Influence of Crop Phenology and Seasonality on Soil Conditions Across Depth Profiles

Abstract

The regulation of nutrient availability and microbial processes in agroecosystems are strongly mediated by soil physico-chemical factors. Yet, their seasonal dynamics in different crops are not fully understood. This study monitored pH, redox potential (Eh), electrical conductivity (EC), and nitrite (NO₂⁻) in soils grown with clover, maize, and triticale from November to May. Monthly samples were collected in four depth layers (0–20, 20–40, 40–60, 60–80 cm) and analyzed to reveal patterns over time and space. Soil pH remained near neutral, with slight decreases in spring, and it appeared that maize maintained more stable values than clover or triticale. Eh was highest in winter, indicating oxidizing conditions, but decreased in spring, especially at depth under triticale. EC showed moderate variation, with higher surface values under maize. NO₂⁻ was uniformly low in winter but increased in spring, especially in deeper soils with triticale, while clover had lower accumulation. Overall, clover supported greater soil stability, maize increased surface EC, and triticale enhanced nitrite accumulation at depth. These results highlight the need for crop-specific, depth-aware management to maintain soil quality and optimize nitrogen cycling in agricultural systems.

1. Introduction

The continuous growth of the global population has intensified the demand for food production [1], even if the area of arable land declines due to progressive soil degradation [2]. To meet these increasing demands, the application of chemical fertilizers has become a key strategy for enhancing crop yields and maintaining agricultural productivity [3,4]. Moreover, nitrogen-based fertilizers are widely used worldwide, despite the ongoing efforts to reduce dependence on mineral nitrogen sources [5,6]. However, the intensive and prolonged use of synthetic fertilizers, particularly when combined with irrigation practices, has been associated with significant environmental risks, including the deterioration of soil fertility and water quality. It also increases greenhouse gas emissions, and it negatively impacts on biodiversity and human health [7,8]. In the long-term, such practices may lead to adverse changes in soil properties, including decreased fertility, increased bulk density, and reduced organic matter content, ultimately compromising soil health and the sustainability of agricultural systems [9].

Nevertheless, the excessive use of nitrogen-based fertilizers or their inefficient application also substantially reduces nitrogen use efficiency and generates significant environmental hazards. One of the most critical consequences is the intensification of greenhouse gas emissions, most notably nitrous oxide (N₂O), a highly potent greenhouse gas and a major driver of climate change [10,11,12,13]. Understanding the processes that govern nitrogen cycling in soils is therefore essential for developing sustainable crop management practices.

The dynamics of N in soils is governed by complex interactions among climatic conditions, soil properties, microbial communities, and crop management practices [14]. Crop residues, for instance, represent a significant source of organic N that can stimulate microbial processes such as denitrification and dissimilatory nitrate reduction to ammonium, thereby influencing N₂O emissions [15]. The type of crop grown strongly affects residue quality, root exudation, and the composition of microbial communities [16], thereby shaping seasonal patterns of N cycling and greenhouse gas fluxes.

In addition to nutrient cycling, cropping systems influence key physicochemical properties of the soil, such as pH, electrical conductivity (EC), and redox potential (Eh). These parameters are widely recognized as sensitive indicators of soil health, as they reflect the balance between aeration, moisture status, and microbial activity [17]. Seasonal variations in temperature and precipitation can cause short-term fluctuations in these indicators, while crop-specific traits, such as rooting depth, water use, and residue composition, can generate distinct vertical profiles within the soil column [18]. Despite their importance, relatively few studies have considered crop type, seasonality, and soil depth together when assessing soil chemical properties and nitrogen dynamics. Addressing these interactions is essential for designing sustainable, crop-specific, depth-sensitive soil management strategies.

In this context, triticale (Triticosecale wittmack), maize (Zea mays L.) and clover (Trifolium pratense) represent three contrasting cropping systems, with different root architectures, residue qualities and seasonal growth patterns. Triticale, a hu-man-developed hybrid of wheat and rye, is valued for its adaptability to marginal conditions and its potential roles in animal feed, biofuel and cover crops [19]. Maize is a high-yielding cereal with substantial nutrient requirements, often leaving large amounts of residue after harvest [20,21]. Clover, as a legume, contributes biologically to fix nitrogen and can improve soil structure and organic matter content [22]. Examining these crops side by side provides an opportunity to capture a broad spectrum of functional traits and their influence on soil chemical dynamics.

The aim of this study was to assess how different cropping systems (clover, maize, triticale) affect soil quality, taking into account temporal and vertical variations. Specifically, we aimed to: (i) determine crop-specific influences on key soil health indicators; (ii) assess seasonal changes in soil properties over a seven-month period (November–May); and (iii) investigate depth-related patterns of soil quality indicators within the 0–80 cm soil profile. The innovative aspect of this work lies in the combined assessment of crop type, seasonality, and soil depth, which together provide a multidimensional understanding of soil chemical dynamics. Such an integrated approach is rarely addressed in short-term studies and provides valuable information for designing crop-specific and depth-sensitive soil management practices.

