Post-Agricultural Shifts in Soils of Subarctic Environment on the Example of Plaggic Podzols Chronosequence

Abstract

This study investigates the post-agricultural transformation of Plaggic Podzols in a Subarctic environment, focusing on the Yamal region, Western Siberia. Agricultural practices historically altered the natural Histic Entic Podzols, leading to their conversion into anthropogenic soils with enhanced organic matter and nutrient profiles. Using a chronosequence approach, soil profiles were analyzed across active and abandoned agricultural fields to assess changes in soil properties over 25 years of abandonment. Results revealed a significant decline in SOC (2.73 → 2.21%, r² = 0.28) and clay (5.26 → 12.45%, r² = 0.84), which is reflected in the values of SOC/clay and SOC/(silt + clay) ratios. Nevertheless, the values of the ratios are still above the thresholds, indicating that the “health” of the soils is satisfactory. We detected a decrease in N_t (0.17 → 0.12%, r² = 0.79) and consequently an increase in the C:N ratio (18.6 → 22.1), indirectly indicating a decrease in SOM quality. Nutrient losses (NPK) with increasing abandonment periods were pronounced, with their concentrations indicative of soil quality degradation. Trace metal concentrations remained below pollution thresholds, reflecting minimal ecological risk according to Igeo, RI, and PLI indexes. The results highlight the necessity for further research on organo-mineral interactions and SOM quality assessment. The findings provide insights into the challenges of soil restoration in Polar regions, emphasizing the role of climate, land-use history, and management practices in shaping soil health and fertility.

1. Introduction

The Arctic and Subarctic regions are traditionally characterized by extremely low temperatures, permafrost events, and short growing seasons [1,2,3]. However, global climate change has resulted in changes in this paradigm, including changes in the duration of the vegetation period, the amount of precipitation, and a decrease in the permafrost depth [4,5,6]. In addition to negative effects, climate change provides new opportunities for the development of regions in the Arctic and Subarctic Belt, one of which is agriculture [2,7,8]. In view of these climatic changes, the opportunities for agriculture in the Arctic are being reevaluated. Warmer temperatures and longer vegetation periods make it possible to grow crops and raise livestock in regions that were previously too harsh for traditional agricultural practices [2,7]. For example, Iceland, Norway, and Russia have already begun experimenting and implementing agricultural practices adapted to Arctic conditions [6,8]. The development of agriculture in the Arctic can make a significant contribution to local food security by reducing dependence on imported food supplies, which are often expensive and logistically complicated [9].

For example, a farmer survey in the Circumpolar North of Europe and America showed that local farmers are already observing the effects of climate change and are making improvements in their farming practices—testing new crop varieties and changing the timing of field work [2]. Regional characteristics are important; recent studies emphasize that high-latitude and mountainous regions could expand their lands by 10 million km² and ~1 million km², respectively, by 2060–2080 if the climate changes according to the RCP 8.5 scenario [6]. In the Arctic zone of Russia, according to the latest estimates, by 2100 the area of land suitable for crop production by climatic conditions will increase by more than 1.7 million km², of which 346 thousand km² will fall on the Yamal-Nenets Autonomous District [7]. Also, however, it is noted that soils in the Arctic zone of Russia will significantly change their soil fertility parameters, such as hydrological regime, groundwater table, and soil organic matter state of soils [7].

It is necessary to note that the agricultural sector of the economy is currently present in the Russian Arctic and Subarctic [8,10]. In particular, potato growing and cultivation of forage crops are relatively highly developed in the Yamal-Nenets Autonomous District [11]. Private farming practices (vegetable gardens, small orchards, etc.) are also gaining popularity; these practices are not only an additional source of food for the local population but also fulfill the function of cultural (esthetic) ecosystem services [12].

According to official statistics, in the Yamal-Nenets Autonomous District (as of 2022), the total area of agricultural land ranges from 41.2 thousand hectares to 223.7 thousand hectares, with hayfields and pastures occupying the largest area of these lands [13]. The area of arable land is from 0.4 to 0.9 thousand hectares, we consider this information to be somewhat underestimated, since the official reports do not include fallow and abandoned agricultural lands, which also have the potential for re-involvement in agricultural turnover [13]. After all, it is the initial development of virgin soils of tundra and forest tundra that is the most labor-intensive activity in the organization of open-pit Polar agriculture [7]. After the Soviet Polar agriculture system collapse, which was developed from the 1930s to the 1990s (according to some data, the area under open field cultivation reached 50 thousand hectares in the 1970s), the Russian Arctic left mosaically scattered areas of potentially fertile land, but their exact quantity and quality are poorly studied [13]. Such soils may not only have the potential for re-involvement in agriculture but may also serve as model objects for studying the processes of degradation of fertile soils in harsh climatic conditions.

