal loop of dependence on chemicals will begin. I will not use fertilizer (even organic) in La Evidencia.
- Plant former vegetable species that have retained the ability to engage in mycorrhizae.
2. The nodisities
Proteins are essential for all living organisms. In the form of enzymes, they regulate all aspects of metabolism. For example, in mammals, structural proteins such as keratin and collagen make skin, claws, bones, tendons and ligaments; muscle proteins produce movement; hemoglobin carries oxygen; and membrane proteins regulate the movement of substances through cells. Proteins are the main component of cells, representing more than 50% of their dry weight. The form of proteins is very variable: it ranges from long fibres in the connective tissue and hair to compact, soluble blood cells that can pass through the cell membrane. They are specific to each living species and organ. It is estimated that there are about thirty thousand different proteins in humans, of which only 2% have been described. An important protein compound is nitrogen. Nitrogen (N) is an atom present in all amino acids, constituting proteins. Amino acids are organic compounds containing the amino group (-NH2) and the acid group (-COOH), as well as a side chain (Group R) specific to each amino acid.
A sequence of three letters (i.e., a codon) of the genetic code of living beings codes for one of the 20 amino acids. The amino acids are assembled by cellular machinery, bound to each other and structured into proteins.
The atmosphere is the main source of nitrogen (N) on Earth, since 78.1% of it is N2, a gas. However, plants do not have the ability to use this source of nitrogen directly because the two nitrogen atoms are bound by three very stable atomic bonds, impossible to break by the plant metabolism, N≡N. In fact, multicellular living beings can only use nitrogen for their metabolism in forms such as NH4 + or NO3- which involves only simple bonds. So the first step for being able to use this source of atmospheric nitrogen is to break the three atomic N2 bonds. Plants are not able of breaking such atomic bonds; some bacteria are the only organisms on Earth able of separating the two nitrogen atoms from atmospheric N2 thanks to their enzyme called nitrogenase. Marginally, lightning also provides sufficient energy to decompose N2 to NO3-. Some bacteria in the soil and ocean have evolved to breathe N2; these bacteria transform N2 into NH4+. Then plants will be able to use the NH4+ formed by these bacteria to make their proteins.
But the problem is that nitrogenase does not work in the presence of the second most abundant gas in the atmosphere, O2. This problem is solved by a single family of plants, the Fabaceae which live in symbiosis with bacteria of the genus Rhizobium able of decomposing N2 at the level of the roots where they form nodules that exclude oxygen. Thanks to this symbiosis, plants give sugars to bacteria and bacteria give nitrogen (NH4+) to plants. It is a symbiosis of mutualism without which there would be very few terrestrial plants, and therefore almost no consumer animals or ecosystems.
This symbiosis is essential to Life on Earth in that it allows, in two different ways, to provide, and even constantly increase, the amount of nitrogen available for organisms to build their proteins: -) When Fabacea is eaten by a predator, all the nitrogen in its proteins passes directly into the predator’s body. -) When the plant residues begin to decompose, the nitrogen passes through the decompressor body and is mineralized to become biologically available in the soil for other plants that depend on it because they do not engage this symbiosis.
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