voir la définition de Wikipedia
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Organic matter (or organic material, Natural Organic Matter, or NOM) is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds. The definition of organic matter varies upon the subject for which it is being used.
Organic matter is broken down organic matter that comes from plants and animals in the environment. Organic matter is a collective term, assigned to the realm of all of this broken down organic matter. Basic structures are created from cellulose, tannin, cutin, and lignin, along with other various proteins, lipids, and sugars. It is very important in the movement of nutrients in the environment and plays a role in water retention on the surface of the planet. These two processes help to ensure the continuance of life on Earth.
All living and growing matter on this planet contains organic components. Different types of matter include humans, animals, plants, and microorganisms. After the living matter dies, it decomposes. The organic matter from them and their excretions is broken down through an unknown reactive process into natural organic matter. Larger molecules of organic matter can be formed from the polymerization of different parts of already broken down matter. The relative size, shape, and composition of a molecule of organic matter is very random. "NOM can vary greatly, depending on its origin, transformation mode, age, and existing environment, thus its bio-physico-chemical functions vary with different environments."
Natural organic matter is present throughout the ecosystem. After degrading and reacting, it can then move into soil and mainstream water via waterflow. NOM forms molecules that contain nutrients as it passes through soil and water. It provides nutrition to living plant and animal species. NOM acts as a buffer, when in aqueous solution, to maintain a less acidic pH in the environment. Little is known why this occurs but research shows the buffer acting component to be crucial acid rain.
A majority of NOM not already in the soil comes from groundwater, which is water under the surface of the earth. When the groundwater saturates the soil or sediment around it, NOM can freely move between the phases. But, the groundwater has its own sources of natural organic matter too:
Note that one source of groundwater is soil organic matter and sedimentary organic matter. The major method of movement into soil is from groundwater, but NOM from soil moves into groundwater as well. Most of the NOM in lakes, rivers, and surfaced water areas comes from deteriorated material in the water and surrounding shores. However, NOM can pass into or out of water to soil and sediment in the same respect as with the soil.
Natural organic matter uses all these different phases (soil, sediment, water,and groundwater) to move throughout the environment. This action of movement creates a cycle. Things decompose into NOM, travel through water flow or soil, and then are free to spread through the phases. If it were not for this cycle, important nutrients such as minerals, vitamins, and metals would not be as easily spread throughout the surface of the Earth. Furthermore, this shows there are no independent processes in the environment, which means everything is connected in some regard. Physical, biological, and chemical systems work together to create natural processes.
The organic matter in soil derives from plants and animals. In a forest, for example, leaf litter and woody material falls to the forest floor. This is sometimes referred to as organic material. When it decays to the point in which it is no longer recognizable it is called soil organic matter. When the organic matter has broken down into a stable substance that resist further decomposition it is called humus. Thus soil organic matter comprises all of the organic matter in the soil exclusive of the material that has not decayed.
One of the advantages of humus is that it is able to withhold water and nutrients, therefore giving plants the capacity for growth. Another advantage of humus is that it helps the soil to stick together which allows nematodes, or microscopic bacteria, to easily decay the nutrients in the soil.
There are several ways to quickly increase the amount of humus. Combining compost, plant or animal materials/waste, or green manure with soil will increase the amount of humus in the soil.
These three materials supply nematodes and bacteria with nutrients for them to thrive and produce more humus, which will give plants enough nutrients to survive and grow.
Organic matter may be defined as material that is capable of decay, or the product of decay (humus), or both. Usually the matter will be the remains of recently living organisms, and may also include still-living organisms. Polymers and plastics, although they may be organic compounds, are usually not considered organic material, due to their poor ability to decompose. A clam's shell, while biotic, would not be considered organic matter by this definition because of its inability to decay.
Measurements of organic matter generally measure only organic compounds or carbon, and so are only an approximation of the level of once-living or decomposed matter. Some definitions of organic matter likewise only consider "organic matter" to refer to only the carbon content, or organic compounds, and do not consider the origins or decomposition of the matter. In this sense, not all organic compounds are created by living organisms, and living organisms do not only leave behind organic material. A clam's shell, for example, while biotic, does not contain much organic carbon, so may not be considered organic matter in this sense. Conversely, urea is one of many organic compounds that can be synthesized without any biological activity.
