МИГРАЦИЯ ЦИКЛИЧЕСКИХ ЭЛЕМЕНТОВ В СОЛОНЧАКАХ ЦЕНТРАЛЬНОЙ ФЕРГАНЫ

УДК 631.4.

Abstract

Studying the formation processes of solonchak soils in Central Fergana and the patterns of microelement accumulation within them is a pressing task for improving soil fertility and ensuring agroecological stability. The primary objective of the study is to evaluate hydromorphic solonchaks as a geochemical system and to determine the migration, accumulation, and redistribution of microelements across the soil profile. The research was conducted at the Fergana Experimental Station using V.Dokuchaev's morphogenetic approach, agrochemical methods, and neutron activation analysis. A high level of arsenic (As) accumulation in the soil horizons (Clarke concentration 6.18–8.12), as well as partial enrichment with molybdenum (Mo) and antimony (Sb), was detected. It was proven that halophytes actively absorb molybdenum and cobalt with high biological absorption coefficients (1.66–11.31 and 1.12–1.38, respectively). The migration of elements is directly dependent on the chemical properties of the soil and pedogeochemical barriers. The obtained results serve as an important scientific and practical basis for managing soil fertility, preventing soil degradation, and conducting regional environmental monitoring.

Аннотация

 Изучение процессов формирования солончаковых почв в Центральной Фергане и закономерностей накопления в них микроэлементов является актуальной задачей для повышения плодородия почв и обеспечения агроэкологической устойчивости. Основная цель исследования — оценка гидроморфных солончаков как геохимической системы, определение миграции, аккумуляции и перераспределения микроэлементов по почвенному профилю. Исследования проводились на Ферганской опытной станции с использованием морфогенетического подхода В. Докучаева, агрохимических методов и нейтронно-активационного анализа. Выявлено высокое накопление мышьяка (As) в почвенных горизонтах (кларк концентрации 6,18–8,12), а также частичное обогащение молибденом (Mo) и сурьмой (Sb). Доказано, что галофиты активно усваивают молибден и кобальт с высокими коэффициентами биологического поглощения (1,66–11,31 и 1,12–1,38 соответственно). Миграция элементов напрямую зависит от химических свойств почвы и педогеохимических барьеров. Полученные результаты служат важной научно-практической основой для управления плодородием, предотвращения деградации почв и проведения регионального экологического мониторинга.

Keywords: hydromorphic, microelements, geochemistry, migration, accumulation, halophyte, humus, pedogeochemistry, mineralization.

Ключевые слова: солончак, гидроморфный, микроэлементы, геохимия, миграция, аккумуляция, галофит, гумус, педогеохимия, минерализация.

Introduction

Soils, as geochemical landscapes, constitute open systems in which the flows of matter and energy are closely interconnected with the atmosphere, vegetation cover, surface waters, and groundwater. Within landscapes, soils regulate the processes of substance migration and exhibit buffering capacity against pollutants. Acidic soils are capable of neutralizing alkaline compounds, whereas carbonate-rich soils neutralize acidic precipitation. [1].

Within the soil profile of saline soils, the fluxes of chemical elements and their compounds encounter several soil–geochemical barriers. These include evaporative, bidirectional gypsum, gley, evaporative, and other types of barriers.

At present, it has been established that numerous elements significantly influence soil formation processes and the development of saline soils in desert regions. The distribution, concentration, and qualitative composition of various microelements across the genetic horizons of soils affect the formation processes of saline soils and solonchaks. During the formation of humus-accumulative horizons, microelements tend to accumulate in the upper part of the soil profile.

During the processes of solonchak formation and soil genesis, microelements are redistributed along the soil profile. The distribution and redistribution of microelement salts in soils and solonchaks depend on several factors, including soil–climatic conditions, as well as the physical, chemical, and biogeochemical properties of soils.

The primary objective of this study is to evaluate solonchak soils as a geochemical system and to determine the characteristics of microelement accumulation and redistribution along the soil profile.

Materials and Methods

As the object of the study, a solonchak massif (protected area) of the Fergana Experimental Station’s cotton-growing farm, as well as lands located along the border of the Buvayda and Yozyovon districts, were selected.

The methodological framework of this study is based on the approaches presented in Carter and Gregorich's book, "Soil Sampling and Analysis Methods."[3] Elemental analysis was conducted using the neutron activation analysis (NAA) method at the VVR-SM research nuclear reactor (Russia) of the Institute of Nuclear Physics of the Academy of Sciences of the Republic of Uzbekistan. Following irradiation, the microelement content in the samples was measured using gamma-ray spectrometers equipped with high-purity germanium (HPGe) detectors (USA).

