Soil lipids in the humic substance system
Recent research has once again emphasized the enormous role of non-specific components of soil humus in the formation of soil fertility and soil genesis. This group of compounds includes various physiologically active substances, carbohydrates, amino acids, and various pigments.
Substances extracted from soils by alcohol-benzene mixture occupy a special position in this group. A generally accepted term has not yet been established for this group, 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. Similar terminological uncertainty exists in geological literature, where, in particular, the term "bitumens" denotes a mixture of hydrocarbons and their derivatives, formed through 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 slightly altered compounds found in plant residues and microbial cells, i.e., waxes, resins, fatty acid glycerides, and fatty acids. This provides a basis for calling the entire group of substances under discussion **lipids**.
Currently, in plant biochemistry, the term lipids unites a large group of substances, "...fats and fat-like substances (lipoids) are combined under the general term lipids. Substances in this group dissolve in various organic solvents. Fat-soluble pigments may also be included in this group. The composition of lipids usually distinguishes between:
- 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 inclusion of hymatomelanic acid and alcohol-soluble proteins in this group cannot be excluded. However, based on the above definitions and the available data on the composition of alcohol-benzene extracts, we consider it possible and more accurate to call this 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 excluded. Soil and plant lipids have a certain similarity.
In assessing the significance of lipids in biochemical systems, the following points must be considered: the content of this group of substances in soil humus ranges from 2 to 14%, and according to some data, in peat soils, tundra, and mountainous soils, there is a clear tendency towards increased accumulation of this group up to 20–24%, and sometimes even more.
By chemical structure, lipids sharply differ 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 peculiar "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 soil type and plant associations. After removing the solvent, a yellowish-brown mass with a faint balsamic odor remains, which melts at temperatures from 63° to 87°.
According to literature data, the composition of lipids, in addition to C and H, includes O, N, P, S, and many macro- and microelements in fractions of a percent. The approximate ratio of the latter varies within significant limits 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 esters typical for waxes and anhydrides characteristic of resins. Up to 56% of acids are included in the wax component of bitumen, among which cerotic $C_{25}H_{50}O_{2}$, carboceric $C_{27}H_{54}O_{2}$, and a hydroxy acid of composition $C_{30}H_{60}O_{3}$ have been identified. In addition, the composition of waxes contains up to 44% of unsaponifiable substances; among them, saturated hydrocarbons — tritriacontane $C_{33}H_{68}$ and pentatriacontane $C_{35}H_{72}$, constituting up to 15%, have been determined. The saturated alcohol — heptacosanol $C_{27}H_{55}OH$ with a melting point of 74–75° has also been isolated. A large number of hydrocarbons have been identified: n-decane, n-undecane, n-hexadecane, naphthalene, methylnaphthalene, diphenyl, acenaphthene, fluorene. Steroids and tannins have been found.
A component of peat "bitumens" is represented by complex esters of cyclic alcohols and cyclic acids, from which unsaturated acids of composition $C_{12}H_{22}O_{2}$ and $C_{14}H_{26}O_{2}$ have been isolated. Furthermore, triterpenoids, which are widely represented in the plant kingdom, were identified.
The composition of soil alcohol-benzene extracts has been little studied so far, 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 main genetic soil types. Lipids were extracted from air-dry soil samples, from which roots were previously removed, as well as from litters of fresh fall and plant leaves. The solvent used was an alcohol-benzene mixture (1:1), and the extraction was carried out in Soxhlet and Greffe apparatuses.