2. Materials and Methods

2.1. Study Area

The study area is situated in the northwestern part of Transylvania, Romania, near the town of Târgu Lăpuș (Maramureș County) (Figure 1), within the Lăpuș basin, which is a small “tectono-erosive” depression located at the forefront of the Preluca regional fault system [23]. This basin is traversed by the Lăpuș River, which acts as an important fluvial agent transporting a wide array of mineral and lithological materials, including hydrotermal ore fragments. These fragments are associated with Neogene metallogenic processes and are enriched in heavy metals, originating from both natural geological sources and historical mining activities in the region. The latter has significantly increased the influx of heavy metals into the river system, through both dissolved forms and particulate matter. As the Lăpuș River flows through diverse lithological units (crystalline, volcanic, and sedimentary formations) in both mountainous terrain and the hilly region locally known as “Ţara Lăpuşului” (Lăpuş Land), it contributes to the geochemical enrichment of downstream environments. Before entering the narrow gorges of the Lăpuș canyons, the river deposits its sediment load within a broad alluvial plain, which serves as a transitional sedimentary environment with high accumulation potential [23]. The soil profiles analyzed in this study are located on this alluvial plain, where we focused specifically on assessing the concentration and vertical distribution of physico-chemical parameters and nitrites concentration. These parameters were selected due to their relevance as indicators of both natural geochemical background and potential anthropogenic influence, particularly related to agricultural practices and legacy mining activities in the region.

2.2. Soil and Vegetation Samples Prelevation

To evaluate the environmental impact of organic fertilizer application and of natural materials input under different crop systems (triticale, maize, clover), soil samples were collected and monitored over a seven-month period (November–May).

Three sampling sites were selected for soil collection (Figure 2) corresponding to different crop types: Site 1 (samples 1–4) from a clover (Trifolium spp.) field (Figure 2); Site 2 (samples 5–8) from a maize (Zea mays L.) field (Figure 2); and Site 3 (samples 9–12) from a triticale (Triticosecale wittmack) field (Figure 2). At each location, soil samples were taken at four depth intervals: 0–20 cm, 20–40 cm, 40–60 cm, and 60–80 cm. Sampling locations were carefully chosen to avoid areas with evident soil erosion, proximity to drainage channels or fences, and to reflect consistent fertilizer application and crop type.

All soil sampling procedures were carried out in accordance with the Romanian Ministry of Agriculture and Rural Development Order No. 278, published in the Official Gazzete [24].

2.3. Analytical Methods

The analyzed physico-chemical parameters included pH, redox potential (Eh), electrical conductivity (EC), and total dissolved solids (TDS). These measurements were performed in a soil/distilled water suspension in the ratio of 1:5, by using a portable multiparameter instrument (WTW Inolab 320i, Troistedt (Turingia), Germany), ensuring minimal alteration of sample characteristics prior to the analysis. The pH and Eh values were determined with an electrode calibrated against standard buffer solution, while EC was measured using the instrument’s integrated conductivity cell. The TDS was calculated by dividing the EC value to a conversion factor of 0.6.

For the electrochemical determination of nitrites in soil, carbon paste electrodes (CPE) modified with zeolite adsorbed with Toluidine Blue O (referred to as CPE-Z-TBO) were employed [25]. The electrochemical investigation technique used was amperometry. Measurements were performed in a three-electrode electrochemical cell comprising:

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Working electrode: carbon paste electrode modified with zeolite adsorbed with Toluidine Blue O (CPE-Z-TBO);

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Reference electrode: Ag/AgCl/KClsat electrode;

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Counter electrode: platinum wire with a large surface area.

The amperometric cell was connected to a voltametric analyzer (Autolab PGSTAT 10, Utrecht, The Netherlands) interfaced with a computer, for potential control and data acquisition from electrochemical experiments. Amperometric measurements were conducted in a magnetically stirred solution, under a constant applied potential (880 mV vs. Ag/AgCl/KCl_sat), using the computer-controlled voltammetric analyzer. All measurements were carried out at room temperature, with 0.1 M phosphate-buffered solution serving as the supporting electrolyte.

The modified carbon paste electrodes were prepared by thoroughly mixing 50 mg of natural zeolite adsorbed with Toluidine Blue O with 50 mg of graphite powder and 10 μL of paraffin oil in an agate mortar for 30 min, until a homogeneous paste was obtained. The resulting paste was repeatedly used to refresh the surface of the working electrode prior to nitrite determination. The standard addition method was employed for nitrite determination.

