Rendzinas in the taiga zone are intrazonal soils; moreover, all of their processes occur in ways that are different from podzolic soil formation, which is typical for the zonal taiga boreal ecosystem. At the same time, the habitats of carbonate soils are known as places in which there is a concentration of biodiversity in the more southern regions, as they are drier, are insolated, and have a higher trophic state than zonal podzols. The biotopes on carbonate soils are becoming more southern and are dominated by nemoralis species of flora, including abundant calciphilous plant species. Carbonate soils regulate biogeochemical processes within their distribution and in the geochemically subordinate landscapes of Northwest Russia. They are associated with the existence of a number of specially protected natural areas, as well as the implementation of a number of important ecosystem services. Carbonate soils of the southern taiga are endangered and require special protection. The belt of the carbonate soils in the northwestern Russian and Baltic regions extends to Poland and is the basis for the formation of a special landscape–ecological framework with specific biodiverse, biogeochemical, and geographical characteristics.
1. Introduction
Shallow Hyperskeletic fertile soils with a high content of carbonate inherited from parent materials are known as rendzinas. For the first time, officially, rendzinas were mentioned by a famous follower of Dokuchaev, N.M. Sibirtsev, in his soil taxonomy system
[1][2] as kind of intrazonal soil found in all natural zones confined to the carbonate soil-forming rocks of different geneses and composition
[2]. N.M. Sibirtsev wrote: “Such humus carbonate soils have intrazonal character, i.e., they are caused by one dominant formation—parent material. Type of such soils is “rendzina” (rendzina) or “borowina” of Polish kingdom. They always appear where there are outcrops of chalk, limestone, dolomite, or marl (no matter the type of geological system)”
[2] (p. 387). N.V. Sibirtsev noted an increase in the content of humus in these soils (from 2 to 7%), as well as in clay matter and in their suitability for wheat. It is noted in Sibirtsev’s textbook that “similar humus-carbonate soils are also known abroad, for example in Germany and Austria on lime and dolomite rocks”
[2] (p. 388). When describing humus–carbonate soils in the text, N.V. Sibirtsev used the terms “rendzina” and “soils of semi-rendzina character”. Vilensky
[3] clarified the genesis of the word rendzina: “In 1903 the work of Y. Mazanowski on the humuscarbonate soils of Poland, in which he wrote that the Polish word “rendzina” includes the following meaning—clay soil, viscous soil, clay soil. It appears to be derived from the word—rzendzie, rzezie—to shudder, and is related to the behavior of the plow when plowing such lime-rich crushed lime-rich soils”. Duchaufour
[4] conducted a deep and detailed investigation of rendzinas under forests and other types of vegetation cover, with a special emphasis on soil evolution
[4]. Reintam
[5] generalized information about the development and evolution of rendzinas, mainly in Estonia; at the same time, and for many years, he collaborated with the Department of Soil Science of Leningrad University in his investigation of rendzinas. Thus, Gagarina
[6][7][8] has published many papers on rendzinas of the Leningrad region, which are quite similar to the Estonian types; meanwhile, the most recent of these are similar in their genesis to Polish carbonate soils. Rendzinas have also been investigated in various conditions and types of climates in Eurasia
[9][10], Africa
[11], and other continents. They may be found both on plains and mountains, as well as in flat and sloped positions
[12]. The rendzinas of all climatic belts were characterized by their increased gravimetric concentrations of organic matter in superficial soil horizons. Thus, Pestryakov
[13] declared these soils to be the most productive for the Leningrad region, and the botanist Nitsenko
[14] remarked that the vegetation cover of rendzinas is characterized by southern features in comparison with the zonal south taiga of the Leningrad region. The presence of rendzinas in the soil cover of the central part of the Leningrad region was due to the fact that the carbonate soils were the first to be involved in agricultural development
[12]; this was also the reason that the first large settlements appeared on the Izhora Uplands, also known as the Ordovician Plateau, since the lands were owned by Sweden. Thus, although native and uncommon in the southern taiga, these soils play a significant role in the biodiversity of taiga forests, as well as in the history of the agricultural development of the region and the formation of the geochemical features of the macro-landscape.