Extraction in the Soxhlet apparatus is time-consuming and does not ensure the complete yield of wax-resins. Intensification of the extraction process using the Greffe–Zaychenko apparatus significantly increases the amount of wax-resins extracted from the soil (Table 1).
| Soil, land use | Horizon | Depth, cm | Soxhlet | Greffe | ||
|---|---|---|---|---|---|---|
| lipid content, % of soil | lipid carbon content, % of total carbon content | lipid content, % of soil | lipid carbon content, % of total carbon content | |||
| Southern Chernozem, arable land, Kherson | $A_{pach}$ | 0–23 | 0.09 | 4.35 | 0.28 | 14.04 |
| Chocolate Chernozem, arable land, Romania | $A_{pach}$ | 0–20 | 0.10 | 0.71 | 0.12 | 12.32 |
| Mountain-meadow soil, spirea-ryegrass meadow, Kherson | $A₁$ | 6–20 | 0.45 | 6.61 | 0.76 | 11.06 |
| Mountain-forest brown earth, fallow, Kherson | $A₁$ | 1–18 | 0.07 | 6.24 | 0.21 | 19.56 |
| Crusted Solonchak, pasture, Kherson | $B₁$ | 2–15 | 0.26 | 7.32 | 0.40 | 11.22 |
The significant increase in the yield of substances when extracted in the Greffe apparatus can 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 assumed that the previously unmeasured portion of wax-resins was part of the non-hydrolyzable residue and constituted a part of the so-called humin, although their presence in humic acids is 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 the organic C. In the upper humic horizons of many automorphic soils, the proportion 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. A relative accumulation of lipids 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 relationship of these indicators with ecological conditions are examined in more detail using the example of individual soils: profile 106 — light loamy, silty krasnozem; profile 127 — the same krasnozem under arable land; profile 123 — subtropical podzol, medium loamy; profile 120 — light loamy, sandy-silty zheltozem. Since the predominant composition of the litter above profiles 127 and 120 consisted of tea leaves and fern, 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 widely. The $A₁$ or $A_{pach}$ horizons of subtropical podzol in krasnozem on a plateau and slope contain up to 33–38% resins. Resins are the predominant group of organic compounds in the lipid composition of zheltozem under fallow and arable land (Table 2). In lipids from various litters, the resinous part predominates, constituting 58–68% of them.
| Object | Horizon | Depth, cm | Lipid Carbon | Waxes, % | Resins, % | |
|---|---|---|---|---|---|---|
| % of sample weight | % of total C | |||||
| Tea leaves, profile 127 | — | — | 22.78 | — | 29.12 | 70.78 |
| Fern leaves, profile 120 | — | — | 7.65 | — | 80.21 | 19.79 |
| Forest litter, profile 106 | $A₀$ | — | 4.36 | — | 33.06 | 66.94 |
| Tea and litter, profile 127 | $A₀$ | — | 2.92 | — | 42.44 | 57.56 |
| Tea and litter, profile 120 | $A₀$ | — | 4.90 | — | 31.99 | 68.01 |
| Forest litter profile 123 | $A₀$ | — | 5.18 | — | 37.50 | 62.50 |
| Light loamy, silty Krasnozem, profile 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 loamy, silty Krasnozem, profile 127 | $A_{pach}$ | 0–10 | 0.14 | 4.1 | 65.00 | 36.00 |
| Light loamy, sandy-silty, Gleyed Zheltozem, profile 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 loamy, silty Zheltozem, profile 120 | $A_{pach}$ | 0–10 | 0.23 | 11.3 | 29.69 | 70.31 |
| Subtropical, Podzolized, medium loamy, Coarse-silty, profile 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-Gley | $A_{t}$ | 0–33 | 3.05 | 13.10 | 65.1 | 34.9 |
| Chocolate Chernozem | $A_{pach}$ | 0–20 | 0.079 | 6.71 | — | — |
| Southern Chernozem | $A_{pach}$ | 0–23 | 0.0618 | 4.35 | — | — |
| Sod-Podzolic | $B₂$ | 48–69 | 0.0397 | 10.36 | — | — |
| Solonetz | $A_{pach}$ | 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 | $B₁$ | 2–15 | 0.1866 | 4.68 | — | — |
| $A₁$ | 6–20 | 0.3252 | 11.26 | — | — | |
| $A_{pach}$ | 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 prevail (80%) in the composition of lipids from fern leaves, while lipids from tea leaves mostly contain resins (71%). However, processes related to the complication of lipid structure already occur in the litter, resulting in the predominance of resinous, higher molecular weight components in all litters.
