Soil Lipids in the Humic Substances System
SOIL LIPIDS IN THE HUMIC SUBSTANCES SYSTEM
Recent studies have once again emphasized the enormous role of non-specific components of soil humus in the formation of soil fertility and the genesis of soils. These compounds include various physiologically active substances, carbohydrates, amino acids, and various pigments.
A special position in this group is occupied by substances extracted from soils by alcohol-benzene mixture extraction. An generally accepted term for this group has not yet been established, and its position in the system of humic substances remains unclear. Substances extracted from soils by an alcohol-benzene mixture are called "bitumens" or wax-resins in the literature, and in many cases, they are simply referred to as a group of compounds extracted by an alcohol-benzene mixture, without using a special term.
Most authors use the term lipids. A similar terminological ambiguity exists in geological literature, where, in particular, the term "bitumens" denotes a mixture of hydrocarbons and their derivatives formed by the transformation of waxes, resins, and fatty acids over geological periods.
Soil humus is a relatively young formation, and the substances in the alcohol-benzene extracts of soils should be represented by little-altered compounds found in plant residues and microbial cells, i.e., waxes, resins, fatty acid glycerides, and fatty acids. This gives a reason to call the entire group of substances under discussion **lipids**.
Currently, in plant biochemistry, the term lipids encompasses a large group of substances, "...fats and fat-like substances (lipoids) are combined under the general term lipids. Substances in this group are soluble in various organic solvents. Fat-soluble pigments can also be included in this group. Within the composition of lipids, the following are usually distinguished:
- neutral fats, waxes, and steroids,
- phospholipids,
- glycolipids and other complex lipids,
- as well as a number of other compounds, including acids, alcohols, vitamins, higher hydrocarbons, including carotenes and carotenoids, etc.
It is likely that in some cases, soil alcohol-benzene extracts may contain some other compounds; in particular, the presence of hymatomelanic acid and alcohol-soluble proteins in this group cannot be excluded. However, based on the definitions above and the available data on the composition of alcohol-benzene extracts, we consider it possible and more appropriate to call the indicated group of substances **soil lipids**.
Lipids enter the soil with plant residues; furthermore, the possibility of lipid synthesis directly in the soil by various microorganisms, such as paraffin-oxidizing yeasts, cannot be ruled out. Soil and plant lipids have a certain similarity.
In assessing the importance of lipids in biochemical systems, the following points must be taken into account: the content of this group of substances in soil humus ranges from 2 to 14%, and according to some data, in peat-rich soils, tundra, and mountainous soils, there is a clear tendency for increased accumulation of this group up to 20—24%, and sometimes even more.
By chemical structure, lipids differ sharply from other groups of humic substances, having a significant proportion of aliphatic structures and hydrophobic groups in their composition. The latter leads to a number of specific chemical properties. This group is of particular interest due to its unique "labeling" by the presence of skeletal forms such as paraffin (fats, waxes), steroids, terpenes, carotenoids, chlorophyll, and other porphyrin pigments. These compounds are relatively easily detected by specific electronic or molecular spectra, which makes it possible to trace the pathways and mechanisms of transformation of these substances during humification and diagenesis.
Alcohol-benzene extracts from soils are liquids of various colors, which range from light yellow (milky) to brownish-orange, depending on the type of soil and plant associations. After removing the solvent, a yellow-brown mass with a faint balsamic odor remains, which melts at a temperature between 63° and 87°.
According to literature data, the composition of lipids, in addition to C and H, contains O, N, P, S, and trace amounts of many macro- and microelements. The approximate ratio of the latter varies significantly in different soils (58—68% C, 8—10% H; 22—32% O; 0.4— 2.0% N).
Soil and peat lipids mainly consist of waxes and resins, which, in turn, contain free acids and saponifiable substances, represented by typical wax esters and anhydrides characteristic of resins. The wax component of bitumen includes up to 56% acids, among which cerotic acid C25H50O2, carboceric acid C27H54O2, and an oxyacid with the composition C30H60O3 have been identified. In addition, the waxes contain up to 44% unsaponifiable substances; among them, saturated hydrocarbons—tritriacontane C33H68 and pentatriacontane C35H72, constituting up to 15%, have been identified, as well as a saturated alcohol—heptacosanol C27H55O with a melting point of 74–75°. A large number of hydrocarbons have been identified: n-decane, n-undecane, n-hexadecane, naphthalene, methylnaphthalene, diphenyl, acenaphthene, fluorene. Steroids and tannins have also been found.
