Luvisols and Cambisols are two types of sub-boreal soils, which are known as continental and humid-ocean types of soils in deciduous forests respectively. Morphological features of soil, which are frequently used as the main argument in solum diagnostics, are subjective and do not give final decision that continental Cambisols are a specific type of soils different from Luvisols. These soils were studied in a mountain massive—Jiguli ridges of Samara region, Russia, East European part. Humid climate of northern slopes leads to formation of brown type of humic acids (HA), while the conditions of eastern slopes assist to formation of gray HA. These HAs of different soils are different in elemental composition (C and N are higher in Cambisols, O is higher in Luvisols), carbon species according to 13-C NMR (aromaticity is higher in Luvisols, while the aliphatic, carbonylic and carboxylic compounds are higher in Cambisols). Cambisols are characterized by dominance of fulvic acids (FAs) on HAs, while the ratio of HA to FA groups in Luvisols is about 1,0. Essential differences in humus composition and humic acids properties confirm that local humid climate in continental forest-steppe leads to formation of Cambisols instead of zonal Luvisols.
Deciduous forest soils of subboreal zone on the Russian plain are represented by Brown soils (Cambisols) and Gray—Dark Gray soils (Luvisols) [
The main differences between conditions of soil formation are climate and parent materials. On the scale of Russian plain Cambisols are typical for western part, while Luvisols are typical for the central part of the Russian plain. Soil combination on the territory of one landscape region is very untypical and can be revealed only in case of mountain regions, characterized by slopes of different exposition. Such type of soils’ combinations were revealed in Samara Jiguli ridges, where Cambisols and Luvisols were formed on different slopes. These two soils are different in morphology, clay fraction chemistry, and mineralogy and are supposed to be very different in content, distribution, and composition of soil organic matter.
The geography of these Cambisols and Luvisols on the central part of the Russian plain is problematic and causes the discussion on the possibility of Cambisol’s presence in continental part of forest-steppe zone with some deviations of local climate. This discussion started in 1970–1980 and hasn’t finished yet [
Cambisols are known as soils with prevalence of fulvic acids and dominance of alkylic-C carbon species in molecules with essential part of low molecular part [
The objective of this study was to reveal differences in soil organic matter and humic acids properties of Cambisols and Luvisols in order to determine if the Cambisols are really different from Luvisols and thus give additional arguments, which could enable us to treat Cambisols as a specific soil type in the central part of the Russian plain.
Jiguli ridges are situated in Samarskaya Luka, Samara Region, Midlle Povolgye, Russian plain. These ridges were formed in Pliocene and now their maximal height is 371 m. Now these ridges are a part of a protected area—Jiguli State Reserve, which is characterized by unusually high diversity of soils, presented by Cambisols, Luvisols, Rendzic Leptosols, Chernozems, and Albeluvisols. This diversity is caused by both great spatial heterogeneity of parent materials and redistribution of precipitation and insolation on slopes of different expositions.
Climate of Samara Ridges is classified as continental [
The soils investigated (Figure
Morphology of two soils investigated: (a) Cambisol and (b) Luvisol.
The vegetation cover is represented by lime (
At least 3 soil pits were used for each type of soil for soil morphology description, determination of soil taxonomy, and sampling of individual samples. Soil pits were made according to detailed soil map of slopes; it means that 3 pits for each type of soil were made in one polypedon. Then, in laboratory, 3 or 4 individual samples of each horizon were mixed and homogenized. These samples of soil horizons of Cambisol and Luvisols with related profiles of Rendzic Leptosols in catenas were dried, grounded, and sieved through 2-mm sieve.
Humic acids (HA) powders were isolated according to IHSS standart method.
The total organic carbon content was determined by Tuyrin dichromate-oxidation method (almost the same as Walkey and Black method); total nitrogen was measured in accordance with [
According to this procedure [
The elemental analysis of HA was conducted on 185B-Hewlett Packard analyzer. Data was corrected on water and ash content.
The C-13-NMR spectra were registered on Brucker Ultra-Shield spectrometer with TMSPNA as a standard in liquid state.
The thickness of humus-accumulation horizons in Cambisol was 15 cm; the horizons were brown-colored, with high porosity, and overlaid by thick forest floor, which consisted of good expressed L and F subhorizons with total thickness of about 5 cm. Luvisol profile revealed the short forest floor—about 2.5–3.0 cm, underlaid by thick (25) dark-gray-colored humus horizon.
The middle parts of the two profiles were also different, that is, BW horizon of Cambisol was characterized by light-brown color, increased density in comparison with over- and underlayed horizon, and absolute absence of cutans. A layer of Luvisol is changed by EL—eluvial bleached horizon with abundant spots of silica accumulation; the lower transitional layer contains siltans, skeletons, and argillans, and leached spots of eluviation. This transitional part was underlayed by BT—clay-illuvial horizon with abundant agrillic cutans.