The territory of the Yamal-Nenets Autonomous District was a pioneer in the field of the Soviet experience in the “northward shift” (we are referring to the northward shift of the farming frontier) of farming, and around the city of Salekhard (the capital of the region), many fields were organized where various crops were grown in the open ground [12]. We managed to find such abandoned fields and estimate the age of their abandonment using archival satellite images and interviews with the local population. Thus, the main objective of this study is to determine the processes of post-agrogenic transformation of agricultural soils at different stages of their abandonment.

We assume that most of the lands are still suitable for agriculture. For this purpose it is necessary to (1) investigate the morphological organization and thickness of the arable horizon at different stages of abandonment; (2) assess the structural state and the degree of soil “health” by indirect signs (SOC/clay and SOC/(silt + clay) ratios; (3) to identify changes in fertility parameters by the dynamics of concentrations of main nutrients in a chronoseries of abandoned soils; and (4) assess the content of microelements and trace elements, as well as well as to assess the ecotoxicological state of soils by several pollution indices.

2. Materials and Methods

The location of the field work is in the vicinity of the city of Salekhard in the Yamal-Nenets Autonomous District (YNAO, Russia), in the zone of distribution of permafrost, at the Arctic Circle (66°30′ N 66°42′ E) at the southern boundary of the forest tundra distribution (Figure 1). The climate is Subarctic (Dfc), with an average air temperature of −5.1 °C (average January temperature—−23.1 °C; July—+15 °C). The period with air temperature below 0 °C is up to 240 days, the vegetation period is less than 70 days, and precipitation is about 500 mm per year with a strong summer maximum [14]. The investigated abandoned and active agricultural fields are located on the right high bank of the Ob River. Based on the analysis of archival satellite images and interviews with the local community, these areas were actively used for growing various crops in the 70–80s of the last century and belonged to the Yamal experimental agricultural station [15]. However, they were gradually withdrawn from the agro-industrial sector and were gradually transformed into fallow land. The oldest fallow field was abandoned more than 25 years ago.

Sampling strategy. One full soil profile was established in each abandoned and active field under study; soils were described according to the international soil classification [16]. In addition, three sampling points from the topsoil horizon (5–15 cm without O horizon) were selected for each field. Further, from the four obtained sub-samples from the topsoil horizon (after drying at room temperature), a representative sample was formed. The representative sample was thoroughly mixed and sieved through a sieve with a mesh diameter of 2 mm. The representative sample was then submitted for further chemical analysis.

Laboratory analysis. Most soil chemical analyses were carried out according to Standard Operating Procedures (SOPs) recommended by FAO and GLOSOLAN [17]. Bulk density (BD) was measured by the cutting ring (cylinder) method [18]. Determination of soil pH was carried out in soil–water (soil to water ratio 1:2.5 w/v) and soil–salt (soil to 1.0 M KCl ratio 1:5 w/v) solution [19]. Soil organic carbon (SOC) was measured by the Walkley–Black colorimetric method [20]. For mature peat soil, SOC content was measured by the loss on ignition method at 550 °C [21]. Total nitrogen (N_t) was measured by the Kjeldahl method with a titration end point [22]. Carbon and nitrogen stocks (kg m⁻²) in the 5–15 cm layer were calculated as (SOC or N_t × BD × 10)/10. Clay (<0.001 mm) and silt (<0.01 mm) and particle-size distribution were measured by pipette method without preliminary removal of soil organic matter (SOM) but with dispersion of the sample with a 4% Na₄P₂O₇ solution [23]. Determination of mobile forms of phosphorus (P) and potassium (K) was carried out according to the Kirsanov method (10 g of soil was filled with 50 mL of 0.2 M HCl, shaken for 15 min, and then filtered). Then, phosphorus was determined on a colorimeter (Truog method), potassium on a flame photometer in aliquots from the filtrate [24]. Nitrate (N-NO₃) and ammonium (N-NH₄) nitrogen content was measured using 0.01 M CaCl₂ solution as an extractant in the soil to solution ratio 1:10 w/v. N-NO₃ was on a colorimeter at a wavelength of 543 nm. Ammonium nitrogen was also determined on a colorimeter at a wavelength of 660 nm by passing the reaction of “indophenol green” [24]. Determination of Cu, Zn, Ni, Pb and Cd was carried out by Atomic Absorption Spectrometry (AAS) using Kvant-2MT (Cortec, Russia) spectrometer. Extraction of metals from soil was performed by the “hot digestion” method—0.5–1 g of soil was flooded with 7.5 to 10 mL of 37% HCl and 2.5 to 5 mL of 65% HNO₃, and the solution boiled for 5 h at 95–105 °C [25].

