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Addressing the persistent environmental threat of organic chemicals with a fresh approach to degradation and transformation processes.
Table of contents

By-products from growing roots and plant residues feed soil organisms. In turn, soil organisms support plant health as they decompose organic matter, cycle nutrients, enhance soil structure and control the populations of soil organisms, both beneficial and harmful pests and pathogens in terms of crop productivity. The living part of soil organic matter includes a wide variety of micro-organisms such as bacteria, viruses, fungi, protozoa and algae.

It also includes plant roots, insects, earthworms, and larger animals such as moles, mice and rabbits that spend part of their life in the soil. The living portion represents about 5 percent of the total soil organic matter. Micro-organisms, earthworms and insects help break down crop residues and manures by ingesting them and mixing them with the minerals in the soil, and in the process recycling energy and plant nutrients. Sticky substances on the skin of earthworms and those produced by fungi and bacteria help bind particles together. Earthworm casts are also more strongly aggregated bound together than the surrounding soil as a result of the mixing of organic matter and soil mineral material, as well as the intestinal mucus of the worm.

Thus, the living part of the soil is responsible for keeping air and water available, providing plant nutrients, breaking down pollutants and maintaining the soil structure. The composition of soil organisms depends on the food source which in turn is season dependent. Therefore, the organisms are neither uniformly distributed through the soil nor uniformly present all year.

However, in some cases their biogenic structures remain. Each species and group exists where it can find appropriate food supply, space, nutrients and moisture Plate 2. Organisms occur wherever organic matter occurs Ingham, Therefore, soil organisms are concentrated: around roots, in litter, on humus, on the surface of soil aggregates and in spaces between aggregates. For this reason, they are most prevalent in forested areas and cropping systems that leave a lot of biomass on the surface.

Plate 2 Termites create their own living conditions near their preferred food sources. Inside the colony life is highly organized.

Persistent organic pollutant

The activity of soil organisms follows seasonal as well as daily patterns. Not all organisms are active at the same time. Most are barely active or even dormant. Availability of food is an important factor that influences the level of activity of soil organisms and thus is related to land use and management Figure 3. Practices that increase numbers and activity of soil organisms include: no tillage or minimal tillage; and the maintenance of plant and annual residues that reduce disturbance of soil organisms and their habitat and provide a food supply.

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Different groups of organisms can be distinguished in the soil Brussaard and Juma, Table 1 classifies them by size. Table 2 classifies them by function. Fresh residues consist of recently deceased micro-organisms, insects and earthworms, old plant roots, crop residues, and recently added manures.

Crop residues contain mainly complex carbon compounds originating from cell walls cellulose, hemicellulose, etc. These carbon chains, with varying amounts of attached oxygen, H, N, P and S, are the basis for both simple sugars and amino acids and more complicated molecules of long carbon chains or rings.

Depending on their chemical structure, decomposition is rapid sugars, starches and proteins , slow cellulose, fats, waxes and resins or very slow lignin. Figure 3 Fluctuations in microbial biomass at different stages of crop development in conventional agriculture compared with systems with residue retention and high organic matter input. Source: adapted from Swift, Heal and Anderson, Bioturbating invertebrates and plant roots, mycorrhizae and some other micro-organisms.

Mostly micro-organisms and plant roots, some C protected in large compact biogenic invertebrate aggregates. Mostly micro-organisms and plant roots, some soil- and litter-feeding invertebrates. Various saprophytic and litter-feeding invertebrates detritivores , fungi, bacteria, actinomycetes and other micro-organisms. Plants, mycorrhizae and other fungi, nematodes, bacteria and various other micro-organisms, collembola, earthworms, various predators.

Plant roots, various insects crickets, beetle larvae, ants, termites , earthworms, vertebrates, micro-organisms and their by-products. Rhizobia, mycorrhizae, actinomycetes, diazotrophic bacteria and various other rhizosphere micro-organisms, ants. Direct effects: plant roots, rhizobia, mycorrhizae, actinomycetes, pathogens, phytoparasitic nematodes, rhizophagous insects, plant-growth promoting rhizosphere micro-organisms, biocontrol agents Indirect effects: most soil biota.

During the decomposition process, microorganisms convert the carbon structures of fresh residues into transformed carbon products in the soil. There are many different types of organic molecules in soil. Some are simple molecules that have been synthesized directly from plants or other living organisms. These relatively simple chemicals, such as sugars, amino acids, and cellulose are readily consumed by many organisms. For this reason, they do not remain in the soil for a long time. Other chemicals such as resins and waxes also come directly from plants, but are more difficult for soil organisms to break down.

Humus is the result of successive steps in the decomposition of organic matter. Because of the complex structure of humic substances, humus cannot be used by many micro-organisms as an energy source and remains in the soil for a relatively long time. Non-humic organic molecules are released directly from cells of fresh residues, such as proteins, amino acids, sugars, and starches.

This part of soil organic matter is the active, or easily decomposed, fraction. This active fraction is influenced strongly by weather conditions, moisture status of the soil, growth stage of the vegetation, addition of organic residues, and cultural practices, such as tillage. It is the main food supply for various organisms in the soil.

Carbohydrates occur in the soil in three main forms: free sugars in the soil solution, cellulose and hemicellulose; complex polysaccharides; and polymeric molecules of various sizes and shapes that are attached strongly to clay colloids and humic substances Stevenson, The simple sugars, cellulose and hemicellulose, may constitute percent of the organic matter in most soils, but are easily broken down by micro-organisms.

Polysaccharides repeating units of sugar-type molecules connected in longer chains promote better soil structure through their ability to bind inorganic soil particles into stable aggregates. Research indicates that the heavier polysaccharide molecules may be more important in promoting aggregate stability and water infiltration than the lighter molecules Elliot and Lynch, Some sugars may stimulate seed germination and root elongation.

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Other soil properties affected by polysaccharides include CEC, anion retention and biological activity. The soil lipids form a very diverse group of materials, of which fats, waxes and resins make up percent of soil organic matter. The significance of lipids arises from the ability of some compounds to act as growth hormones. Others may have a depressing effect on plant growth. Small amounts exist in the form of amines, vitamins, pesticides and their degradation products, etc.

The rest is present as ammonium NH 4 - and is held by the clay minerals. Humus or humified organic matter is the remaining part of organic matter that has been used and transformed by many different soil organisms. It is a relatively stable component formed by humic substances, including humic acids, fulvic acids, hymatomelanic acids and humins Tan, Campbell R The structure and dynamics of microbial populations in soil.

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Persistent organic pollutant - Wikipedia

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The Fate of Chemicals in Soil

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2 Environment Degradation