HUMIC SUBSTANCES IN THE BIOSPHERE*

Humic substances (hereinafter "HS") are formed in soils, peats, coals and other natural bodies. They accumulate nutrients and energy, participate in the migration of cations, reduce the negative effect of toxic substances, affect the development of organisms and the thermal balance of the planet.

They are stable, high molecular weight, polydisperse, contain various functional groups, amino acids, polysaccharides, benzoid fragments.

There are several groups of HS:

1. humic acids, soluble only in alkaline solutions;
2. himatomelanic acids extracted from the crude residue (gel) of humic acids with ethyl alcohol;
3. fulvic acids, soluble in water, alkaline and acidic solutions;
4. humin is an almost insoluble and non–extractable organic substance from natural bodies and compost.

All these groups of humic acids are usually spoken of in the plural (for example, humic acids), since their composition and properties vary depending on the source of HS, but even in preparations obtained from the same source (one type of soil, peat, coal), they are heterogeneous, polydisperse and represented by a large set of similar structures, but non-identical molecules.

Humic substances perform many functions in the biosphere, of which the following are the most important.

1. Accumulative function.It consists in the accumulation of chemical elements and energy necessary for living organisms. Practically, this means that HS responsible for the life support of soil biota and hydrobiota, but since they persist for a long time due to their stability (according to radiocarbon dating hundreds and thousands of years), they thereby guarantee a continuous supply of plants and microorganisms with energy and building material. The composition of humic substances contains from 40 to 60% C, 3-5% N, 30-40% O, as well as hydrogen, sulfur, phosphorus, many metal cations, including the so-called trace elements. It is no coincidence that dark gray and black colored soils have always been considered fertile by the people and called, although not always correctly, chernozems. The color of such soils is given by HS. Humic substances give living organisms the nutrients they need gradually, as they are consumed, thereby preserving the necessary supply of these elements for future generations. In this way, they differ significantly from many mineral compounds that can supply plants with nutrients, but are usually represented by easily soluble substances that are quickly consumed or washed out of the soil. At the same time, some of the mineral elements are included in the crystal lattice of aluminosilicates, they are inaccessible to living organisms and only after the destruction of minerals are consumed by plants.
2. Transport function.It consists in the formation of geochemical flows of mineral and organic substances, mainly in aqueous media, due to the formation of stable, but relatively easily soluble complex compounds of humic acids with metal cations or hydroxides. The transport function to some extent contradicts the accumulative function, since their results are directly opposite, but the inconsistency of the action provides a variety of effects of humic substances on the mineral components of soils and rocks.
3. Regulatory function. This function combines many different phenomena and processes and refers to soils, waters and other natural bodies. In the regulatory function of humic substances , several main components can be distinguished:
   1. formation of soil structure and water-physical properties of soils;
   2. regulation of ion exchange reactions between solid and liquid phases;
   3. influence on acid-base and redox regimes;
   4. regulation of the nutritional conditions of living organisms by changing the solubility of mineral components;
   5. regulation of the thermal regime of soils and the atmosphere, including manifestations of the greenhouse effect.
4. The protective function,which consists in the ability of humic substances to bind toxic and radioactive elements into sedentary or difficult-to-dissociate compounds, as well as compounds that negatively affect the environmental situation in nature, including they can incorporate some pesticides, hydrocarbons, phenols. The protective function of humic substances is so great that soils rich in them can completely prevent lead ions and other toxic substances from entering the groundwater.
5. Physiological function.Many researchers have found that various humic substances, especially humic acids and their salts, can stimulate seed germination, activate plant respiration, increase the productivity of cattle and poultry. Moreover, it has been shown that some preparations of humic substances restrain the development of malignant tumors, increase the resistance of organisms to various kinds of inflammatory processes.

On the method of extraction of humic acid from peat F. Ahard wrote as follows: "Extracts from peat obtained by caustic alkali, I saturated with vitriolic acid. The mixture darkened and seemed dark brown, almost black, the sediment sank to the bottom." This method is used almost to this day to isolate HS from any natural bodies. In other words, HS is extracted with alkali solutions, then humic acids and himatomelanic acids are precipitated with acid, while fulvic acids and non-specific substances remain in the solution.

The content of various chemical elements in these substances has been well studied. The carbon content in mass fractions ranges from 40 to 60%, depending on the origin and source of HS. Nitrogen is always there, it was proved by the Russian scientist R. Hermann in the middle of the last century, but it is not enough – 3-5%. Hydrogen usually contains 3-6%, and oxygen – 33-37%. Be sure to include sulfur – up to 0.7-1.2% and phosphorus – up to 0.5%. There are always different metals, although it is difficult to say whether they are mandatory for HS or just an impurity, since it is not easy to clean HS. For example, fine-crystalline quartz SiO2, fine-crystalline goethite FeOOH were found in the products of HS, which have to be recognized as obvious impurities.

Any HS contains a large set of functional groups, they are multifunctional. Their molecules contain carboxyl groups –COON, phenolic –OH, quinone =C=O, amino groups –NH2, etc. Firstly, their number is large, and secondly, they are distributed unevenly across molecules of different sizes, and even molecules of the same size may differ in the content of functional groups. Moreover, HS molecules differ in the number of amino acid residues included in their composition (there are 17-20 in total), in the number of carbohydrate residues and the nature of their location.

The content of functional groups, expressed in mM ⋅ kg-1 according to M. Schnitzer, varies in humic acids within the following limits: – COON – 1500-5700, acidic – OH – 2100-5700, slightly acidic and alcoholic – OH – 200-4900, quinoid – C=O – 100-5600, ketonic – C=O – about 1700, – OSN3 – 300-800. In addition, groups –NH2 play an important role. The variety of acidic functional groups is so great that L.I. Glebko proposed not to identify them, but only to divide them according to the apparent dissociation constants, pK = −lgK, where K is the dissociation constant.

Table 1. Average elemental composition of humic acids from various natural bodies, %

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*D.S. Orlov, Moscow State University