Nitrite determination in vegetation was performed electrochemically on the sap extracted from clover and triticale. Approximately 10 g of each vegetation sample was weighed and ground using a mortar and pestle. The homogenized material was transferred into a Berzelius beaker, to which 50 mL of graphite solution was added. The sample was sonicated for 15 min in an ultrasonic bath containing distilled water, followed by centrifugation at 700 rpm for 15 min, with a magnetic stir bar placed in each beaker. The resulting supernatants were filtered through filter paper to obtain the sap [26].

The nitrite transfer coefficient from soil to vegetation was calculated as the ratio between the nitrite content in the vegetation and the nitrite content in the soil from a depth of 0–20 cm.

2.4. Statistical Analysis

The statistical analyses were performed using IBM SPSS Statistics 26 and OriginPro 2024. Principal Component Analysis (PCA) was applied to identify the main gradients explaining variability across different crop types, soil depths, and sampling months. The suitability of data for PCA was verified using the Kaiser–Meyer–Olkin (KMO) measure of sampling adequacy and Bartlett’s test of sphericity. The analysis was conducted using the correlation matrix and oblique (Direct Oblimin) rotation to allow for correlation between components. For factor extraction, considering the sample size, the scree plot was used. Differences in PCA scores across crop types, soil depths, and sampling months were assessed using one-way ANOVA with Tukey HSD or Games–Howell tests, depending on variance homogeneity. The significance level was chosen at α = 0.05.

3. Results and Discussion

3.1. Physico-Chemical Parameters of Soil

3.1.1. Clover Crop

Soil pH under clover cropping system ranged from 6.2 to 7.5, indicating slightly acidic to neutral conditions (Figure 3). At shallower depths (0–40 cm), pH values tended to be slightly lower compared to deeper layers (60–80 cm), where higher pH levels were particularly evident in January and May. Between November and January, soil pH remained relatively stable across all depths, with slightly higher values observed in the subsoil. A general decline in pH was recorded from February through April, more pronounced in the 40–80 cm layers. This trend was followed by a marked increase in May, especially at 60–80 cm depth, where pH peaked around 7.5, potentially reflecting a buffering effect or reduced acidification at greater depths. These seasonal soil pH fluctuations likely reflect residual biological effects from the preceding clover crop. Legumes such as clover are known to acidify the soil by exuding organic acids like citric and fumaric acids in their rhizosphere, leading to a decline in pH [27] and through nitrogen fixation processes that further contribute to acidification [28]. In early April, the spring thaw can stimulate soil microbial activity and organic matter mineralization, temporarily increasing acidity [29]. Simultaneously, winter precipitation may leach base-forming cations from the subsoil, further lowering pH [30]. Consequently, the clover legacy produces a pronounced and temporally variable influence on soil acidity, particularly within surface horizons.

Throughout the monitoring period, Eh values exhibited both temporal and spatial variation, a pattern well documented in soil redox literature, where fluctuations occur across seasons and depths due to changing environmental conditions [31,32]. During the colder months (November through February), negative Eh values predominated across most depths, particularly below 40 cm (Figure 3). This aligns with observations that water saturation and low temperatures reduce oxygen diffusion, leading to more reducing conditions [33]. The most reduced conditions were recorded in January at 60–80 cm depth, with Eh values approaching −70 mV, reflecting combined effects of low temperature and high moisture limiting aeration, consistent with hydrologically driven redox dynamics and seasonal redox transitions [34]. From March onwards, a gradual increase in Eh in upper layers (0–40 cm) corresponds with drying soils and increased oxygen availability, as described in wet-dry transition studies [35]. By April, positive Eh values (>30 mV at 40–60 cm) suggest improved aeration and potential microbial stimulation, as microbes consume oxygen and promote oxidative conditions [34,36]. The sharp decline in Eh again in May, particularly in the 60–80 cm layer, indicates re-saturation or enhanced microbial oxygen demand outpacing diffusion, matching patterns seen in dynamic hydrological system [34,35]. These seasonal redox dynamics reflect the interplay between climatic conditions, soil depth, and biological processes in the post-harvest to pre-sowing interval under clover cover, with implications for nutrient availability and soil microbial activity.

The variation in soil electrical conductivity across four depths from November to May reflects key changes in soil conditions during the transition from the post-harvest to the early stages of crop growth. Seasonal rainfall and evaporation dynamics strongly influence salt movement, precipitation in winter promotes downward salt leaching, whereas dry periods can cause accumulation near the surface [37,38]. Throughout this interval, EC values generally remained stable between November and February, with slight fluctuations across depths (Figure 3), consistent with studies showing minimal EC shift during non-growing, moisture-limited periods [39]. A notable peak in January, particularly at the 60–80 cm depth, suggests leaching of salt from upper layers, probably driven by early winter precipitation moving solutes downward [38]. In March, EC values decreased across all layers, except for a slight increase again in the 60–80 cm layer, possibly due to continued salt leaching and organic residue decomposition influencing soluble salt release and EC profiles [40,41]. By May, elevated EC persisted at the surface, although lower than in April, perhaps reflecting residual salts combined with enhanced microbial and root activity mobilizing ions near the surface [42]. Overall, this pattern underscores the dynamic nature of soil salinity and EC during the post-harvest to early crop growth transition, highlighting how climatic conditions and soil processes interact across depths. The pronounced spring EC increase at the surface may pose osmotic stress risk to young plants, emphasizing the importance of monitoring EC trends to inform soil and crop management strategies [42].