Very little is currently known about natural organic material. Scientists are unable to crystallize it. This is important because once you can crystallize the material, it can be isolated and studied with x-ray crystallography. This method is standard for determining unknown compounds. NOM has not been characterized either and no unique structure is known. The best way to characterize NOM is by discovering chemical, physical, and thermodynamic properties of the matter. Analytical techniques are currently being discovered to allow this to happen. The only information scientists have is that NOM is heterogeneous and very complex. Generally, NOM, in terms of weight, is:
The molecular weights of these compounds can vary drastically, depending on if they repolymerize or not, from 200-20,000 amu(4). It is also important to know that 10-35% of the carbon present forms aromatic rings. These rings are very stable due to resonance stabilization, so they are difficult to break down. The aromatic rings are also susceptible to electrophilic and nucleophilic attack from other electron-donating or electron-accepting material, which explains the possible polymerization to create larger molecules of NOM.
There are also reactions that occur with NOM and other material in the soil to create compounds never seen before. Unfortunately, it is very difficult to characterize these because so little is known about natural organic matter in the first place. Research is currently being done to figure out more about these new compounds and how many of them are being formed.
The same capability of natural organic matter that helped with water retention in soil creates problems for current water purification methods. In water, NOM can still bind to metal ions and minerals. These bound molecules are not necessarily stopped by the purification process, but do not cause harm to any humans, animals, or plants. However, because of the high level of reactivity of natural organic matter, byproducts that do not contain nutrients can be made. These byproducts are much larger and can induce biofouling, which essentially breaks down water filtration systems in water purification facilities. The larger molecules clog the water purification filters intended to keep material like that out of drinking water. The fact that these byproducts are removed through purification is very good news, but having to replace filters constantly to maintain effectiveness is costly for water treatment businesses. This byproduct problem could be treated by the disinfection technique known as chlorination, which often breaks down residual material clogging systems, but research has shown that the natural organic matter also forms byproducts with this method.
A large breakthrough could be underway after a paper published in the Applied and Environmental Microbiology journal showed proof that water with natural organic matter could be disinfected with ozone-initiated radical reactions. The ozone(three oxygens) has very strong oxidation characteristics. It can form hydroxyl radicals (OH) when it decomposes, which will react with the natural organic matter to shut down the problem of biofouling. The article did this on a very small scale of water and natural organic matter, so further research is being done to scale up the reaction. This journal article does show that solutions are on the horizon to prevent our water purification systems from being broken down by NOM.
Many water quality groups, such as the North Carolina State University Water Quality Group, believe that having too much natural organic material will cause deoxygenation and essentially remove oxygen from the water. Although organic material, which consists of many hydrocarbon and cyclic carbon chains, is susceptible to attack by oxygen, it would be sterically unfavorable to attach oxygens to every single carbon. Basically, molecules do not enjoy other molecules being too close to them when they have the same electronegativity. Most of this is because of electrostatic charge, which says that opposite charges are attracted and like charges are repelled. If you have oxygens (with a negative charge in theory) bonded to carbons next to each other, they will want to be as far away from each other as possible. Also, a larger molecule like oxygen (relative to carbon) does not want to attach to a carbon that already has oxygens on it when it could attach to a carbon without oxygens on it.
Of course, there are exceptions, such as varying the temperature at which these reactions occur. As the temperature becomes much higher, there is a better chance that an unfavorable reaction will occur because molecules move around faster increasing the randomness of the system (entropy). Yet, as we consider the cold water in the natural environment, it is logical to see that all the oxygen in the water will not be consumed by NOM.
This can be proved numerically. The approximate volume of liquid water on the Earth is 1.333*10^9 km³, or 1.333*10^24 milliliters. Since the density of water is roughly 1g/mL, this is 1.333*10^24 grams of water. Dividing this number by 18.02 grams per mole gives 7.399*10^22 moles of water. Each mole of water has one mole of oxygen and each mole of oxygen has 6.022x10^23 oxygen molecules. Multiplying 7.399*10^22 moles by 6.022*10^23 molecules/mole gives 4.456*10^46 oxygen molecules in the liquid water on the surface of the planet. (Please note this is an estimate taken from average data.)
NOM is not going to use up all the oxygen on the earth and remove water.
The equation of "organic" with living organisms comes from the now-abandoned idea of vitalism that attributed a special force to life that alone could create organic substances. This idea was first questioned after the artificial synthesis of urea by Friedrich Wöhler in 1828.
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