Results and Discussion

The studied desert solonchaks are characterized as hydromorphic typical alluvial meadow solonchaks and belong to the group of progressively salinized soils. Typical meadow solonchaks are distinguished by a high degree of salinity throughout the entire soil profile; however, a significant accumulation of salts occurs in the upper horizons. In the upper layers, the salt content (expressed as dry residue) ranges from 2–5 %, whereas in the lower horizons it varies between 1.3–4.0 %.

The surface of these solonchaks is covered by a salt crust with a thickness of 0–3 cm. The maximum concentration of salts is observed in the crust and subcrust horizons. The accumulation of water-soluble toxic and non-toxic salts in the upper 0–3 cm and 3–40 cm layers reaches 2.7–3.1 %. At the same time, the content of toxic salts in these horizons reaches 1.8–2.2 %, respectively.

The pattern of salt distribution in the upper part of the soil profile indicates that salt accumulation in these soils alternates with periods of temporary desalinization. The salinity type is chloride–sulfate. In terms of mechanical composition, the studied solonchaks are classified as light and medium loamy soils.

The humus content in the upper 3–40 cm horizon reaches 0.65 %, after which it decreases sharply to 0.26%. In these layers, the carbon-to-nitrogen ratio varies within the range of 5.2–6.1(N:C).

In solonchaks, the carbonate content ranges from 10.2 % to 15.1 %. The regime of soil–groundwater corresponds to the meadow (hydromorphic) type. Under such conditions, the concentration of cyclic elements varies along both the soil and solonchak profiles; moreover, this variation is not uniform across different horizons.

Therefore, characterizing the concentration of cyclic elements within the solonchak profile is of considerable theoretical and practical importance for improving soil fertility. Determining the average concentration of cyclic elements in solonchaks is associated with significant difficulties, which is not surprising, as it requires accounting for the influence of numerous factors, including the concentration of water-soluble salts, the quantity and quality of toxic salts, the composition and properties of soil solutions, as well as the composition and properties of groundwater and mineralized waters.

In the hydromorphic solonchaks of Central Fergana, the concentrations of a number of cyclic chemical elements in the upper 0–3 cm layer vary within the range of 1.7–28,500 mg/kg. In terms of abundance, the microelements are arranged in the following order: Fe > Mn > Sr > Zn > Cr > Ni > Co > As > Mo > Sb. This pattern is generally preserved in the lower horizons as well.

Figure 1. Clarke concentration and distribution indices of elements in solonchak soils

In the horizon that is in contact with soil–groundwater, a slight increase in the concentrations of iron, manganese, strontium, nickel, and molybdenum is observed, which is associated with gley-related geochemical barriers.

The presented data demonstrate specific patterns in the distribution of cyclic elements within the solonchak profile. Therefore, in addition to determining the total content and average concentrations of microelements, it is also necessary to consider their capacity for accumulation and dispersion across the horizons of the studied solonchaks.

For the quantitative assessment of chemical elements in the lithosphere, a special indicator—the concentration Clarke (CC)—is used; it characterizes the deviation of the content of a chemical element in a given object from its Clarke value [3].

Data on the concentration Clarke (CC) and Clarke distribution (CD) in solonchaks are presented in Figure 1. Based on this principle, it can be stated that strontium (Sr) gradually accumulates in the lower, relatively gypsum- and carbonate-rich layers of solonchaks. The Clarke concentration of Sr in these horizons varies within the range of 1.56–2.09.

An increase in the CC values of zinc (Zn) is also observed, ranging from 1.11 to 1.45. Unfortunately, in all horizons of solonchaks, arsenic (As) shows a pronounced increase in CC values, reaching 6.18–8.12. This situation is undoubtedly associated with the presence of nearby mercury–antimony ore occurrences. In addition, elevated CC values of molybdenum (Mo) and antimony (Sb) are also observed. The uptake of Sb by plants from soil depends upon soil type and plant species. Some plants accumulate high amount of Sb and its compounds in their tissues. Plant bioavailable form of Sb is antimonate (Sb(V)) and antimonite (Sb(III)) (Földi et al., 2018). The speciation of Sb is highly dependent on soil pH. Antimony is slightly or less mobile under neutral pH conditions in soil and sediments. [9]

The concentrations of the remaining studied elements (Fe, Mn, Cr, Ni, Co) are lower than their Clarke values and, do not tend to accumulate in solonchaks.