3.1.2. Maize Crop

Maize (Zea mays L) is a major cereal crop cultivated extensively across the globe, serving as a key resource for food, animal feed, and various value-added products [43]. In Europe, maize is typically grown as a row crop with a standard inter-row spacing of 70 cm [44]. The growth and yield performance of maize are influenced by multiple agronomic and environmental factors, including plant spacing [44], light availability [45], temperatures fluctuations, and droughts stress [46].

Throughout the monitoring period, soil pH values under the maize cropping system remained relatively stable, ranging from 6.4 to 6.9 (Figure 4). These values indicate slightly acidic to near-neutral soil conditions (I). This pH stability aligns with previous observations reported by [44], who documented similar pH ranges under comparable cropping systems. In contrast to the clover system, pH values were remarkably uniform across all soil depths and months, indicating a homogeneous vertical pH profile under maize. Temporal fluctuations were minimal, with a modest increase observed between December and January, followed by a steady or gently increasing trend through May. This stability may be attributed to the slow decomposition rate of maize residues [47], which typically possess a high carbon-to-nitrogen (C:N) ratio [48,49] and lower biochemical reactivity. Consequently, the release of organic acids during residue breakdown is limited, resulting in minimal acidification. In fact, some studies have observed increases in soil pH after crop residue application, due to decomposition-driven release of alkaline cations (e.g., Ca²⁺, Mg²⁺, K⁺) or OH⁻ via decarboxylation of organic anions [50,51,52]. Furthermore, the absence of an active crop during the winter, combined with moderate microbial activity under cold, low-input conditions, contributed to the overall chemical stability of the soil. Evidence indicates that lower temperatures constrain microbial activity, thereby slowing residue decomposition and associated chemical transformations during the off-season [53]. The consistent pH profile may also reflect the soil’s inherent buffering capacity. Soils rich in organic matter tend to resist pH shifts [54], and biotic agents like earthworms have a high acid-base buffering capacity [55]. Collectively, these findings suggest that corn residues exert a negligible short-term impact on soil acidity, thereby maintaining a stable chemical environment in the absence of active vegetation.

Overall, Eh values remained close to or slightly below 0 mV throughout the monitoring period (Figure 4), reflecting mildly reducing conditions in the soil profile, a common observation in soils with limited oxygen availability during colder, wetter months [36,56]. In the early winter months (November and December), Eh values showed minor depth-dependent variations, with slightly positive values in the upper layers (0–20 cm and 20–40 cm) and small negative shifts in deeper horizons. In January, a marked drop in Eh was observed across all depths, with the lowest values recorded at 0–20 cm and 60–80 cm layers, reaching approximately −35 to −38 mV. These values suggest the onset of anaerobic conditions across the soil profile, potentially driven by low temperatures, increased soil moisture, and microbial oxygen consumption under crop residue decomposition [57]. Eh values began to increase slightly in February and March, with the 40–60 cm depth showing a transient positive value (~5 mV) in March, possibly indicating improved aerobic conditions at mid-depths. Nonetheless, redox potential remained generally negative across the profile during this time. In April and May, Eh values stabilized near zero or slightly negative in the upper three layers, while deeper soil (60–80 cm) again exhibited a decline in redox potential (below 20 mV in May). These results reflect persistent reducing conditions at depth, likely influenced by water retention and slower oxygen diffusion [58]. Compared to the clover system, the maize cropping system’s more consistently negative Eh, particularly in the surface layer, may reflect higher microbial activity and oxygen depletion under residue-driven conditions without active roots [36,59,60].

Soil electrical conductivity values in November were moderate across all depths (Figure 4), with a gradual increase observed in December, particularly in the 0–20 cm and 40–60 cm layers. A marked peak occurred in January at the 60–80 cm depth, where EC reached 120.6 µS/cm, suggesting significant leaching and accumulation of salts in the deeper soil layers, likely due to seasonal precipitation. This contrasts with the relatively stable EC values in the upper layers during the same period and aligns with reports of heavy rainfall displacing salts downwards, increasing deeper-layers EC [42]. In February, EC increased again in the surface layer (0–20 cm), while decreasing in the deeper layers, indicating upward salt movement or redistribution within the profile, consistent with soil salinity dynamics described in rainfall-irrigation model that show variable EC responses based on water flux direction [38]. March presented a general stabilization of EC across all depths, followed by a rise in April at the 40–80 cm depths, which may be related to increased microbial activity or nutrient mobility associated with early root development. By May, EC values decreased slightly across all depths, possibly due to nutrient uptake by young corn roots or dissolution from spring rainfall. Overall, the deeper layers (particularly 60–80 cm) exhibited a more pronounced fluctuation, highlighting their role as zones of salt accumulation and redistribution over time.