The distribution pattern of waxes and resins across genetic horizons is different. In zheltozem, waxes and resins are evenly distributed throughout the profile; no tendency for accumulation or depletion of any group is observed in it. In the subtropical podzol, conversely, the resin content increases with depth: 34% in the $A₁$ horizon, 50% in the $A₁A₂$ horizon, and 64% in the $BC₃$ horizon. In krasnozem, resins slightly accumulate 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 related to the intensive 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 confirmed by the analysis of thick chernozem, mountain-meadow, and peaty-gley soils. In chernozem, which is characterized by one of the highest levels of biological activity, the share of resins reaches 75%, while in peaty-gley 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 lipids from tea litter, which is consistent with literature data (Table 3). Hydrogen content varies from 4 to 10%, oxygen from 22 to 25%. The studied fractions 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.
| Extracted from what object | Elemental composition, % of ash-free dry matter | Author | |||
|---|---|---|---|---|---|
| C | H | O | N | ||
| Tea leaves under krasnozem, profile 127 | 62.97 | 6.54 | 23.90 | 6.59 | Our data |
| Tea litter of krasnozem, profile 127 | 67.18 | 3.90 | 25.37 | — | Our data |
| Krasnozem, profile 127, $A_{pach}$ hor. | 67.25 | 5.63 | 23.92 | — | Our data |
| Podzolic loam, $A₁$ hor. | 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 |
| Thick Chernozem, steppe | — | — | — | 1.57 | Our data |
| Typical Sierozem: a) grass mixture | — | — | — | 1.33 | Our data |
| Typical Sierozem: b) virgin land | — | — | — | 0.22 | Our data |
The acid numbers of lipids (Table 4) range from 1 to 12 mEq/g. Increased content of "free organic acids" compared to the upper horizons of other soils is noted in lipids from zheltozems under fallow in the $A₁$ hor.. Lipids from this horizon are also distinguished 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 the $A₁$ hor. of krasnozem, as well as the small values of other characteristics (acid number — 1, iodine number — 6 mEq/g), are explained by the predominance of sands, which carry a smaller number of functional groups. The range of variation of the iodine number is 0.5–66 mEq/g. A slight negative correlation is observed between the ester and iodine numbers in krasnozem lipids: the fewer unsaturated compounds, the greater the number of ester groups. Additionally, there is a dependence between the acid numbers, ester numbers, iodine numbers, and the content of waxes and resins.
| Soil | Horizon (Depth, cm) | Acid Number, mEq/g | Ester Number, mEq/g | Iodine Number, mEq/g |
|---|---|---|---|---|
| Subtropical podzol, medium loamy, coarse-silty, profile 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 loamy, sandy-silty, gleyed zheltozem, profile 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 loamy, silty krasnozem, on variegated clay, profile 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), profile 127 | $A₀$ | 1.45 | 19.48 | 2.61 |
| Light loamy, silty krasnozem on slope, profile 127 | $A₁$ (0-10) | 2.70 | 71.25 | 17.00 |
| Meadow Chernozem | $A₁$ (3-21) | 7.32 | 86.86 | 66 |
| Peaty-Gley | $A₁$ (0-33) | 1.36 | 0.96 | 2 |
| Mountain-meadow | $A₁$ (2-10) | 2.42 | 1.88 | 8 |
| Tundra | $A₁$ (3-10) | 2.45 | 7.07 | 2 |
| Sod-Podzolic | $A₁$ (5-9) | 3.26 | 9.92 | 10 |
Lipids from the zheltozem horizon under fallow and from chernozem have the highest values. The resinous part also predominates in these soils (Table 2). Therefore, it can be concluded that the lipids in this soil have the most complex structure, meaning all active groups characterized by the acid, ester, and iodine numbers are predominantly due to "resins".
