Morphological features of soils in post-pyrogenic forest plantations

Nureev Nail' Bilalovich ORCID: 0000-0003-1797-0700 PhD in Biology Associate Professor; Department of Ecology, Soil Science and Environmental Management; Volga State Technological University 424000, Russia, Republic of Yoshkar-Ola, Lenin Square, 3 amimalinur@mail.ru

Morphological soil features are among the most important in field study and correct identification of not only soil type, but also supertype categories (KiDPR, 2004). The external features of soils depend on a whole range of natural soil-forming factors, and can change dramatically under anthropogenic or other influences. In some cases, this may even lead to a change in their classification position at the type or subtype level.

A feature of forest ecosystems is their heterogeneity and the variety of relationships between different components, including soils, even within the same climatic zone, which is the subject of interest to a large number of researchers. The aspects of the influence of various soil properties on vegetation have been studied quite well, and a large number of works have been devoted to this issue. Whereas the effect of vegetation on the soil cover as a whole is much more multifaceted and poorly studied ^[1]. One of the most important distinguishing features of forest soils is the presence of forest litter on the surface as an independent biohorizon, which performs many different functions – a source of nutrients, a habitat for various organisms, a water and heat regulating role, etc., thereby representing a rather valuable component of the soil. It is the forest floor that acts as a barrier between any external factors and the mineral layer. One of these impacts is forest fires, which, depending on their intensity and duration, can affect either only the surface organic layer, or entail more serious post-pyrogenic changes due to thermal effects, including the development of soil erosion due to loss of surface organic matter ^[2]. At the same time, post-pyrogenic restoration of vegetation and soil cover is of great interest to researchers ^[3-5]. Since the above processes largely depend on various factors, forest fires are also one of the important environmental factors of modern soil formation. M.I. Ushakov pointed out that natural forest fires have always been the most important environmental factor changing the appearance of landscapes ^[6]. Many researchers have noted significant changes in both morphological and physico-chemical parameters of soils ^{[3; 4; 7; 10;11]}. In this paper, the impact of forest fires on the morphological features of soils is considered.

The purpose of the work is to assess the impact of grass-roots forest fires on the morphological features of soils of light granulometric composition using the example of pine plantations in the Republic of Mari El.

Research materials and methods

The research was carried out in lichen pine forests that were affected by fires in 2021 in the territory of the Republic of Mari El. 4 full-profile soil sections were laid at the study site, one of which (P-4) was located in a control area that was not affected by fire and 3 sections in pyrogenic areas. The degree of fire intensity in the surveyed areas was relatively uniform and low, as evidenced by fragmentary damage to the bark of trees up to 1-2 m high (Fig.1). A detailed description of morphological features along genetic horizons was carried out in the soil sections.

Fig.1 Post-pyrogenic forest plantations of the Republic of Mari El

In all the surveyed areas, the predominant tree species was common pine (the proportion of 8-10 units), a similar type of forest conditions was A2. The soil-forming rocks are ancient alluvial sands of fluvial origin, on which soils of light granulometric composition were formed accordingly. The description and diagnostics of soils were carried out according to the "Classification and Diagnostics of soils of Russia" (KiDPR) (2004) ^[11], as well as the "Field Soil Determinant" (2008) ^[12].

Research results and discussion

Soil section 1 (R-1) was laid on an elevation, the damaged burnt stand was cut down. The relief is bumpy, the microrelief is represented by dead wood. There is a noticeable presence of self-seeding of pines and birches 2-4cm with uneven placement, TL – A2. The living ground cover is represented by anemone, sedges, very rare, the degree of coverage is 5-10%.

The soil profile has the following structure:

Orig 0-3cm – the litter is represented by charred remains (ash), 3cm thick, dry, the transition to the mineral horizon is sharp.

We 3-7cm is a coarse humus horizon of dark gray color, dry, slightly tinged, loose lumpy structure, sandy loam, slightly compacted, small roots, noticeable transition.

BF 7-24cm is an alfegumus horizon of brownish color, with a fragile nutty structure, sandy loam, dense, with an admixture of small roots and streaks of humus along the roots, fresh, the transition is clear.

BCf 24-48cm is a transitional horizon of yellowish–brown color, there is no structure, the grain composition is loose-sanded, compacted, roots, glandular spots are present, moist, the transition is noticeable.

C1d 48-136cm – the parent rock is represented by ancient alluvial sand, yellowish-fawn in color, the structure is absent, the granulation is loose sand, slightly compacted, visibly illuminated with clay-ferruginous material, roots, the lower part of the horizon is glazed, moist, the transition is noticeable.