3.1.3. Triticale Crop

Triticale, a man-made hybrid derived from crossing wheat (Triticum spp.) and rye (Secale cereale L), has been suggested as a promising alternative cereal for livestock nutrition due to its potential to combine the favorable feeding qualities of wheat with the winter hardiness and disease resistance traits characteristic of rye [61,62]. While it has limited use in baking, its adaptability to diverse environments, environmental and genotype-dependent quality traits, and potential is sustainable agriculture make it a valuable model crop for research and future food and fuel production challenges [63].

Soil pH values under triticale crops ranged from 6.2 to 7.0, reflecting slightly acidic to neutral conditions (Figure 5), typical for cereals and well within triticale’s tolerance to mild acidity, for which yield response to liming is generally weaker than in barley or wheat at comparable pH, indicating relative acidity tolerance [64]. Across months and the topsoil profile, modest variability (~0.2–0.3 pH units) is consistent with managed arable soils where buffering, nitrate-driven rhizosphere alkalinization, and limited acid inputs dampen short-term swings [65,66]. Minor vertical differences without progressive acidification at depth align with studies showing residue- and nitrate-mediated alkalinity moving downward and mitigation subsurface acidity under cereals [66]. The slight late-spring rise can reflect seasonal shifts in residue decomposition and nitrate uptake that transiently raise pH, a pattern observed where biological activity and N form modulate rhizosphere acidity [65,67]. Compared with legumes, cereals typically exert a weaker residual acidifying effect because their residues have higher C:N and lower base cation release, leading to slower organic acid inputs and smaller net acid loads [68,69]. Stable pH across depth and time under appropriate management (balanced fertilization, residue return) mirrors field observations that surface residue management can create only small pH gradients and overall stable bulk soil pH in cereal systems [70].

Eh values were uniformly positive, in November, across all soil depths, ranging from +8 to +15 mV (Figure 5), indicative of oxidizing conditions typical of well-aerated soils before winter saturation [36,71]. However, a noticeable decline occurred in December and January, especially at deeper layers. The 60–80 cm depth experienced the most significant reduction, with Eh values approaching −25 mV in January, suggesting a suboxic shift t mildly anaerobic conditions, consistent with seasonal cooling, higher water content, and restricted gas diffusion [56]. In February, Eh values increased markedly in the upper layers (0–40 cm), reaching +10 to +13 mV, due to resumed root activity and soil thaw improving aeration, a phenomenon also noted in winter cereals where biological oxygen release can transiently re-oxidize surface horizons [36]. In contrast, deeper layers (particularly 60–80 cm) remained in a reduced state, reinforcing the influence of depth-related oxygen gradients and waterlogging effects [56]. From March to May, Eh values gradually decline again across all depths. While the surface layer (0–20 cm) showed minor fluctuations around 0 mV, deeper layers consistently maintained negative values. The persistence of reducing conditions in the lower profile layers may be attributed to intense microbial respiration coupled with low oxygen penetration [71]. Overall, the redox behavior under triticale differed from that of maize and clover systems. Triticale maintained more oxidizing conditions in the upper layers during active growth (especially in February), likely due to its winter-growing nature and early-season biological activity, while deeper soil layers showed prolonged reducing conditions, characteristic of oxygen gradients under saturation.

At the beginning of monitoring period, the EC values were generally low across all layers, with the 0–40 cm depths showing slightly higher values than the deeper 40–80 cm layers (Figure 5). A modest increase in EC was observed in December, particularly in 60–80 cm layers, suggesting early seasonal changes in salt distribution, likely due to water movement [42]. January showed a sharp EC increase at the 40–60 cm and 60–80 cm depths, with values peaking around 55 µS/cm, indicating leaching from the surface and accumulation in the deeper profile, a common phenomenon under winter precipitation and limited evapotranspiration [40,72]. In contrast, EC at the surface (0–20 cm) remained relatively unchanged. In February, EC values became more uniform across all depths, suggesting redistribution of salts through the soil profile. From March onward, a consistent decline in EC was observed across all depths, with values stabilizing at lower levels. This decline may reflect salt dilution by rainfall and uptake by actively growing triticale roots. Notably, the 0–20 cm depth maintained slightly higher EC values during the spring months, hinting at surface salt retention possibly related to fertilizer application or reduced downward movement.