The component of peat "bitumens" is represented by complex esters of cyclic alcohols and cyclic acids, from which unsaturated acids with the composition C12H22O2 and C14H26O2 have been isolated. Furthermore, triterpenoids, which are widely represented in the plant world, were successfully identified.
The composition of soil alcohol-benzene extracts is still poorly studied, although according to several authors, fatty acids, fats, waxes, resin acids and their esters, sterols, triterpenoids, hydrocarbons, etc., may be present here.
We investigated the lipid fractions of the major genetic types of soils. Lipids were extracted from air-dry soil samples, from which roots were previously removed, as well as from litter of fresh fallen leaves and plant leaves. The solvent used was an alcohol-benzene mixture (1 : 1), and the extraction was carried out in Soxhlet and Graefe apparatuses.
Extraction in a Soxhlet apparatus takes a long time and does not ensure a complete yield of wax-resins. Intensification of the extraction process using the Graefe-Zaichenko apparatus significantly increases the amount of wax-resins extracted from the soil (Table 1).
| Soil, Land Use | Horizon | Depth, cm | Soxhlet | Graefe | ||
|---|---|---|---|---|---|---|
| lipid content, % of soil | lipid carbon content, % of total carbon | lipid content, % of soil | lipid carbon content, % of total carbon | |||
| Southern Chernozem, arable land, Kherson | Apakh | 0—23 | 0.09 | 4.35 | 0.28 | 14.04 |
| Chocolate Chernozem, arable land, Romania | Apakh | 0—20 | 0.10 | 0.71 | 0.12 | 12.32 |
| Mountain Meadow Soil, meadowsweet-ryegrass meadow, Kherson | A₁ | 6—20 | 0.45 | 6.61 | 0.76 | 11.06 |
| Mountain Forest Brown Soil, fallow land, Kherson | A₁ | 1—18 | 0.07 | 6.24 | 0.21 | 19.56 |
| Crusted Solonchak, pasture, Kherson | В₁ | 2—15 | 0.26 | 7.32 | 0.40 | 11.22 |
The significant increase in the yield of substances during extraction in the Graefe apparatus may significantly change our understanding of the role of this fraction in soil biochemistry. It remains to be seen which fraction accounts for the observed difference. It can be tentatively suggested that the previously unmeasured portion of wax-resins was included in the non-hydrolysable residue and constituted part of the so-called humin, although their presence in humic acids is also not excluded.
In the studied soils (Table 2), the lipid content ranges from 0.02 to 0.50% of the soil, and from 2.0 to 80.0% of organic C. In the upper humus horizons of many automorphic soils, the share of lipids is 2–10% of the total carbon (based on Soxhlet extraction). Increased lipid content is associated with soils of increased moisture (hydromorphic), peaty, tundra, and mountainous soils. Relative accumulation is often observed in deeper horizons. In some soils, the absolute content of this fraction remains constant throughout the soil profile depth. The relative accumulation of lipids in the B and C horizons is apparently related to the latter.
The physical and chemical characteristics of lipids and the connection of these indicators with ecological conditions are examined in more detail using the example of individual soils: profile 106 — light clay, silty Krasnozem (Red Earth); profile 127 — the same Krasnozem under arable land; profile 123 — subtropical Podzol, medium loamy; profile 120 — light clay, sandy-silty Zheltozem (Yellow Earth). Since the predominant composition of the litter above profiles 127 and 120 consisted of tea and fern leaves, we considered it necessary to trace the content and composition of lipids in the plant—litter—soil system.
The lipid fraction is easily separated into waxes and resins by extracting the latter with acetone. The content of waxes and resins in lipids varies within wide limits. The A₁ or Apakh horizons of subtropical Podzol in Krasnozem on a plateau and slope contain up to 33–38% resins; in the composition of Zheltozem lipids under fallow land and arable land, resins are the predominant group of organic compounds (Table 2). The resinous part, accounting for 58–68% in the lipids from various litters, prevails.