2. Rendzinas of the Russian Northwest
2.1. Geography, Morphology, and Taxonomy
The total area of rendzina soils (in the broad meaning of this term) in Russia is estimated to be 3%, or about 500,000 km
2 [15]. The carbonate-containing soils of Northwest Russia are presented in two categories. The first is thin soils with a thin soil profile that is formed upon hyperskeletic parent materials, which, in some cases, are covered by local carbonate moraines
[16]. These soils are mostly concentrated on the Izhora Upland. Some areas of rendzina soils are described for the Pskov and Novgorod regions
[7][17][18]. Rendzinas and highly fertile luvic rendzinas are present in the territory of the Dnovsky and Pechorsky districts of the Pskov region
[18], where they are located on expositions of Hyperskeletic limestones. Another type of soil cover, deep-leached umbric Albeluvisols (retisols), originate from rendzina soils during a long-term history of weathering of local carbonate moraines located in the Pskov and Novgorod regions and are represented on the following map by a green color. These soils are not really rendzinas; however, they are their closest relatives and are also characterized by high fertility.
Rendzina soils were named and classified in various soil taxonomy systems. In the USSR’s soil taxonomy system
[19] was type of sod–carbonate soil (“dernovo–carbonatnye” in Russian) with three subtypes—typical, leached and podzolized. These subtypes are close to each other in terms of their geneses and evolution in time and space. The new Russian soil taxonomy
[20][21] divided this type on the basis of various trenches of soils—primary soils (lithozems) and postlithogenic soils (organo-accumulative clay-illuviated carbonate soils). Thus, a previously single type of soil formation became divided by the new Russian classification
[15].
2.2. The Role of Parent Materials in Soil Genesis
As a result of the stage-by-stage glacial retreat in the territory of the northwest, different parts have different absolute surface ages; hence, the duration of soil-forming processes on each is different
[7]. The formation of soil parent material can be conventionally considered as the initial state of the soil system and the zero moments of soil formation. Meanwhile, in the periglacial zone, the consolidation of soil-forming features was often limited to the deposition of new material or other processes
[22]. Soils formed on moraine deposits are most common in the northwest. On carbonate moraines, the soil-forming process deviated from the zonal one. The gradual leaching of carbonates was accompanied by weathering, browning of the upper horizons, and loessivage. Two types of soil chronologies formed on carbonate moraines were found within the northwest. The first chronology is very traditional—sod–carbonate typical→sod–carbonate leached→sod–carbonate podzolized soil. These soils are typical for low carbonate-containing moraines of poor mineralogical composition.
2.3. The Development of Soil Profile in Time
There is a question as to whether rendzinas are an independent though intrazonal soil type, or if they represent an initial stage of soil evolution, developing further into zonal soils. The first case is true for rendzinas formed on dense rocks, for example, on dense limestones of the Izhora Upland (Leningrad region) or on the upper slopes of the Zhigulevsky mountains (Samara region). The second case is more suitable for pararendzinas, which contain fewer stones and more loose material. Here, the decarbonatization and then leaching of cations proceeds relatively quickly; calcareous material is dissolved intensively and insoluble residue remains, which is the mineral base for the subsequent soil type. This is well described in the studies of Gararina
[6][7][8], Ergina
[23], Reintam
[24], Sukhanov
[11], and Valkov
[9]. Russia’s soil taxonomy
[20] has various subtypes of humus horizons designated as such for their dependence on the soil organic matter content and pH range. These subtypes are the following: dark humus AU horizon with a neutral or alkaline pH and a humus content of more than 4–5%; gray humus AY horizon with a low humus content and an acidic pH range and light humus AJ horizon with a low humus content and alkaline pH values