3.2. Nitrites in Soil Under Clover, Maize and Triticale Crops

Across the sampling period, nitrite concentration under clover crop, exhibited marked temporal fluctuations (Figure 6), with the highest values consistently observed in April, particularly in the upper soil layers (0–60 cm), where concentrations approached or exceeded 2.9 mg kg⁻¹. This seasonal peak likely corresponds to increased microbial activity and enhanced nitrification rates associated with rising soil temperatures and moisture content during early spring. Temperature and soil moisture are well-known regulators of nitrifying microbial communities and enzymatic activity involved in ammonium oxidation [73,74]. In contrast, the lowest concentrations were generally recorded in February, especially in the uppermost layers (0–40 cm), consistent with reduced microbial activity under cold winter conditions [75,76]. Throughout the study period NO₂⁻ concentrations were typically higher in the surface and subsurface layers (0–40 cm) compared to the deeper strata (40–80 cm). This vertical gradient likely reflects the concentration of organic matter and oxygen availability in the soil upper horizons, which are critical for supporting aerobic microbial processes such as ammonium oxidation and nitrification [14]. The deepest layer (60–80 cm) consistently displayed the lowest nitrite levels, with a pronounced minimum in December (below 1.0 mg kg⁻¹) potentially indicating limited substrate availability and anaerobic conditions that suppress nitrification or promote denitrification [77]. Interestingly, an anomalous peak was observed in February at the 40–60 cm depth, where NO₂⁻ concentration reached 2.6 mg kg⁻¹, surpassing values recorded in the upper layers during that month. This inversion in the typical depth profile may indicate transient downward migration of soluble nitrogen compounds or a localized delay in denitrification processes at that depth due to fluctuating redox conditions [78]. From March onwards, nitrite levels increased uniformly across all depths, culminating in the highest concentrations in April. A slight decline was observed in May, yet NO₂⁻ remained high relative to the winter months. These patterns highlight the strong influence of seasonal climatic factors on nitrogen cycling in soil ecosystems, particularly in temperate regions where freeze–thaw cycles and spring rewetting events can rapidly re-activate microbial communities and nitrogen fluxes [75]. Overall, the data underscores the importance of both temporal and depth-resolved monitoring of soil nitrogen dynamics. The seasonal accumulation of nitrite in the upper soil profile during spring may reflect the combined effects of increased mineralization [79], nitrification [80,81], and potentially fertilizer application, which can supply additional ammonium for microbial oxidation [82,83]. In contrast, the stratified distribution with depth illustrates the role of soil physico-chemical properties and microbial ecology in regulating nitrogen transformations. These findings are relevant for optimizing nitrogen management practices and mitigating potential environmental impacts associated with reactive nitrogen in agricultural landscape [84,85].

Under the maize crop, higher NO₂⁻ concentrations were frequently recorded in the deeper layers (up to 3.7 mg kg⁻¹ at 60–80 cm—Figure 6), particularly in March and May, indicating pronounced downward migration of nitrogen following crop harvest, when plant uptake is minimal. This accumulation in subsoil layers is consistent with leaching processes described in previous studies, which report substantial winter losses of mineral nitrogen from the upper profile (0–60 cm) due to precipitation-driven percolation in the absence of active root adsorption [86]. In contrast, the surface layers (0–20 cm) exhibited comparatively lower NO₂⁻ values during winter months (1.3 mg kg⁻¹ in December), probably due to reduced nitrification under low temperatures, dilution by infiltrating water, and the transient conversion of NO₂⁻ to NO₃⁻. The gradual increase in NO₂⁻ concentrations observed toward spring in the upper layers may reflect the reactivation of microbial nitrification with rising soil temperatures and moisture availability [87].

The NO₂⁻ concentrations, under the triticale crop, exhibit seasonal fluctuations (Figure 6), with notable peaks in early spring (March-April), particularly in deeper soil layers (60–80 cm—4.4 mg kg⁻¹) (Figure 6). In contrast, during autumn and winter (November through February), NO₂⁻ values are comparatively lower and more uniform across depths. The pronounce elevation of NO₂⁻ in early spring reflects increased nitrification activity under warming and moistening soil conditions. As soil temperature rise and moisture increases, ammonia-oxidizing microorganisms become more active, facilitating the oxidation of NH₄⁺ to NO₂⁻, the rate-limiting step in nitrification, leading to transient accumulation of nitrite in deeper profiles [88]. This pattern aligns with broader findings that nitrification and net nitrite production often peak with favorable abiotic conditions, such as higher soil temperature, moisture, and sufficient oxygen supply [89]. Elevated NO₂⁻ in deeper layers (particularly 60–80 cm) during spring suggests slower downstream processing (e.g., oxidation to NO₃⁻) or limited microbial capacity for complete nitrification or denitrification at depth. Reduced microbial biomass and activity at such depths can result in nitrite persistence [90]. During colder months (November-February), microbial metabolism and nitrification rates drop substantially, resulting in uniformly low soil NO₂⁻ levels across all depths. This is consistent with the general decline in nitrogen transformation activity during periods of low temperature and reduced biological uptake [91].