| Object | Horizon | Depth, cm | Lipid Carbon | Waxes, % | Resins, % | |
|---|---|---|---|---|---|---|
| % of sample | % of Total C | |||||
| Tea leaves, p. 127 | — | — | 22.78 | — | 29.12 | 70.78 |
| Fern leaves, p. 120 | — | — | 7.65 | — | 80.21 | 19.79 |
| Forest litter, p. 106 | A₀ | — | 4.36 | — | 33.06 | 66.94 |
| Tea and litter, p. 127 | A₀ | — | 2.92 | — | 42.44 | 57.56 |
| Tea and litter, p. 120 | A₀ | — | 4.90 | — | 31.99 | 68.01 |
| Forest litter p. 123 | A₀ | — | 5.18 | — | 37.50 | 62.50 |
| Light clay, silty Krasnozem, p. 106 | A₁ | 0—14 | 0.39 | 8.5 | 61.11 | 38.89 |
| AB | 14—26 | 0.09 | 11.3 | 66.92 | 33.08 | |
| B₁ | 26—52 | 0.14 | 36.8 | 67.50 | 32.50 | |
| B₂ | 52—76 | 0.11 | 38.0 | 60.24 | 39.76 | |
| C₁ | 76—140 | 0.09 | 53.0 | 58.46 | 41.54 | |
| C₂ | 140—190 | 0.14 | 77.7 | 55.26 | 44.74 | |
| Light clay, silty Krasnozem, p. 127 | Apakh | 0—10 | 0.14 | 4.1 | 65.00 | 36.00 |
| Light clay, sandy-silty, Gleyic Zheltozem, p. 120 | A₁ | 0—15 | 0.18 | 9.5 | 36.73 | 63.27 |
| AB | 15—36 | 0.19 | 36.1 | 32.59 | 67.41 | |
| B₁ | 36—57 | 0.02 | 5.7 | 29.69 | 70.31 | |
| B₂ | 57—115 | 0.23 | 76.7 | 28.26 | 71.74 | |
| B₃ | 115—150 | 0.12 | 80.0 | 29.51 | 70.49 | |
| BC | 150—200 | 0.04 | 66.6 | 30.34 | 69.66 | |
| C | 200—220 | 0.10 | 77.0 | 31.71 | 69.29 | |
| Light clay, silty Zheltozem, p. 120 | Apakh | 0—10 | 0.23 | 11.3 | 29.69 | 70.31 |
| Subtropical, Podzol., medium loamy, Coarse silty, p. 123 | A₁ | 0—10 | 0.28 | 10.3 | 66.23 | 33.77 |
| A₁A₂ | 10—20 | 0.12 | 14.5 | 50.00 | 50.00 | |
| A₂ | 20—34 | 0.02 | 20.0 | 49.00 | 51,00 | |
| BC | 34—57 | 0.06 | 24.0 | 37.75 | 62.25 | |
| BC₁ | 57—100 | 0.14 | 93.4 | 38.00 | 62.00 | |
| BC₂ | 100—120 | 0.06 | 75.0 | 36.00 | 64.00 | |
| BC₃ | 120—140 | 0.21 | 87.6 | 35.58 | 64.42 | |
| Meadow Chernozem | A₁ | 3—21 | 0.22 | 2.69 | 24.6 | 75.4 |
| Mountain Meadow (Teberda) | A₁ | 2—10 | 0.59 | 7.40 | 44.8 | 55.2 |
| Peaty-Gleyic Soil | At | 0—33 | 3.05 | 13.10 | 65.1 | 34.9 |
| Chocolate Chernozem | Apakh | 0—20 | 0.079 | 6.71 | — | — |
| Southern Chernozem | Apakh | 0—23 | 0.0618 | 4.35 | — | — |
| Sod-Podzolic Soil | B₂ | 48—69 | 0.0397 | 10.36 | — | — |
| Solonetz | Apakh | 0—17 | 0.0987 | 12.88 | — | — |
| Crusted Solonchak | B₁ | 45—62 | 0.0397 | 7.32 | — | — |
| Mountain Meadow (Crimea) | A | 0—5 | 0.5033 | 6.61 | — | — |
| Brown Forest Soil | B₁ | 2—15 | 0.1866 | 4.68 | — | — |
| A₁ | 6—20 | 0.3252 | 11.26 | — | — | |
| Apakh | 0—28 | 0.0497 | — | — | — | |
| B₁ | 54—84 | 0.0439 | — | — | — | |
The study of the composition of lipids from fern and tea leaves indicates a different nature of their constituent compounds. Waxes (80%) predominate in the composition of lipids from fern leaves, while lipids from tea leaves mostly contain resins (71%). However, processes associated with the complication of lipid structure already occur in the litter, as a result of which resinous, more high-molecular-weight components prevail in all litters.