The main differences identified by soil macromorphology are the dominance of local weathering process in Cambisol and prevalence of eluvial-illuvial differentiation of soil profile in Luvisol. There was also morphological evidence of higher intensity of humus accumulation in Luvisol as compared to Cambisol. This is frequently explained by a more intensive development of humification process in dry and continental conditions, while the wet and humid climate assists to formation of acid-brown-mull humus with low degree of humification [
Total stock of fresh forest floor identified as L horizon is estimated as 340 and 22 g/m2 for Cambisol and Luvisol, respectively, while the stock of organic matter in fermentation (F) subhorizon was estimated as 351 and 280 g/m2. The root stock was estimated as 124 and 88 g/m2 for these soils. This data shows that Cambisol is characterized by slower processes of mineralization and humification of organic matter as compared to Luvisol.
The absolute total content of organic carbon (Table
Total carbon content, portion of HAs, humus type ratio and humus richness by nitrogen.
Horizon | Ctotal, g/kg | C/N | ||
---|---|---|---|---|
Cambisol | ||||
A | 37.0 | 19. | 0.66 | 15.5 |
AB | 10.2 | 18.6 | 0.38 | 19.9 |
BW | 3.4 | 23.6 | 0.61 | 6.6 |
Rendzic leptosol related to cambisol | ||||
A | 38.0 | 26.4 | 1.10 | 16.7 |
Luvisol | ||||
A | 49.0 | 18.0 | 0.86 | 12.3 |
BEL | 3.6 | 25.0 | 1.00 | 18.0 |
BT | 4.6 | 38.0 | 1.04 | 8.2 |
Rendzic leptosol related to luvisol | ||||
A | 37.6 | 31.3 | 2.31 | 14.5 |
The groups of HAs in soils: Percentage of humic acid carbon to total C content.
Horizon | Forms of HAs | ||
Free | Ca-connected | Clay-connected | |
Cambisol | |||
A | 4.0 | 7.8 | 7.8 |
AB | 2.9 | 9.8 | 5.9 |
BW | 5.9 | 5.9 | 11.8 |
Rendzic leptosol related to cambisol | |||
A | 10.4 | 5.3 | 10.7 |
Luvisol | |||
A | 2.8 | 8.9 | 6.3 |
BEL | 1.0 | 16.0 | 8.0 |
BT | 2.0 | 35.0 | 26.0 |
Rendzic leptosol related to luvisol | |||
A | 1.6 | 18.6 | 11.1 |
The profile distribution of humic acids was more or less gradual in Cambisol solum, while it decreases sharply with depth in Luvisol. Previously it was shown that if HA portions increase with depth, then some migration of HA is possible [
The same differences were revealed for Rendzic Leptosols of ridges crests related to Cambisol and Luvisol in catenas. The data in Table
According to the classification of HAs by Tuyrin
The difference between soils in HAs elemental composition (Table
HA elemental composition.
Element content, atomic, | Cambisol | Luvisol | RL related to Cambisol | RL related to Luvisol |
---|---|---|---|---|
C | ||||
H | ||||
N | ||||
O | ||||
C/N | 12.79 | 16.92 | 15.39 | 13.17 |
H/C | 1.00 | 1.13 | 0.94 | 1.06 |
O/C | 0.62 | 0.72 | 0.62 | 0.62 |
On the base of 13-C NMR (Table
Carbon species of humic acids on the base of 13-NMR spectra, Percentage of each carbon species to carbon in molecule.
Carbon species | Cambisol | Luvisol | RL related to cambisol | RL related to luvisol |
---|---|---|---|---|
Aromaticity | 0.6 | 0.7 | 0.6 | 0.7 |
Aromatic C | 29.4 | 42.3 | 23.4 | 34.9 |
Aliphatic C | 18.9 | 13.3 | 18.2 | 18.4 |
Carbonylic | 5.8 | 4.4 | 3.5 | 3.4 |
Carboxylic | 20.3 | 16.8 | 17.5 | 18.8 |
Phenolic | 6.9 | 7.7 | 5.3 | 7.7 |
Aldehyde | 19.1 | 16.0 | 32.9 | 17.0 |
Russian steppe and forest steppe are usually considered as plain-type macrolandscapes with continental climate with dominance of Chernozems in steppes and Luvisols in forest-steppes. These soils are characterized by dark-colored humus accumulative horizons, prevalence of well-aromitisized HAs of “dark type” [
The main result of this paper is the demonstration of the fact, that humid climate assists in the formation of Cambisols, which are ocean type of soils, even in central part of the Russian plain, which for the first time gives evidence of radical changes of humus and humic acids type.