The ecotoxicological status of soils was assessed by several pollution indices and were calculated according to [26].

Geoaccumulation Index (Igeo):

$$I_{geo}={\log }_2\left[{\displaystyle \frac{Cn}{1.5 GB}}\right]$$

(1)

where Cn—concentration of individual heavy metals, GB—value of the geochemical background, and 1.5—a constant, which allows the analysis of the variability of heavy metals as a result of natural processes.

Pollution Load Index (PLI):

$$PLI=\sqrt[n]{{PI}_1\times {PI}_2\times \dots {PI}_n}$$

(2)

where n—the number of analyzed heavy metals and PI—calculated values for the Single Pollution Index (Cn/GB).

Potential ecological risk (RI):

$$RI={\sum }_{i=1}^n{E}_r^i$$

(3)

where n—the number of heavy metals and Er—the single index of the ecological risk factor calculated based on the following equation:

$$E_r^i=T_r^i\times PI$$

(4)

where $T_r^i$—the toxicity response coefficient of an individual metal and PI—calculated values for the Single Pollution Index.

Toxicity response coefficients for studied metals: Cu—5; Zn—2; Ni—5; Pb—5; and Cd—30 [27]. Regional mean values (value of geochemical background—GB) of metal concentrations in sandy and sandy loam soils of the Priuralsky district of YNAO were taken from long-term monitoring data of technogenically and anthropogenically undisturbed landscapes. The concentration values are as follows: Cu—8.52; Zn—30.52; Ni—11.29; Pb—7.12; and Cd—0.27 mg kg⁻¹ [28].

Aerial photography was performed using a DJI Mavic 2 Pro (Dà-Jiāng Innovations, Shenzhen, China) quadcopter. Statistical processing (descriptive statistics, linear regression) and visualization of data were carried out using GraphPad Prism version 10.2.3 (or Windows, GraphPad Software, Boston, MA, USA) and QGIS 3.34.1 (QGIS Geographic Information System. QGIS Association) software. The k-means clustering was performed by Origin (Pro), Version 2024 (OriginLab Corporation, Northampton, MA, USA).

3. Results and Discussion

Studied soils represent a chronosequence of agrogenically transformed soils (Figure 2) with a similar type of land use but different periods of abandonment, from zero to twenty-five years. The main crop cultivated on these fields has always been potatoes. In the process of agrogenic transformation, natural Histic Entic Podzol under forest-tundra vegetation was transformed by uprooting the natural vegetation cover and mixing the surface soil horizon with the mineral stratum. Subsequently, organic and mineral fertilizers (mainly peat, substrates from animal bedding, and cow manure) were applied after primary preparation.

Therefore, the transformation of the native soil horizon Histic into Hortic and, on a longer period of abandonment, into Plaggic was realized. Studied soils according to the international soil classification system [16]:

• S5—Hortic Podzol (Arenic, Cordic)—0 years of abandonment (field);
• S6—Plaggic Albic Podzol (Arenic, Cordic)—5 years of abandonment;
• S8—Plaggic Ortsteinic Podzol (Arenic)—10 years of abandonment;
• S4—Plaggic Turbic Gleyic Ortsteinic Podzol (Arenic)—16 years of abandonment;
• S2—Plaggic Turbic Ortsteinic Podzol (Siltic)—17 years of abandonment;
• S3—Plaggic Podzol (Siltic, Cordic)—20 years of abandonment;
• S1—Plaggic Ortsteinic Podzol (Siltic)—25 years of abandonment;
• Mature soil—Histic Entic Podzol (Folic)—no disturbed soil.

Similar scenarios of agriculture on poor and infertile soils are common in numerous northern regions and often lead to the formation of Plaggic anthrosol [29,30]. Such soils are characterized by an increased content of biophilic elements, reduced acidity, and increased organic carbon stocks relative to their agrogenically untransformed analogs [31,32,33].

3.1. Dynamics in SOC and “Soil Health” Ratios

Figure 3 shows the basic chemical properties of the studied soils. Based on the results of regression analysis, it can be seen that the time of abandonment of agrogenic soils differently affects their chemical properties. There is a statistically significant trend for a decrease in SOC content (r² = 0.28) and an increase in clay content (r² = 0.84). The SOC/clay ratio decreases (r² = 0.95), which indirectly indicates a change in the structural organization of soils over time [34,35,36]. The bulk density of soil was also found to increase with time (r² = 0.05), but this trend was not supported by statistical tests.