The distribution pattern of waxes and resins across the genetic horizons is different. In Zheltozem, waxes and resins are distributed uniformly throughout the profile, and there is no tendency for either group to accumulate or deplete. In subtropical Podzol, on the contrary, the resin content increases with depth: 34% in the A₁ horizon, 50% in the A₁A₂ horizon, 64% in the BC₃ horizon. In Krasnozem, resins accumulate slightly down the profile (from 39% in the A₁ horizon to 45% in the C₂ horizon), but waxes are the predominant group in this soil. This feature is probably associated with intense microbiological decomposition processes of humus components, as a result of which waxes, as a more inert group, accumulate in the Krasnozem profile.
This statement is supported by the analysis of deep Chernozem, mountain meadow, and peaty-gleyic soils. In Chernozem, which is characterized by one of the highest levels of biological activity, the proportion of resins reaches 75%, whereas in peaty-gleyic soil, it does not exceed 35%. Thus, the level of biological activity of soils is reflected not only in the total lipid content but also in their qualitative composition.
Elemental analysis data for some lipids show a very high carbon content: from 63% in the lipid fraction from tea leaves to 67% in the lipids from tea litter soil, which is consistent with literature data (Table 3). Hydrogen content varies from 4 to 10%, and oxygen from 22 to 25%. The fractions studied contain quite a lot of nitrogen. Apparently, the increase in nitrogen content in some lipids is due to the nature of plant residues, which may contain heterocyclic compounds of the pyrrole type and its derivatives, such as chlorophyll, which are extracted by alcohol-benzene.
| Object from which isolated | Elemental composition, % of ash-free dry matter | Author | |||
|---|---|---|---|---|---|
| C | H | O | N | ||
| Tea leaves under Krasnozem, p. 127 | 62.97 | 6.54 | 23.90 | 6.59 | Our data |
| Tea litter of Krasnozem, p. 127 | 67.18 | 3.90 | 25.37 | — | Our data |
| Krasnozem, p. 127, hor. Apakh | 67.25 | 5.63 | 23.92 | — | Our data |
| Podzolic loam, hor. A₁ | 68.17 | 9.74 | 21.72 | 0.37 | Our data |
| Chernozem A₁ | 66.27 | 8.71 | 24.32 | 0.70 | Our data |
| Sod-Podzolic: a) forest | — | — | — | 2.02 | Our data |
| Sod-Podzolic: b) fallow | — | — | — | 0.41 | Our data |
| Deep Chernozem, steppe | — | — | — | 1.57 | Our data |
| Typical Serozem: a) grass mixture | — | — | — | 1.33 | Our data |
| Typical Serozem: b) virgin land | — | — | — | 0.22 | Our data |
The acid numbers of lipids (Table 4) vary from 1 to 12 mg-eq/g. An increased content of "free organic acids" compared to the upper horizons of other soils is noted in lipids from Zheltozem under fallow land, hor. A₁. Lipids from this horizon are also characterized by high ester numbers (92) and iodine numbers (21). High ester numbers and iodine numbers are also characteristic of Chernozem lipids. The minimum value of the ester number in lipids from hor. A₁ of Krasnozem, as well as the low values of other characteristics (acid number — 1, iodine number — 6 mg-eq/g) are explained by the predominance of sands, which carry fewer functional groups. The range of change in the iodine number is 0.5— 66 mg-eq/g. In Krasnozem lipids, there is some negative correlation between the ester and iodine numbers; the less unsaturated compounds, the more ester groups there are. In addition, there is a relationship between the acid, ester, and iodine numbers and the content of waxes and resins.