As can be seen from the presented sequences of Clarke distribution (CD), the properties of Clarke distribution patterns inversely mirror the distribution regularities of Clarke concentration values. In other words, in the upper 0–3 cm layer, the studied elements follow the following distribution order: Ni > Mn > Fe > Sr > Co > Cr > Zn > Sb > Mo > As.

Changes in cobalt and iron are observed in the soil–groundwater contact zones. Vegetation cover plays a significant role in the formation and distribution of solonchaks. Solonchak vegetation includes species such as sarsazan (Holocnemum strobilaceum), saltwort (Salsola spp.), saxaul (Haloxylon aphyllum), yulg‘un (Anabasis salsa), kermek (Limonium gmelinii), plantago (Plantago salsa), aster (Aster tripolium), as well as couch grass, wormwood, and others, which are characterized by a high ash content. Their ash is dominated by chlorides, sodium sulfates, and compounds of alkaline earth elements.

Halophytes are well adapted to living in solonchak environments during their ontogenetic development. Depending on their age and environmental conditions, they contain varying quantities and qualities of microelements along with water-soluble salt cations and anions. Based on Clarke concentration and Clarke distribution patterns, the living biomass of solonchak plants is capable of absorbing and accumulating certain cyclic elements.[6]

Figure 2. Biological absorption coefficient (BAC) of solonchak plants

This ability of plants is characterized by the biological absorption coefficient (BAC). The values of this indicator for solonchak plants are presented in Figure 2, which reflects the intensity of microelement biological uptake. Under these conditions, the migration of microelements is significantly influenced by the quantity and quality of salinized soils. The distribution of microelements varies considerably across horizons with different degrees of salinity.

The redistribution of microelements along the soil profile, as well as their concentration and migration within soils, depends on soil formation conditions, soil properties, and the specific characteristics of the elements themselves. The complex interaction of soil-forming factors and the differences between soil horizons determine variations in both the quantity and quality of microelements, including their content and distribution in plants. In desert zones and solonchaks, the biomass typically ranges from 5 to 15 centners per hectare (c/ha). A characteristic feature of desert flora is the relatively intensive biogenic accumulation of sodium, chlorine, and sulfur, as well as potassium and phosphorus. Among these elements, sodium, chlorine, and sulfur accumulate more in the aboveground organs than in the belowground organs. The total ash content is relatively high.

This distribution is associated with comparatively high biological absorption coefficients. At the same time, alkaline and alkaline earth elements are more strongly accumulated in desert plants than in steppe vegetation.  Halophilic chemical elements are characterized by high concentrations in both the aboveground and belowground parts of plants. These elements return to the soil annually. In wind-active zones such as Central Fergana, plants partially lose excess elements through deflation processes.

In the oxidation and evaporation pedogeochemical barriers of desert environments, within the group of saline soils—especially in solonchaks—the decomposition of plant residues occurs relatively intensively. The resulting organic matter is rapidly mineralized, and as a consequence, organic carbon and humus are almost not accumulated.

It is well known [5, 6] that the chemical composition of plants varies depending on the geochemical properties of soils and soil-forming rocks, plant species and varieties, climatic conditions, and other factors. Some cyclic elements are actively absorbed by solonchak vegetation. According to the data presented in Figure 2, the most intensively absorbed elements include, first of all, molybdenum (Mo: _BAC11,3-1,7) and cobalt (Co: _BAC1,3-1,1 ).

The remaining studied elements are weakly involved in plant uptake processes and do not accumulate in plant tissues.

Conclusion

In conclusion, the migration and distribution of cyclic elements in the desert hydromorphic solonchak soils of Central Fergana exhibit specific characteristics. According to the research results, a high level of arsenic accumulation was identified in the genetic horizons of solonchaks (concentration Clarke 6.8–8.12, Clarke distribution 0.12–0.16).

In addition, a significant enrichment of molybdenum and antimony was observed. In solonchak vegetation, the biological absorption coefficients of molybdenum and cobalt are relatively high (1.66–11.31 and 1.12–1.38, BAC respectively), indicating their tendency to accumulate. The remaining studied elements (Ni > Mn > Fe > As > Sr > Zn > Sb > Cr) are only weakly absorbed and assimilated by plants. These results are of important theoretical and practical significance for a deeper understanding of solonchak soil formation mechanisms, improving their fertility, and conducting environmental monitoring and reclamation activities.

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