The threshold values of the SOC/clay ratio are considered to be 1:8 (0.125), 1:10 (0.10), and 1:13 (0.075). These values indicate the boundaries between ‘very good’, ‘good’, ‘moderate’, and ‘degraded’ levels of soil structural conditions, respectively [37]. It is noted that the SOC/clay ratio can be used as an indicator of soil health when taking into account local climatic conditions and land use [37]. It is also noted that when comparing soils with different pH values and land-use patterns, this indicator gives mixed results [38]. A threshold value of 1/20 (0.05) was adopted for the SOC/(silt + clay) ratio to reflect organomineral interactions relevant to soil physical properties [39].

Taking into account that the soils studied in this research are located in the same climatic conditions and do not differ significantly in pH values), we can use this index to detect changes in soil health. As can be seen in Figure 3C, the value of the SOC/clay ratio for all studied soils is higher than 0.125, which indicates a “very good” structural condition of soils. At the same time, the values rapidly decrease depending on the time of soil abandonment; in the soil used in agriculture at present, the SOC/clay ratio is 0.52 ± 0.01, and at the abandonment period of 25 years, it decreases to 0.18 ± 0.09. Moreover, there is a trend of trend in decrease content in these soils from 2.73 ± 0.07% to 2.21 ± 0.06% in the oldest abandoned soil of 25 years of age. The clay content changes in the opposite direction from 5.26 ± 0.26% to 12.45 ± 0.88%. Spearman r between SOC and clay is −0.25 (p > 0.05); SOC/clay and SOC are 0.45 (p < 0.05); and SOC/clay and clay are –0.91 (p > 0.001).

Previously published research has already noted that agrogenic soils in Polar conditions are not characterized by high SOC content; data from the Yamal experimental agricultural station show that SOC content varies widely from 0.89 to 5.12%. This is associated with different doses of organic fertilizers applied to the experimental field [20]. Extremely high SOC content (>15%) is characteristic of agrosoils of private households, where composts rich in organic substrate are used for fertilization [11]. High content of particles < 0.01 mm in surface soil horizons at the initial stage of agricultural development is also not characteristic of agrosoils of the Yamal region on sandy parent materials. Frequently their content does not exceed 7% [12,40].

Gradual decrease in SOC content in the studied soils can be primarily related to harsh climatic conditions. After the initial enrichment of soil with organic matter in the form of fertilizers and composts, the input and humification of organic matter are limited by climatic conditions during the transfer of soil into a fallow state [41,42]. As can be seen in Figure 2, the formation of the O horizon (Figure 2, S4–S1) with a weak degree of decomposition of plant remains takes place. It can also be assumed that the previously accumulated organic matter is gradually removed from the soil in the process of its mineralization or washed down the soil profile. However, it is worth noting that our results largely contrast with the previously published data on post-agricultural restoration in soils of other climatic zones [43,44,45]. In these works, carbon accumulation in soils after their transition to abandonment is predominantly reported; in the climatic gradient from mid-temperate to subtropical zones (23-year post-agricultural restorations), carbon accumulation (+33–+60%) is noted mainly in the non-protected SOC fraction [44]. In soils of temperate broadleaved forest areas of European Russia, the growth of carbon stocks of active and passive pools in a chronoserial scale of 120 years duration was noted [45]. For a more objective assessment of our results, it should be emphasized that in this study only post-agrogenic and agrogenic soil horizons were studied; the material from the litter horizon O was not analyzed. Nevertheless, studies on the fractionation of carbon pools are necessary for a more explicit understanding of the behavior of carbon stocks during post-agricultural restoration in Arctic and Subarctic conditions.

The basic characteristics of undisturbed soil in the vicinity of Salekhard are presented in

Table 1. Basic topsoil (5–15 cm) properties of mature soil—Histic Entic Podzol (Folic). Data on porosity, clay content, and related indices are not given here, as it is not possible to determine them in peat. Abbreviations are the same as Figure 3 and Figure 5.

Parameter	BD	pHw	pHs	SOC	Nt	C:N	P	K	N-NH4	N-NO3
Units	g cm−3	-		%		-	mg kg−1
Value	0.22	3.3 ± 0.1	2.3 ± 0.1	32.6 ± 0.1	1.4 ± 0.1	26.3	977 ± 29	80 ± 9	13.6 ± 0.6	32.3 ± 1.7

. It can be seen that these peat soils are rich in organic matter (32.6 ± 0.1%) and nitrogen (1.4 ± 0.1%), and peat also contains a significant amount of available phosphorus (977 ± 29 mg kg⁻¹). As it was mentioned earlier, during the agrogenic development of these soils under forest–tundra vegetation was transformed by uprooting the natural vegetation cover and mixing the surface soil horizon with the mineral stratum. Also, peat from such soils is used for fertilizer and fertility maintenance on agricultural fields in the Yamal region [12].