[15]. All of these types can be revealed in different natural zones; however, for the south taiga, only AU and AY are possible in its soil profiles. The following horizon sequences and soil profile formulas, in terms of the WRB and Russian soil taxonomy, are the most common after generalizing the published data of Reintam
[24], Gagarina
[6][7][8], and Pestryakov
[13] for the south taiga regions, on the basis of an example from the Baltic region. The formula of the soil profile before brackets is according to the WRB system; the horizon sequence in brackets is given according to Russian soil taxonomy: Aα—ACα-Cα-Rα (AUca-ACca-Cca-R)—typical rendzina formed on Rα (Rca)—dense limestone, covered by the products of its weathering—Cα (Cca), where Acα (ACca) is a transitional horizon of low humus accumulation and intensive decarbonatization of skeletal parts; thus stony lime materials become fine earth. The whole thickness of these soils is about 20–30 cm. The Aα (AUca) horizon fizzes from hydrochloric acids from the superficial part of the soil. This is a typical rendzina in the sense suggested by Sibirtsev
[2]. The next stage of this soil evolution is A-ACα-Cα (AU-ACca-Cca-R), where A (AU) is leached from carbonates; this is why it is not Aα (AUca), but A. This is still a typical rendzina, while the next stage is presented by AE-AC-Cα-Rα (AUe-AC-Cca-Rca)—leached rendzina that features the eluviation of fine particles from the top horizon to the middle part of the soil profile. In this case, the whole thickness of the solum increases and reaches 40–50 cm; thus, the soil cannot be named as a primary soil
[20] or leptosol
[25]; this is the next step to zonal soils. And, from this stage, soil formation diverges towards the zonal soils of the respective natural zones, which was proven earlier by Valkov
[9] and Sukhanov
[11].
The morphological organization of natural rendzinas of the Northwest Russian Federation is presented in
Figure 1.
Figure 1a shows two soil-forming rocks simultaneously—limestone and the local carbonate moraine overlaying it. In this case (Leningrad region), the thickness of the local carbonate moraine is tens of centimeters to the first and second meters; the moraine is very strongly enriched in carbonates and is characterized by the weak leaching of carbonates.
Figure 1b shows typical plant communities—alvares, which Nitsenko
[14] characterized as steppe meadows, i.e., a peculiar intrazonal vegetation type. To the south, in the Pskov and Novgorod regions, carbonate moraines are more leached and thicker, which contributes to the formation of more differentiated soil profiles (
Figure 1e).
Figure 1. (a) Ordovician limestones overlapped by local carbonate moraine, central part of Izhora Upland (Ordivician Plateau); (b) alvares (dry herbaceous vegetation communities) of Izhora Upland; (c) rendzic leptosols on hyperskeletic debris of limestones, Izhora Upland, (d) rendzic leptosols, Izhora Upland; (e) leached rendzina on deep weathered carbonate debris, Izhora Upland; (f) umbric albeluvisol on deep leached and weathered carbonate moraine, Borovitchy, Novgorod region.
2.4. Soil Organic Matter in Presence of Carbonates
Generally, superficial soil horizons of rendzinas are more enriched with total organic carbon than zonal podzolic or retic soils. It is well known that the presence of carbonate in fine earth stimulates soil organic matter stabilization and accumulation in topsoils
[5][26][27][28]. The leaching of carbonates in rendzinas with illuviation features normally results in decreasing humus content in topsoils, and the reaccumulation of organic matter in deeper horizons. There is a kind of dilution of humus in increasing amounts of fine-grained deposits; the latter occurs due to the destruction of carbonate rubble and a general increase in the fine-grained layer of soils, including through deepening the profile
[29]. It is well known that the system of soil organic matter in rendzinas is completely different from that of zonal podzolics. The reason is that the presence of calcium results in the formation of so-called “dark” fractions of humic acids, strongly bound with calcium cations, with increased humin fraction content than in acidic soils
[30].