| Soil | Horizon (Depth, cm) | Acid Number, mg-eq/g | Ester Number, mg-eq/g | Iodine Number, mg-eq/g |
|---|---|---|---|---|
| Subtropical Podzol, medium loamy, coarse silty, p. 123 | A₁ (0-10) | 4.55 | 52.60 | 10.39 |
| B (34-57) | 12.50 | 88.75 | 3.12 | |
| BC₁ (120-140) | 2.88 | 43.27 | 0.48 | |
| Light clay, sandy-silty, Gleyic Zheltozem, p. 120 | A₁ (0-15) | 10.20 | 92.45 | 21.35 |
| B₁ (36-57) | 4.00 | 75.00 | 20.60 | |
| C (200-220) | 3.66 | 95.12 | 2.44 | |
| Light clay, silty Krasnozem, on zebra-like clay, p. 106 | A₁ (0-14) | 0.93 | 6.48 | 6.02 |
| B₁ (26-52) | 3.75 | 71.25 | 2.12 | |
| C₂ (140-190) | 1.32 | 67.11 | 0.52 | |
| Leaf (tea) | — | — | — | 2.23 |
| Litter (tea), p. 127 | A₀ | 1.45 | 19.48 | 2.61 |
| Light clay, silty Krasnozem, on slope, p. 127 | A₁ (0-10) | 2.70 | 71.25 | 17.00 |
| Meadow Chernozem | A₁ (3-21) | 7.32 | 86.86 | 66 |
| Peaty-Gleyic Soil | A₁ (0-33) | 1.36 | 0.96 | 2 |
| Mountain Meadow Soil | A₁ (2-10) | 2.42 | 1.88 | 8 |
| Tundra Soil | A₁ (3-10) | 2.45 | 7.07 | 2 |
| Sod-Podzolic Soil | A₁ (5-9) | 3.26 | 9.92 | 10 |
Lipids from the hor. A₁ of Zheltozem under fallow land and from Chernozem have the highest number values. The resinous part also predominates in these soils (Table 2), hence, it can be concluded that the lipid structure in this soil is the most complex, i.e., all active groups, characterized by ester, acid, and iodine numbers, are primarily due to "resins."
From 7 to 22 or more peaks were found on pyrolysis-gas chromatograms, which are tentatively attributed to the following compounds:
- CO
- CH₄
- N₂
- CO₂
- benzene
- toluene
- p-xylene
- phenol
- pyrocatechol
- ethyl-benzene, etc.
Lipids from tea leaves have simple benzene rings in their composition, substituted by numerous alkanes and carrying many functional groups of acidic nature, which yield undefined pyrolysis products.
The chromatogram from the tea litter is the most complex among those obtained, in which both individual components of the tea leaf and products of their interaction are present. There are few simple aromatic nuclei in this fraction, and a high degree of aromaticity is characteristic of it. The composition of lipids from tea litter contains many functional substituents that yield unseparated products upon pyrolysis: fatty acids, amino acids.
The structure of soil lipids from Krasnozem is labile, "loose," consisting of a large number of benzene rings connected by —CH₂—C—O— bridges; there are a large number of substituents in the form of normal and branched alkanes.
Thus, according to pyrolysis-gas chromatography data, the simplest structure belongs to lipids from tea leaf; in the litter, its structure becomes significantly more complex. Simultaneous processes of decay and synthesis of new organic compounds occur in the soil, as a result of which soil lipids acquire specific characteristics.
The general appearance of the visible spectra of lipids from hor. A₁ of subtropical soils, litter, and vegetation is very specific. The spectra have distinct absorption maxima for porphyrin-group substances (in particular, pheophytin, chlorophyll a and b) in the region of 418—420 nm and 668—670 nm, as well as small maxima at 510 nm, 540 nm, and 610 nm. Consequently, chlorophyll and other porphyrin-group substances are contained in the lipids. The extinction coefficients of lipids from the upper soil horizons range from 0.001 to 0.003.
Studies of alcohol-benzene extracts from tea leaves, tea litter, and Krasnozem allow us to speak of some similarity in their structure. The composition of the tea leaf contains various substances (Table 5), a significant part of which will pass into the alcohol-benzene extract. The alcohol fraction from tea leaf lipids likely contains alkaloids (caffeine), which are characterized by absorption maxima at 212 and 286 nm. These maxima are absent in the litter and soil.
| Substances in the tea composition | Approximate content, % of dry matter | |
|---|---|---|
| A. Phenolic substances | ||
| 1. Tannins: tannins, derivatives of polyhydric phenols. | 2 | |
| 2. Flavonols-glycosides of the diphenylpropane nucleus. | 1—2 | |
| B. Non-phenolic substances | ||
| 1. Carbohydrates | 0.2 | |
| 2. Pectic substances | 3 | |
| 3. Alkaloids (caffeine, theophylline, theobromine) | 3—5 | |
| 4. Protein substances and amino acids (alcohol-soluble proteins are present) | 30 | |
| Chlorophyll and associated pigments (carotene and xanthophyll) | up to 1 | |
| Organic acids | ||
| Resinous substances (resin acids) | 7—8 | |
| Vitamins | ||
| 9. Mineral substances | 4—5 | |
| C. Substances that determine the aroma of tea | ||
| Essential oils | 1 | |
| D. Enzymes | ||
Absorption maxima at 205—210 nm correspond to unsaturated organic acids. Weak maxima in the region of 267, 337, and 420 nm correspond to tannins.