We found (Figure 3C,D) that with increasing age of soil abandonment, there is an increase in clay (<0.001 mm) and silt + clay (<0.01 mm) content; regression analysis shows high r² values for both relationships—clay (r² = 0.84, p < 0.0001) and silt + clay (r² = 0.89, p < 0.0001). The SOC/clay and SOC/(silt + clay) ratios associated with these parameters, which reflect “soil health”, are decreasing (Figure 3E,F); the relationships also have high coefficients of determination (r2 > 0.7, p < 0.0001). A decrease in the value of these ratios indirectly indicates a degradation of soil quality [46,47]. However, these indices behave differently depending on the change in land-use types and climatic conditions [37,38,39]. In our study, there is a rapid decline in SOC/clay and SOC/(silt + clay) ratios after the transition to abandonment, but they have not yet fallen below the threshold values of 1/13 (0.075) and 1/20 (0.05), respectively [39,46]. We see a decrease in the SOC/clay ratio by more than two-fold over 25 years, from 0.52 ± 0.01 (S5 soil of an active agricultural field) to 0.18 ± 0.01 (S1 soil abandoned 25 years ago), the same pattern observed for the SOC/(silt + clay) ratio. It is worth elaborating on the possible causes of soil deterioration, firstly—agricultural soils in Arctic and Subarctic conditions are not formed on in situ fertile soils; before their exploitation, it is necessary to apply a large amount of organic fertilizers to saturate the soil with organic matter [11,12]. It was previously observed that manure application leads to a rapid increase in the SOC/clay ratio [46]. Application of manure and compost improves soil structure, increases macroaggregation, and reduces its bulk density, but if applied irregularly, the positive effect may be short-term [48]. Due to the application of organic matter to non-fertile soil, the artificial formation of macro and microaggregates, and the formation of non-complexed and non-protected organic matter, is sensitive to soil management practices in arable soils and is susceptible to decomposition [49,50]. After the transition of soils to the abandoned state in Subarctic conditions, artificial input of organic matter ceases, and the rate of accumulation of fresh plant residues and their humification is limited by climatic conditions, and, as a rule, the rate of primary production is higher than the rate of decomposition of organic residues. The accumulation of coarse organic matter can be seen in the increasing thickness of the O horizon in Figure 2. Furthermore, the rate of organic matter accumulation and decomposition in Subarctic and Arctic conditions is limited by the availability of nutrients, excess precipitation over evapotranspiration, pH, and low mean temperatures [51,52,53]. Consequently, low values of potential (pHs) acidity (Figure 5E) and severe climatic conditions lead to a complex process of degradation of the structural state of the studied soils and an increase in clay dispersion [54,55].

When excluding time dynamics from the analysis, we also see the relationship between SOC content, SOC_stock, SOC/clay, and SOC/(silt + clay) ratios (Figure 4). Using regression analysis, it was found that there is a statistically significant (p < 0.01) relationship between the SOC/clay ratio and the SOC stock, r² = 0.31. A similar trend was found for SOC/clay ratio and SOC content (r² = 0.38, p < 0.05). The coefficients of determination between the SOC/(silt + clay) ratio and similar parameters were higher, for the SOC_stock and the SOC/(silt + clay) ratio r² = 0.67 (p < 0.0001), and for SOC and the SOC/(silt + clay) ratio r² = 0.78 (p < 0.0001). Thus, it is possible to see the relationship between “soil health” and SOC stock/content; the worse the structural condition and clay dispersion of the studied soils, the less they contain and store SOC.