3.3. Statistical Assessment of Soil Parameters

To evaluate the effects of crop type, soil depth, and seasonal variation on soil physico-chemical properties, a comprehensive statistical analysis was conducted. Multivariate techniques, including principal component analysis, were employed to explore patterns and relationships among soil parameters and to visualize the clustering of samples according to crop type and temporal dynamics. This approach allowed for a robust assessment of both individual and interactive effects of biotic and abiotic factors on soil chemical and physical characteristics.

The suitability of the data for PCA was verified using the KMO measure and Bartlett’s test of sphericity. The KMO value of 0.51 indicates marginal sampling adequacy, while the highly significant Bartlett’s test (χ²(10) = 401.77, p < 0.001) confirms the presence of sufficient correlations among variables to proceed with PCA.

Figure 7 presents the global PCA results, where the first principal component (PC1), accounting for 48.5% of the total variance, is primarily associated with pH, electrical conductivity (EC), total dissolved solids (TDS), and redox potential (Eh), variables that are tightly interrelated as indicators of soil chemical reactivity and ionic balance [36]. The second component (PC2) captures 30.5% of the variance and reflects a more balanced contribution from EC, TDS, and Eh, suggesting that salt dynamics and redox processes are key secondary drivers of soil variability under the studied systems. Nitrite concentrations contribute moderately to both components, reflecting their dependence on redox-mediated transformation of nitrogen, which are known to be highly sensitive to shifts in soil aeration and moisture [34]. Samples associated with triticale (blue dots) are mostly positioned on the negative side of both PC1 and PC2, suggesting relatively lower salinity and more stable redox conditions, consistent with the moderate biochemical reactivity of cereal residues and their weaker acidifying legacy compared to legumes [68,69]. In contrast, clover and maize samples show greater dispersion, particularly along PC1, reflecting heterogeneity in EC and TDS. This variability is probably linked to the stronger acidifying effect of legumes through rhizosphere organic acid exudation and nitrogen fixation [27,28], and to maize’s slower residue turnover, due to their higher C:N and lignin contents, leading to early N immobilization and delayed release [92]; when residues are left in discrete layers or bands, decomposition and N cycling concentrate at the residue-soil interface, producing spatially uneven nutrient mineralization [93,94]. Overall, PCA underscores that crop type is a significant driver of soil chemical heterogeneity, with cereals maintaining more stable conditions and legumes and maize inducing greater variability through their contrasting residue quality and nutrient cycling effects.

After performing PCA, the factor scores corresponding to PC1 and PC2 were extracted and used for further statistical testing. One-way ANOVA applied to the PCA scores indicated statistically significant differences for triticale compared to both maize and clover (p < 0.001), as also illustrated in Figure 8. In the case of the second component (PC2), a significant difference was observed only between triticale and maize (p < 0.001), while no significant differences were detected between triticale and clover or between maize and clover.

No statistically significant differences in PCA scores (PC1 or PC2) were detected across the four soil depth intervals, suggesting that vertical gradients in these physico-chemical parameters were minimal or overshadowed by seasonal and crop-related influences. This suggests that strong buffering capacity, homogeneous soil texture, and consistent management practices minimized vertical gradients in soil chemical properties. Moreover, seasonal drivers such as moisture fluctuations and residue decomposition imposed stronger influences than depth per se, resulting in relatively uniform profiles. Our findings are consistent with those of [95], who reported that soil properties exhibited temporal variability across the entire soil profile, but without depth-specific differentiation, highlighting that temporal dynamics exert a stronger influence than vertical stratification on soil chemical variability.

The observed temporal (Figure 9). variation in PC1 scores across cropping systems reflects crop-specific interactions with seasonal soil dynamics. In the case of clover, the pronounced peak followed by a spring decline results from legume-driven processes such as enhanced nitrogen fixation and organic acid exudation during the cold season, which can temporarily alter pH, redox, and ionic balances in the soil [27,28]. The subsequent spring decline coincides with thaw-induced microbial mineralization and leaching processes that reduce soil chemical stability [29]. In maize, the more moderate seasonal variability in PC1 suggests slower residue turnover and weaker rhizosphere-driven acidification, consistent with studies showing that maize residues decompose gradually, leading to dampened short-term chemical fluctuation [65]. By contrast, triticale soils displayed persistently low or negative PC1 scores with limited temporal responsiveness, indicating that this winter cereals exerts a stabilizing influence on soil chemical properties through balanced residue decomposition and moderate rhizosphere activity, a pattern also observed in cereals with lower acidifying capacity [69]. The limited seasonal variation in PC2 scores, with significance detected only between April and May, further suggests that this component reflects secondary processes less sensitive to crop-specific effects and more influenced by transient environmental changes such as moisture redistribution or early plant growth.