From 7 to 22 or more peaks were found on the 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 contain simple benzene rings, substituted with 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, containing both individual components of the tea leaf and products of their interaction. Aromatic nuclei of simple nature are few in this fraction; it is characterized by a higher degree of aromaticity. 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 the tea leaf; its complexity increases significantly in the litter. Simultaneous processes of decomposition and synthesis of new organic compounds occur in the soil, as a result of which soil lipids acquire specific characteristics.
The overall appearance of the visible spectra of lipids from the $A₁$ hor. of subtropical soils, litters, and vegetation is very specific. The spectra have distinct absorption maxima for porphyrin group substances (specifically, 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 suggest some similarity in their structure. Tea leaf composition includes various substances (Table 5), a significant portion 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 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. Pectin substances | 3 | |
| 3. Alkaloids (caffeine, theophylline, theobromine) | 3–5 | |
| 4. Protein substances and amino acids (includes alcohol-soluble proteins) | 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 responsible for the aroma of tea | ||
| Essential oils | 1 | |
| D. Enzymes | ||
Absorption maxima at 205–210 nm correspond to unsaturated organic acids. Tannins correspond to weak maxima in the region of 267, 337, and 420 nm.
Tannins (tannin and catechins) are also clearly identified in the chloroform extract of tea leaf and litter lipids. In addition, absorption of carotenoids and vitamins of group A is found 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 are heterogeneous.
Analysis of the IR spectra confirmed the chemical analysis data from visible and ultraviolet spectroscopy. The bands in the IR spectra of the lipids are narrow, clearly defined, with specific absorption maxima. A broad band in the region of $3100–3400\ cm⁻¹$ corresponds to the vibrations of the OH— group.
A series of bands in the range of $2918–2850\ cm⁻¹$, 1480, $1400–1380\ cm⁻¹$ corresponds to the vibrations of the CH— group. The series of bands in the range of $2918–2850\ cm⁻¹$, 1480, $1400–1380\ cm⁻¹$ is due to the symmetric vibrations of $CH₂$ and $CH₃$ included in saturated and unsaturated hydrocarbons. Furthermore, the characteristic band in the region of $720\ cm⁻¹$ corresponds to the vibrations of terminal $(CH₂)_{n}$, where $n>4$.
Intense bands at $1710–1730\ cm⁻¹$, which lie 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 the CO— groups of aldehydes and ketones present in various compounds (alkaloids, essential oils, etc.).
Intense bands of alcoholic hydroxyls and simple ethers are observed in the interval of $1010–1030\ cm⁻¹$; the latter also appear in the range of $1100–1250\ cm⁻¹$. The spectrum of lipids 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. The 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 $745–765\ cm⁻¹$, likely attributable to $\gamma—CH$ at the pyrrole ring in porphyrins, and $819–832\ cm⁻¹$ due to $\delta—CH$ of di—and tri—substituted aromatic compounds; $1490–1550\ cm⁻¹$ are the vibrations of the pyrrole ring.
Comparison of the IR spectra of lipids from the upper horizons ($A₀$) of krasnozem, zheltozem, and subtropical podzol shows the predominance of $CH₂—$ and $CH₃—$ groups in krasnozem and subtropical podzol, which is confirmed by the high wax content (over 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 demethylation reactions resulting in the formation of a hydroxyl group. Furthermore, the inclusion of finely dispersed silicic acid in the lipid fraction cannot be 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 involvement of aromatic compounds of acids and aldehydes and their reaction 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
- The use of the Greffe 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 of increased moisture (hydromorphic) — podzolic, tundra, mountainous — and reaches a maximum in peats.
- The accumulation of lipids occurs mainly due to waxes, which is attributed to their comparative biochemical stability and inertness.
- The active functional chemical groups (characterized by ester, acid, and iodine numbers) are predominantly due to "resins," and 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 both in the total lipid content and in their qualitative composition.
- Soil lipids, according to pyrolysis-gas chromatography, electronic, and infrared absorption spectra, have some similarities and definite differences from the lipids isolated from plants and litter.