3.2. Dynamics of Soil Fertility and Nutrients Concentrations

Main nutrients concentrations in the studied agrotransformed soils varied within a wide range (Figure 5). Phosphorus concentration (Figure 5A) was found to range from 1115 ± 23 mg kg⁻¹ to 215 ± 11 mg kg⁻¹ from the active field soil to the oldest (25 years old) fallow soil. Phosphorus concentration decreased linearly (r² = 0.49, p < 0.001). The maximum concentration of potassium was found in the soil of the active agricultural field (S5); here, the concentration of potassium in the soil is 1145 ± 20 mg kg⁻¹. In fallow soils, the quantity of potassium is significantly lower, in most cases less than 60 mg kg⁻¹ (Figure 5B). After agricultural land conversion to abandonment, the potassium content decreases sharply and remains consistently low (r² = 0.003, p = 0.83). The content of the ammonium nitrogen form is also highest in the soil of the active agricultural field (S5)—302 ± 20 mg kg ⁻¹. Following the change to fallow condition, the content of ammonium nitrogen decreases sharply to less than 20 mg kg⁻¹ (r² = 0.02, p = 0.57). Concentrations of nitrate nitrogen (Figure 5D) are maximum in the soil of the active field (S5)—16.7 ± 2.0 mg kg⁻¹ and young fallow soil (5 years of abandonment—S6)—12.5 ± 3.0 mg kg⁻¹. In other investigated soils, nitrate nitrogen content is minimal—less than 1.7 mg kg⁻¹, and in fallow soils aged from 10 to 25 years, nitrate nitrogen concentration is stably low (r² = 0.02, p = 0.60). Agrotransformed soils in the Yamal region are characterized by different content of nutrients [11]. It was previously reported that on the soils of Yamal experimental agricultural station, the concentrations of nutrients vary in a wide range: phosphorus from 165 to 1268; potassium from 53 to 294; ammonium nitrogen from 0.9 to 10.3; and nitrate nitrogen from 0.2 to 62.8 mg kg⁻¹. These are associated with different doses of minerals and organic fertilizers [14]. In general, the natural soils of the Yamal Subarctic region are not characterized by high content of nutrients [56,57]. However, during agricultural development of mature soils in the region and their transition to a fallow state, concentrations of nutrients can decrease many times, which was previously shown in several studies [40,53]. At present, the degradation of fertility of agrotransformed soils in the north is poorly studied, but it is known that, for example, phosphorus can be fixed in the soil profile and can play a role in indicating past agricultural impact, which is also related to soil acidity and the type of fertilizers used [58,59].

Potassium content after transfer of soils to a fallow state sharply decreases to the level of its content in the parent material (about 20–60 mg kg⁻¹), which can be associated with its active removal down the profile or active consumption of potassium by agricultural crops; potatoes (the dominant crop on Yamal) need active fertilization with potassium and experience stress at its concentration decrease [15,59]. It is also worth noting that pre-supplemental doses of potassium are introduced into northern soils only in the form of mineral water-soluble fertilizers, while phosphorus, nitrogen, sulfur, and other elements are also supplied in the process of gradual long-term decomposition of organic fertilizers (compost and manure), which determines their relatively long-term fixation in the arable layer [59]. Natural sandy soils of the Yamal region are poor in nitrogen, and during the agrogenic development of soils, nitrogen enters the soil from a number of sources: mineral forms (nitric acid salts and ammonium salts) and organic forms that enter the soil with organic fertilizers (plaggen material, compost, manure, peat, etc.) [11,15]. From the obtained data, it is clear that the dynamics in nitrogen of different forms are different; mineral water-soluble (ammonium and nitrate nitrogen) (Figure 5B,D) forms entering the soil with mineral fertilizers are not fixed in the arable layer and are quickly consumed by vegetation or washed out of the arable layer. Total nitrogen (N_t) content decreases gradually (Figure 6A), and we found that the N_t content of the active field soil was 0.172 ± 0.010% and that of the oldest fallow soil was 0.116 ± 0.004%. N_t content decreased linearly (r² = 0.79, p < 0.0001), and over 25 years the nitrogen content decreased, on average, by 0.05%. Along with the total content, the nitrogen stock also decreases during the transition to abandonment (Figure 6B); it decreased from 0.192 ± 0.016 kg m⁻² to 0.152 ± 0.006 kg m⁻² over 25 years (r² = 0.81, p < 0.0001).

In abandoned agrotransformed soils, we also found that organic matter N enrichment decreases over 25 years, as indicated by a gradual increase in the C:N ratio (Figure 6C). In the active field soils, the C:N ratio is 18.6 ± 1.3, and in the oldest fallow soil 22.1 ± 0.2, although the coefficient of determination is low (r² = 0.13, p = 0.1098), we see a linear trend of increasing C:N ratio. In general, the data on the value of the C:N ratio in agrogenic and post-agrogenic soils of the Yamal region are extremely heterogeneous; the values vary significantly from 10–11 to 26–27 and appear to depend on the quantity and quality of organic fertilizers applied to the soil during their agrogenic transformation [60,61].