2.5. Agricultural and Thechnogenic Soil Transformation
The agrogenic transformation of rendzinas increases their bulk density, changes their porosity, and decreases their water-holding capacity
[31]. How the resistance of sod–carbonate soils changes due to water erosion, which theoretically should be activated during the plowing of watersheds, where, as a rule, rendzinas are spread, is poorly studied. It is well known that the water-holding capacity of soils strongly depends on their stoniness
[32]; therefore, taking into account the hyperskeletic features of many rendzinas, in the arable fields they occupy scholars should expect an increased depreciation of agricultural implements. Cultivated rendzinas represent one of the oldest cultivated soil types in Northwest Russia, although fewer publications have been devoted to them than to agropodzols or agropeatland soils. Limestone is a popular non-metallic mineral that is most often quarried in open pit mines. This is especially common in the Izhora Upland (Leningrad region) and Valday Upland (Novgorod region), where extensive areas of carbonate soils are consequently destroyed. Such operations lead to external dumping and dusting, as well as chemical contamination and alkalinization of adjacent ecosystems
[33]. The dumps of lime-crushing siftings are a prospective ameliorator for acidic soils of the Russian northwest as a local and cheap fertilizer
[34].
2.6. Soil Transforms Local Biodiversity
The presence of carbonate soils leads to nemoralization (shift in plant species list from boreal to sub-boreal type) of taiga flora. This was noted by Nitsenko
[14] for the central part of the Leningrad region. This is expressed through an increase in the proportion of broad-leaved trees and sub-boreal grass species. In the Izhorskaya Upland, there are widespread ecosystems of the alvares type
[35][36], which are dry semi-xerophytic herbaceous communities with sparse juniper (
Juniperus communis) forests. Thus, the presence of carbonates leads to changes in biodiversity, in particular to the appearance of calciphilous species such as
Cyprepedium calceoulus and
Eqiusetum variegatum.
2.7. Soil Regulates Ecosystem Functions and Services
Ecosystem services are known as the benefits that humans receive from using or not using ecosystems
[37]. Since carbonate soil sites are characterized by non-zonal ecosystems, with more southern species, such plant communities perform essential recreational ecosystem services. Thus, such regional natural monuments as Dudergof Heights and Pudost quarry are located on carbonate soils
[38]. Another regional reserve, the Petrovshchinskaya larch grove, is located on the Putilovsky carbonate plateau in the Leningrad region. These sites attract numerous tourists. In addition, calciphilous plant species settle here (
Cyprepedium calceolus,
Epipactis atrorubens), which also attract nature lovers. Carbonate soils are also important in providing food ecosystem services, as it is believed that the best potatoes grow on the rubbly carbonate soils of the center of the Leningrad region. Carbonate soils of the northwest uplands regulate the chemical composition of water; for example, water flowing down from the Izhora Upland is carbonate and more neutral than the acidic water of podzolic and boggy soils
[39]. The quality of drinking water is reduced due to the presence of carbonates
[40].
2.8. Soil Chemical Properties
The total organic carbon content (TOC) was higher in topsoils in comparison with zonal soils; this is a well-known phenomenon
[30] because the presence of carbonates in the fine earth results in carbon compound stabilization in micro- and mezoaggregates, and it is due to the binding of humic materials with calcium cations. Umbric retisols contain a lower TOC than rendzinas; the reason for this is the dilution of humus in increasing proportions of fine earth formed from weathering processes
[29]. As for rendzinas formed at the bottom of abandoned quarries, the humus accumulation rate is low in comparison with that in developed rendzinas. Thus, according to the suggestions of Konyshkov
[22], scholars designated this horizon as AJ instead of AU, according to the Russian Soil Taxonomy
[20]. As for the pH values and the calcium carbonate content, there is a tendency for the pH to decrease in retisols in comparison with rendzinas soils, due to the weathering of carbonates and leaching of alkaline cations. This is also accompanied by increasing skeletal fractions within the depth of the soil. The content of the skeletal fraction is lower in retisols while they form on parent materials with initially fewer stones (local carbonate moraines in the case of the Leningrad region and proximal carbonate moraines in the Novgorod region). As for the fractions of fine earth and clay particles, these values are higher in superficial soil layers due to weathering and lowest in the initial soil of the limestone quarry bottom, where the alteration of stones is at a very initial stage.