Organic matter

thumb|Beetle feeding on organic matter

Organic matter, organic material or natural organic matter (NOM) is the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have come from the feces and remains of organisms such as plants and animals. Organic molecules can also be made by chemical reactions that do not involve life. Basic structures are created from cellulose, tannin, cutin, and lignin, along with other various proteins, lipids, and carbohydrates. Organic matter is very important in the movement of nutrients in the environment and plays a role in water retention on the surface of the planet.

Formation

Living organisms are composed of organic compounds that are secreted or excreted into their environment during life. Following the death of an organism, these materials—along with shed tissues like roots and leaves—are decomposed by microbial and fungal activity. Through a process of oxidative polymerization, these smaller degradation products can then recombine to form complex, larger-scale organic molecules.

The composition of natural organic matter depends on its origin, transformation mode, age, and existing environment, thus its bio-physicochemical functions and properties vary with different environments.

Natural ecosystem functions

Organic matter is common throughout the ecosystem and is cycled through decomposition processes by soil microbial communities that are crucial for nutrient availability. After degrading and reacting, it can move into soil and mainstream water via waterflow. Organic matter provides nutrition to living organisms. Organic matter acts as a buffer in aqueous solutions to maintain a neutral pH in the environment. The buffer acting component has been proposed to be relevant for neutralizing acid rain.

Source cycle

Some organic matter not already in the soil comes from groundwater. When the groundwater saturates the soil or sediment around it, organic matter can freely move between the phases. Groundwater has its own sources of natural organic matter including:

organic matter deposits, such as kerogen and coal.
soil and sediment organic matter.
organic matter infiltrating into the subsurface from rivers, lakes, and marine systems."

Organisms decompose into organic matter, which is then transported and recycled. Not all biomass migrates, some is rather stationary, turning only over the course of millions of years.

Soil organic matter

The organic matter in soil derives from plants, animals and microorganisms. In a forest, for example, leaf litter and woody materials fall 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 resists 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.

An important property of soil organic matter is that it improves the capacity of a soil to hold water and nutrients, and allows their slow release, thereby improving the conditions for plant 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.

Compost: decomposed organic material.
Plant and animal material and waste: dead plants or plant waste such as leaves or bush and tree trimmings, or animal manure.
Green manure: plants or plant material that is grown for the sole purpose of being incorporated with 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. The phenomenon is generally caused by either pulsed or continuous changes to inputs of fresh organic matter (FOM). Priming effects usually result in an acceleration of mineralization due to a trigger such as the FOM inputs. The cause of this increase in decomposition has often been attributed to an increase in microbial activity resulting from higher energy and nutrient availability released from the FOM. After the input of FOM, specialized microorganisms are believed to grow quickly and only decompose this newly added organic matter. The turnover rate of SOM in these areas is at least one order of magnitude higher than the bulk soil. However, the mechanisms which lead to the priming effect are more complex than originally thought, and still remain generally misunderstood.

Decomposition

One suitable definition of organic matter is biological material in the process of decaying or decomposing, such as humus. A closer look at the biological material in the process of decaying reveals so-called organic compounds (biological molecules) in the process of breaking up (disintegrating).

The main processes by which soil molecules disintegrate are by bacterial or fungal enzymatic catalysis. If bacteria or fungi were not present on Earth, the process of decomposition would have proceeded much slower.

Various factors impact the decomposition of organic matter including its chemical properties and other environmental parameters. Metabolic capabilities of the microbial communities play a crucial role on decomposition since they are highly connected with the energy availability and processing. In terrestrial ecosystems the energy status of soil organic matter has been shown to affect microbial substrate preferences. Some organic matter pools may be energetically favorable for the microbial communities resulting in their fast oxidation and decomposition, in comparison with other pools where microbial degraders get less return from the energy they invest. By extension, soil microorganisms preferentially mineralize high-energy organic matter, avoiding decomposing less energetically dense organic matter.

Organic chemistry

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 it 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.

Organic matter is heterogeneous and very complex. Generally, organic matter, in terms of weight, is:

Aquatic

Aquatic organic matter can be further divided into two components: (1) dissolved organic matter (DOM), measured as colored dissolved organic matter (CDOM) or dissolved organic carbon (DOC), and (2) particulate organic matter (POM). They are typically differentiated by that which can pass through a 0.45 micrometre filter (DOM), and that which cannot (POM).

Detection

Organic matter is important in water and wastewater treatment and recycling, natural aquatic ecosystems, aquaculture, and environmental rehabilitation. It is, therefore, important to have reliable methods of detection and characterisation, for both short- and long-term monitoring. Various analytical detection methods for organic matter have existed for up to decades to describe and characterise organic matter. These include, but are not limited to: total and dissolved organic carbon, mass spectrometry, nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, UV-Visible spectroscopy, and fluorescence spectroscopy. Each of these methods has its advantages and limitations.

Water purification

The same capability of natural organic matter that helps with water retention in the soil creates problems for current water purification methods. In water, organic matter can still bind to metal ions and minerals. The purification process does not necessarily stop these bound molecules but does not cause harm to any humans, animals, or plants. However, because of the high reactivity of organic matter, by-products that do not contain nutrients can be made. These by-products can induce biofouling, which essentially clogs water filtration systems in water purification facilities, as the by-products are larger than membrane pore sizes. This clogging problem can be treated by chlorine disinfection (chlorination), which can break down residual material that clogs systems. However, chlorination can form disinfection by-products.

Vitalism

The equation of "organic" with living organisms comes from the now-abandoned idea of vitalism, which attributed a special force to life that alone could create organic substances. This idea was first questioned after Friedrich Wöhler artificially synthesized urea in 1828.

References

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