3.3. Potentially Toxic Elements and Micronutrients Concentrations

In addition, we investigated the content of trace elements Zn, Ni, Cu, Pb, and Cd in soils (Figure 7); these elements in high concentrations are potentially toxic to living organisms, but it is also worth considering that, for example, Zn and Cu in low concentrations play an important role in the mineral nutrition of plants and provide high yields [62]. We found that the mean micronutrient concentrations (n = 21) were Zn, 14.7 ± 4.9; Ni, 7.6 ± 1.4; Cu, 3.88 ± 1.5; Pb, 4.5 ± 1.5; and Cd, 0.33 ± 0.05 mg kg⁻¹. According to previously published data, the average content of trace elements in the world’s unpolluted soil (depending on soil type) is Cd—0.37–0.78; Pb—22–44; Ni—12–34; Zn—45–100; and Cu—13–24 mg kg⁻¹ [63,64]. In general, the concentrations of trace elements obtained by our study are lower compared to the world average concentrations for unpolluted soils. Comparing our data with the results of long-term monitoring of heavy metal concentrations in sandy soils of YNAO (Cu—8.52; Zn—30.52; Ni—11.29; Pb—7.12; and Cd—0.27 mg kg⁻¹), we also see that concentrations of trace elements in agrotransformed soils are lower (except for Cd concentrations, which are slightly higher) compared to the regional background [28]. According to our data (Figure 7A) the content of potentially toxic elements in the reference soil in the vicinity of Salekhard is as follows (n = 3): Zn—7.8 ± 0.4; Ni—8.8 ± 0.3; Cu—3.9 ± 0.5; Pb—2.3 ± 0.3; and Cd—0.26 ± 0.04 mg kg⁻¹; these values are higher compared to the studied agrotransformed soils.

Micronutrient concentrations do not vary significantly with the age of soil abandonment (Figure 7A), but the coefficients of variation of concentrations are rather wide: Cu—41.17%; Zn—34.83%; Ni—19.75%; Pb—35.89%; and Cd—10.26%. We found that Zn (r² = 0.49), Ni (r² = 0.44), Cu (r² = 0.49), and Pb (r² = 0.31) concentrations show a linear trend for increasing concentrations as a function of age of soil abandonment, while Cd concentrations remain consistently low in all soils studied (r² = 0.008). The concentrations of Cd also do not change as a function of SOC (r² = 0.0008), clay (r² = 0.006) and SOC/clay ratio (r² = 0.001). Meanwhile, Zn, Cu, Ni, and Pb show a positive relationship (Figure 7B) with clay content (r² > 0.43) and a negative relationship (Figure 7C,D) with SOC content (r² > 0.28) and SOC/clay ratio (r² > 0.44).

Previously published data show that agrogenic and post-agrogenic soils of the Yamal region are characterized by low concentrations of potentially toxic elements. For the 0–20 cm layer, data (n = 6) on the concentrations of Cu—5.0 ± 1.5, Pb—2.9 ± 0.9, Zn—20.3 ± 7.4, Ni—14.7 ± 6.7, Cd—0.03 ± 0.02 mg kg⁻¹ in active and fallow agriculture soils in the vicinity of Salekhard and Yamgort village [40] have been published. For old plowed soils of Yamal experimental agricultural station, data on higher concentrations (n = 40) of potentially toxic elements Pb—15.4 ± 5.8, Zn—43.0 ± 7.4, Ni—21.5 ± 2.4 mg kg⁻¹ have been published. The authors attribute the increase in concentrations relative to background soils to long-term application of mineral fertilizers [14], which are a potential source of toxic elements in the soil [63].

The ecotoxicological state of agrotransformed soils was assessed using the following soil quality indexes: Igeo—Geoaccumulation Index; PLI—Pollution Load Index; and RI—Potential ecological risk (Figure 8). Calculations showed that the average value of Igeo index for all trace elements and potentially toxic elements is negative: Cu—−1.8 ± 0.5 (CV 26.5%); Zn—−1.7 ± 0.4 (CV 22.8%); Ni—−1.2 ± 0.3 (CV 22.3%); Pb—−1.3 ± 0.4 (CV 34.0%); Cd—−0.32 ± 0.24 (CV 76.4%). Thus, we can say that the content of the studied elements is below the background values and there is no soil pollution; however, it is worth noting that there is an increasing trend (Figure 8A) of Igeo values for Zn (r² = 0.55), Ni (r² = 0.41), Pb (r² = 0.23), and Cu (r² = 0.52) in the chronoseries of soils.

Mean values of the RI index—46.2 ± 3.4 (CV 23.0%) and the PLI—0.63 ± 0.14 (CV 7.4%) indicate a favorable ecotoxicological state of the studied soils (Figure 8B). There is a linear trend of increasing values of ecotoxicological state indices: PLI—r² = 0.54; RI—r² = 0.16; nevertheless, even in the oldest fallow soils, the quality of soils does not exceed the boundary of minimal pollution level. According to previously published data, the ecotoxicological condition of agrosoils in YNAO can be assessed as favorable; in agrogenically transformed soils in the vicinity of Salekhard and Yamgort village, in most cases, no exceedance of Igeo index values was found, and single cases of nickel and cadmium contamination (1 < Igeo < 3) were registered [40]. No considerable exceedances of threshold values of concentrations of potentially toxic elements were found for old plowed soils of the Yamal experimental station [65].