C2ff 136-178 cm is the parent rock of a heterogeneous brownish-red color, the structure is flimsy, lumpy, dense, the granulation is cohesive, pseudofibre layers are noticeable, moist.

There is no groundwater, no boiling from HCl has been detected.

The soil is a protohumus podzolic pyrogenic gley sandy loam on ancient alluvial sands.

Soil section 2 (R-2) is laid on an elevated relief element under a pine stand, which was destroyed by a grass-roots fire in 2021. The relief is bumpy, the microrelief is formed by fallen burnt trunks. The composition of the stand is 10C, age 85-85 years, dsr 30cm, Hsr 24m, TLU – A2, relative completeness 0.6. The height of the observed bark burn is 1-2m. The type of forest is pine – cowberry. The undergrowth is uneven, sparse, consists of pine and birch, the undergrowth is completely burnt out. The living ground cover is very rare – the degree of coverage is 5-10%, it consists of cereals, lingonberries, lily of the valley.

Morphological structure of the soil profile:

Orig 0-3cm – the litter is represented by charred remains (ash), 3cm thick, dry, the transition to the mineral horizon is sharp.

We 3-7cm is a coarse humus horizon of gray color, fresh, slightly tinged, the structure is missing, the grain composition is loose, loose, small roots, the transition is sharp.

BF1 7-22cm is an alphegumus horizon with the presence of ferruginous films of brownish color, the structure is absent, the grain composition is cohesive, loose, with an admixture of small roots and streaks of humus, moist, the transition is noticeable.

VF2 22-46cm is a brownish–red alphegumus horizon, the structure is absent, the granulation is loose-sanded, loose in density, roots and glandular streaks along the roots in the upper part are present, moist, the transition is noticeable.

VS 46-88cm – the transitional horizon is yellowish-fawn, structureless, the grain composition is loosely sanded, roots and rhizomes are present, fresh, friable, the transition is noticeable in color.

Cff 88-178cm – the horizon of the soil-forming rock is yellow-fawn, structureless, the granulation is loose-sanded, root cracks are noticeable, there are many pseudofibres, slightly compacted, moist.

No groundwater has been detected, and there is no boiling from HCl.

The soil is a protohumic slightly saline pyrogenic loosely sandy loam on ancient alluvial sands.

The soil section 3 (R-3) is laid under a pine forest, passed by a grass-roots fire. The relief is uneven, bumpy, and the microrelief is represented by fallen burnt trunks. According to the tax description, the composition of the plantation is 10C, age 65-75 years, dsr 23cm, Nsr 22m, TLU – A2. The height of the observed bark burn is 1-2m. The type of forest is pine-cowberry. Undergrowth and undergrowth are missing, obviously destroyed by fire. The living ground cover in the first post-fire year is characterized as very rare, represented by lingonberries and ferns, burnt lichens.

Morphological structure of the soil profile:

Orig 0-2cm – the litter is represented by charred remains, the degree of decomposition is weak, dry, includes remnants of needles, bark, and the transition to the underlying mineral horizon is clear.

We 2-5cm – the protohumus horizon is weakly expressed with a noticeable "graying effect", gray, the structure is weakly expressed, fragile, dense, the grain composition is loose sand, the presence of humic substances, there are small and medium roots, rhizomes, fresh in moisture, the transition is noticeable.

ABf,e 5-16 cm is a transitional horizon of heterogeneous color with brownish shades and signs of gilding, there is no structure, dense, loosely sanded, brownish and lightened spots create a heterogeneous color, there are roots, roots, fresh, the transition is noticeable.

BF 16-47cm is an alphegumus horizon with a more pronounced brownish coloration, structureless, the grain composition is loose sand, dense, roots, rhizomes, fresh in moisture, gradual transition.

VS 47-69 cm is a transitional horizon with a predominance of brownish-yellow shades, loose-sanded granulation, dense, structureless, contains roots, moist, gradual transition.

Cff 69-156cm is the parent rock of heterogeneous yellowish-gray shades, loose-sanded granulation, dense build, no structure, there are roots and pronounced pseudofibres, moist.

There is no boiling from HCl, no groundwater has been detected.

The soil is a protohumus podburgized pyrogenic sandy on fluvial sandy deposits.

Soil section 4 (R-4 control) is laid in an area not affected by fire, on the territory of the Chernushkinsky forestry. The terrain of the site is flat, with microrelief in the form of mineralized bands. The composition of the stand is 10C, age 82 years, dsr 22cm, Nsr 20m, type of forest conditions (TL) – A2. The undergrowth is sparse from pine and birch, and the area is uneven. There is no undergrowth. The living ground cover is uneven, consisting of lichens, grasses, ferns, lily of the valley, covering about 70%. The type of forest is lichen pine.