PCA applied separately to data subsets (by crop type, depth, or month) confirmed the robustness of the global trends and further highlighted the consistent chemical fingerprint associated with triticale across conditions. Seasonal variation remained the dominant source of variance when analyzing monthly subsets, whereas crop-specific patterns were stable across depths.

This study hypothesized that soil chemical properties and nitrogen dynamics are shaped by crop type, season, and soil depth, and the results support this: clover induced pronounced seasonal and depth-dependent variability, maize maintained relatively stable conditions, and triticale enhanced buffering capacity. These crop-specific patterns highlight the role of functional traits in driving soil chemistry and point to opportunities for targeted, depth-aware management strategies. While this study provides a detailed assessment of crop-specific, temporal and depth-related soil chemical dynamics, certain limitations should be acknowledged. The monitoring period spans seven months (November–May), which captures the main seasonal transitions but does not cover a complete annual cycle. Additionally, the study focuses on a specific site and soil type, which may limit the direct generalization of the findings to other agroecosystems with differing climates, soils, or management histories. Despite these limitations, the information obtained provides valuable guidance for designing crop-specific and depth-sensitive soil management strategies and forms a solid basis for future multi-site or multi-year investigations.

4. Conclusions

This study demonstrates that soil chemical properties and nitrogen dynamics are strongly regulated by crop type, seasonal variation, and their interactions with soil depth. Comparative evaluation of clover, maize, and triticale systems revealed distinct crop-specific patterns in soil pH, redox potential, electrical conductivity, and nitrite concentrations, reflecting differences in residue quality, biological legacies, and temporal factors.

Clover induced pronounced seasonal and depth-dependent variability, highlighting the transient but significant influence of legumes on soil chemistry and the importance of depth- and time-resolved monitoring. In contrast, maize remained relatively stable soil conditions, with only minor fluctuations driven by its slow residue decomposition and the soil’s buffering capacity, resulting in a limited short-term impact on acidity, redox potential, and salinity. Triticale exhibited moderate stability, with slight seasonal variation and a tendency to support more oxidizing surface conditions, highlighting its resilience to acidity and its role as a buffering cereal system.

Nitrite concentrations varied widely across all cropping systems, with winter lows linked to suppressed microbial activity and spring peaks reflecting reactivated nitrification under favorable soil conditions. Clover promoted nitrite accumulation in the topsoil, while maize and triticale favored subsoil accumulation, reflecting contrasting nitrogen cycling pathways and associated risks of leaching.

Multivariate analysis further confirmed that crop type, rather than soil depth, was the dominant factor of soil chemical variability, with seasonal dynamics amplifying these effects.

Collectively, the findings highlight the central role of crop functional traits in shaping soil chemical environments: legumes impose transient but pronounced fluctuations, maize supports chemical stability, and triticale enhances buffering capacity in cereal systems. These insights emphasize the need to incorporate crop-specific soil responses into management strategies to optimize nutrient cycling, mitigate environmental risks, and foster sustainable agricultural production.

Thus, for clover, nitrogen application should be timed to coincide with its peak nitrogen release in spring to minimize nitrite accumulation and reduce nitrate leaching. Shallow-root monitoring and targeted surface amendments can mitigate pH fluctuations induced by legume residues, while grain rotations and strategic cover cropping can utilize excess surface nitrogen and moderate seasonal chemical variations. For maize, management should focus on monitoring subsoil nitrogen and using slow-release fertilizers to meet the steady nutrient demand of the crops, reducing risks of leaching. Retention of maize residues supports chemical stability and buffers redox changes, while regular monitoring of subsoil electrical conductivity, especially the irrigated systems, prevents salinity buildup. Intercropping or rotations with legumes can enhance surface nitrogen availability and long-term fertility. In the case of triticale, its buffering capacity allows for the limited use of alkaline amendments. Monitoring surface electrical conductivity and redox potential ensures optimal microbial activity, while residue incorporation or tillage improves subsoil nitrogen cycling without disturbing surface chemical balance. Legume rotations complement nitrogen dynamics and exploit triticale’s buffering properties. For all crops, depth-resolved soil monitoring for pH, electrical conductivity, redox potential, and nitrogen species is essential to anticipate chemical fluctuations and optimize fertilization. Aligning fertilizer application timing with crop-specific nitrogen dynamics reduces leaching and improves nutrient use efficiency. Implementing legume–cereal rotations can balance chemical stability and nitrogen supply throughout the soil profile, supporting sustainable agricultural production.

Further studies should be extended to more locations and over longer periods to capture spatial and temporal variability. In addition, integrated microbial and greenhouse gas analyses can be performed to clarify how crop traits influence soil nitrogen dynamics. These approaches would strengthen crop-specific management strategies for sustainable nutrient use and environmental protection.