Based on the obtained data matrix, cluster analysis of the data was performed using the k-means method (Figure 9A). Four clusters with similar properties were identified: 1 cluster—soils of active agricultural field (S5); 2 cluster—soils of fallow field 10 years old (S8); 3 cluster—soils of fallow fields 5 to 20 years old (S2, S3, and S6); 4—soils of fallow fields 16 to 25 years old (S1 and S4). From the obtained data it is also seen that soils with close in age periods of abandonment (S4, S2, and S3—16, 17, and 20 years, respectively) are in a single cluster in the factor field and, accordingly, have similar chemical properties. Also, in the two-factor space, the line of variability from soil S6 (5 years of abandonment) to S1 (25 years of abandonment) can be traced, which indicates the general dynamics in changes in chemical properties of soils.

According to the results of principal component analysis, the data matrix was divided into two factors. For PC1, the following parameters had the highest loadings: clay—−0.32; SOC/clay ratio—0.34; N_t—0.31; N_{t stock}—0.25; pHw—0.15; pHs—0.13; and trace elements concentrations of −0.24—−0.29. For PC2, SOC—0.33; SOC_stock—0.32; C:N ratio—0.44; and nutrient content: −0.15—−0.38. Thus, depending on the age of soil abandonment, data clustering occurs mainly linearly; the values of significant factors for the oldest soils are predominantly negative and increase in soils with a shorter period of abandonment.

4. Conclusions

The findings demonstrate that agrogenic activities initially enriched the soils with organic matter and nutrients through the application of fertilizers and organic amendments. However, upon abandonment, the soils exhibited a gradual decline in soil organic carbon (SOC) content, SOC/clay ratios, and nutrient availability, indicative of structural and fertility degradation. Despite these trends, the SOC/clay ratio remained above critical thresholds, suggesting that the structural integrity of these soils has not yet reached a degraded state within the 25-year observation period. Heavy metal concentrations, including Cu, Zn, Pb, and Cd, remained low and below ecotoxicological risk thresholds, confirming minimal anthropogenic pollution in these soils. The following key changes in the soil chronosequence can be identified based on the results of this research: 1—Decrease in SOC concentration (2.73 → 2.21%, r² = 0.28) and SOC stock (3.04 → 2.88 kg m⁻², r² = 0.19) with linearly increasing clay content (5.26 → 12.45%, r² = 0.84). Which is reflected in the values of “soil health” indices (SOC/clay and SOC/(silt + clay)) ratios, which indicate a sharp decrease in the quality of the structure after the transition of fields to abandoned state. The index values are still above the threshold values, but this sharp decline in quality is a cause for concern. 2—Extremely rapid losses of the main nutrients (mobile P and K, as well as mineral forms of N); their concentrations decline sharply following field abandonment. 3—Decrease in the N_t content (0.17 → 0.12%, r² = 0.79) and stocks (0.19 → 0.15 kg m⁻², r² = 0.81), and, as a consequence, an increase in the C:N ratio (18.6 → 22.1), which indirectly indicates a decrease in the quality of SOM. Currently, the nature of these changes is not fully understood, as the climatic conditions of the Arctic and Subarctic usually promote “conservation” and stabilization of soil properties that remain stable over a long period of time. We can identify several possible causes that require further verification. Taking into account a sharp decrease in the SOC/clay ratio and a small amount of clay in general (5.26–12.45%), temperature fluctuations could lead to the destruction of microaggregates formed due to the application of organic fertilizers. This assumption requires deeper studies of the mineralogical composition of these soils for the presence of layered minerals in the surface horizons. Disturbance of organo-mineral interactions on the basis of presumably poor mineralogical composition could also result in a decrease in the quality of SOM and its total content. This is also indirectly related to a sharp decrease in the amount of nutrient elements fixed in soil. The questions posed call for additional research into organo-mineral interactions and evaluation of SOM stability by more advanced methods (e.g., ¹³ C NMR spectroscopy). Nevertheless, the obtained results highlight the sensitivity of Subarctic soils to land-use changes and the challenges of maintaining soil health in Polar environments. While the soils retain some structural resilience, the long-term loss of organic matter and nutrients underscores the need for sustainable management practices tailored to the unique environmental conditions of the Arctic.