Morphological structure of the soil:

About 0-3cm – single-layer forest litter of the moder type, the degree of decomposition is weak, there are no signs of pyrogenicity, dry, consists of needles, foliage, bark, branches, the transition is clear.

We 3-8cm is a protohumus horizon with signs of SiO2 accumulation, gray coloration, no structure, loose build, loose-sanded grain composition, noticeable humus contractions, roots, rhizomes, fresh in moisture, clear transition.

BF 8-39cm is a median alphegumus horizon of homogeneous composition, with a predominance of brown shades, loose-sanded grain composition, structureless, loose build, roots and roots are present, fresh moisture, the transition is noticeable.

BCff 39-86 cm is a transitional horizon with a predominance of yellow–brown morphones, loose, the granulation is loose-sanded, structureless, pseudofibres and root veins are well expressed, fresh, the transition is gradual.

Cff 86-171 cm is a soil–forming rock of a heterogeneous light yellow color, the structure is absent, loose, loosely sanded, pseudofibres are noticeable, fresh, the transition is gradual.

There is no groundwater, there is no boiling.

The soil is protohumus podburgum, podzolic, sandy on fluvial sandy deposits.

The structure of the studied soil profiles with diagnostic horizons and features is presented in Table 1.

Table 1

The structure of the soil profile of the studied soils

According to Table 1, in the areas traversed by the fire (P-1,2,3), pyrogenic forest litter (Orig) is formed in the form of ash residues with a thickness of about 3 cm, which in all cases sharply separates from the mineral underlying layer, which, according to external signs, is not affected by the fire – there are no signs of particle sintering and uncharacteristic color changes, which is evidence a low-intensity fire. A feature of all the studied soils is a light granulometric composition throughout the profile and poor soil-forming rock, represented by ancient alluvial sands. This factor, combined with coniferous acidic fall (typical for similar soils in the region), caused the formation of an underdeveloped, heterogeneously colored protohumus horizon, whose thickness in all sections does not exceed 5 cm, which is one of the main criteria for identifying this horizon according to KiDPR, 2004. The protohumus horizon of the studied sandy soils is characterized by fragmentation and heterogeneity of humus coloring. In all sections within the humus horizon, a whitish powder is noticeable (the effect of grayness), which indicates the presence of fragmentary signs of gilding, which are displayed in the horizon indication using the soil feature (e).

Under underdeveloped humus horizons, as a rule, there is an alpha humus horizon BF (P-1,2,4), or a transitional AB (P-3), in which heterogeneous yellow-brown shades predominate. The intensity of coloring is most pronounced in the upper part of the horizon, which is associated with organic and aluminum-ferruginous compounds coming from above, and the coloring brightens downwards. In the studied soils, the alphegumus horizon in all sections has a thickness in the range of 15-35cm, sandy granulation and no structure.

The soil-forming rock of all the studied soils is represented by relatively homogeneous sandy deposits of fluvial origin, of a light shade. Pseudofibre ferruginous layers are present everywhere, which is generally characteristic of soils of light granulometric composition in a forest zone and indicates a fairly intensive washing of the soil column with the leaching of ferruginous compounds. In the soil of section 1, there are signs of soil waterlogging, which is expressed in the presence of signs of gluing (g).

Thus, all the studied forest soils under pine plantations affected by low-intensity fire have external signs of pyrogenicity only in the thickness of the forest floor (Table.1), which most likely may slightly affect the chemical composition of both the litter itself and the underlying mineral horizon for some time. For a more detailed analysis of the impact of fires on the power of various horizons, a statistical analysis of the data was carried out (Table 2).

Table 2

Indicators of the variability of horizon power

the studied soils in pyrogenic and non-pyrogenic areas

Note: Mx is the average value of the parameter; min, max are the minimum and maximum values; Shx is the standard deviation of the parameter; mx is the error of the average; V is the coefficient of variation, %.

Analysis of the effect of forest fires on the morphological features of soils has shown that the effect of low-intensity fire, as a rule, affects only the upper organogenic horizons, causing charring of the forest floor and, consequently, the formation of ash residues with a decrease in the total thickness of both the forest floor and possibly the humus horizon due to the total loss of organic matter. No signs of sintering of mineral particles were observed.

Conclusions

1. As a result of grass-roots forest fires, the grass and shrub layer is completely destroyed with the formation of a specific Orig horizon on the soil surface, representing charred remnants of vegetation.

2. As a result of gorenje, the total thickness of the forest floor and the underlying humus horizon decreases.

3. There are no signs of sinterability of mineral particles.

4. According to statistical analysis, the variability of the thickness of the upper soil horizons decreases in pyrogenic soils.