Tannins (tannin and catechins) are also clearly identified in the chloroform extract from tea leaf and litter lipids. In addition, absorption of carotenoids and group A vitamins is detected in the ultraviolet region. Carotenoids absorb in the region of 338 and 454 nm, vitamin A₁ at 326 nm, and vitamin A₂ at 287 and 351 nm. Thus, the UV spectra of the studied lipids have a heterogeneous character.
Analysis of IR spectra confirmed the data from chemical analysis, visible, and ultraviolet spectroscopy. The bands of the IR spectra of lipids are narrow, clearly defined, with definite absorption maxima. A wide band in the region of 3100—3400 cm⁻¹ corresponds to OH—group vibrations.
A series of bands in the range 2918—2850 cm⁻¹, 1480, 1400—1380 cm⁻¹ corresponds to CH—group vibrations. The series of bands in the range 2918—2850 cm⁻¹, 1480, 1400—1380 cm⁻¹ is due to the symmetrical vibrations of CH₂ and CH₃, which are part of saturated and unsaturated hydrocarbons. In addition, a characteristic band in the region of 720 cm⁻¹ corresponds to the vibrations of terminal (CH₂)n, where n>4.
Intense bands at 1710—1730 cm⁻¹, lying in the absorption region of carboxyl groups, are characteristic of all spectra. Since the acid numbers in the studied lipids are generally low, the vibrations at 1710—1730 cm⁻¹ may be due to C=O—groups of aldehydes and ketones, which are part of various compounds (alkaloids, essential oils, etc.).
Intense bands of alcoholic hydroxyls and simple ethers are observed in the interval 1010— 1030 cm⁻¹; the latter also manifest in the range 1100— 1250 cm⁻¹. The lipid spectrum from tea is distinguished by the presence of intense clear bands at 1698—1650 cm⁻¹, characteristic of C=C—bonds in conjugated complex structures such as triterpenoids. Maxima in the region of 1650—1557 cm⁻¹ are partially due to the content of aromatic compounds.
A feature of the IR spectrum of the lipid fraction from tea leaves is the clear bands at 745—765 cm⁻¹, probably attributable to γ—CH at the pyrrole ring in porphyrins, and 819—832 cm⁻¹ due to δ—CH of di—and tri—substituted aromatic compounds; 1490—1550 cm⁻¹ are vibrations of the pyrrole ring.
Comparison of the IR spectra of lipids from the upper horizons (A₀) of Krasnozem, Zheltozem, and subtropical Podzol shows a predominance of CH₂— and CH₃—groups in Krasnozem and subtropical Podzol, which is confirmed by the high wax content (more than 60%). It is interesting to note the increase in the intensity of the 1000—1100 cm⁻¹ band in lipids from the lower horizons compared to the upper ones, which can be explained by demethyllation reactions with the formation of an oxy group. Furthermore, the inclusion of fine-dispersed silicic acid in the lipid fraction is not excluded.
A feature of the IR spectra of lipids from the lower horizons is the increase in the width of the 3000—3400 cm⁻¹ band (hydrogen bonds), which indicates a change in the primary molecular structure with depth.
Judging by the IR spectra, the lipid preparations are a mixture of saturated and unsaturated hydrocarbons and alcohols with the participation of aromatic compounds, acids, and aldehydes, and their interaction products, which is consistent with their chemical and spectral characteristics. At the same time, a number of bands in the spectra of alcohol-benzene extracts from tea leaves, litter, and soil can be partially attributed to porphyrin rings or pyrrole derivatives.
CONCLUSIONS
- Using the Graefe apparatus for extraction instead of the Soxhlet apparatus allows increasing the yield of the lipid fraction by 2–3 times.
- The lipid content, which is minimal in meadow-steppe and steppe soils, increases in soils with increased moisture (hydromorphic)—Podzolic, tundra, and mountainous soils, and reaches a maximum in peatlands.
- Lipid accumulation occurs mainly due to waxes, which is determined by their comparative biochemical stability and inertness.
- Active functional chemical groups (characterized by ester, acid, and iodine numbers) are primarily due to "resins"; their content is maximal in Chernozem, which is characterized by one of the highest levels of biological activity.
- Thus, the level of biological activity of soils is reflected in both the total lipid content and their qualitative composition.
- According to pyrolysis-gas chromatography, electronic, and infrared absorption spectra, soil lipids have some similarities and definite differences from the lipids isolated from plants and litter.