The Relationship Between Organic Carbon and Ca in the Profile of Luvisols: A Case Study of a Long-Term Experiment in Pulawy, Poland

Abstract

The aim of this study was to determine the impact of multi-year variations in nitrogen fertilisation (N) and farmyard manure application (FYM) in two 4-year crop rotations, A and B—which differed primarily in terms of their organic matter management systems (A: poor vs. B: enrichment)—on changes in the content of calcium and soil organic carbon at three soil horizons of Luvisols, and on the dynamics of these changes over the past 40 years. Mineral N fertilisation was found to affect the movement of Ca in the profiles of both rotations. Most Ca accumulated in the soil layer at >55 cm in the profiles of both rotations. The total Ca content was similar in both rotations; however, a correlation analysis revealed a stronger relationship between organic carbon and Ca in the enriched-cropping system (rotation B) compared to the poor-organic-matter system (rotation A). In the 0–30 cm soil layer, the correlation coefficient between organic carbon and Ca was r = 0.52 in rotation B, while in rotation A, it was lower at r = 0.23. In deeper layers, this relationship weakened or became non-significant. Under the climatic and soil conditions of Poland, where the phenomenon of nutrient leaching into the soil profile is prevalent, it is extremely important to manage these nutrients in a sustainable way in order to limit their loss. The predominantly sandy texture of the topsoil horizons (0–30 cm) of the Luvisols prevents any significant enrichment of these horizons in Corg, even over a very long period of time, i.e., more than 40 years.

1. Introduction

Luvisols are zonal soils characteristic of the humid temperate warm zone, where precipitation exceeds evaporation. These climatic conditions, along with the moderately acidic reaction of the soils, are conducive to the process of clay eluviation and illuviation, which is characteristic of Luvisols (pH below 5.5). In this process, carbonates, Ca ions, and some humus compounds are primarily leached into the soil profile. As a result of the leaching of clay and free Ca²⁺ and Mg²⁺ ions, they accumulate in the subsurface horizons (argic Bt). Above this level, a lighter-coloured, component-poor (eluvic Et) horizon is formed [1,2,3]. Poland’s geographical location in Central Europe, i.e., a moderately humid climate zone, was favourable for the formation of Luvisols, which account for at least 45% of the soil cover [2]. Luvisols are highly differentiated in terms of their physical and chemical properties, resulting in the distinction of many subtypes of these soils. Their high variability influences their agricultural evaluation and value (bonitation) [4].

The phenomenon of leaching of nutrients such as Ca into the soil profile is of great importance for their rational management, as also mentioned in the latest Directive of the European Parliament and of the Council on the monitoring and resistance of soils [5]. Nutrient leaching is a serious problem for crop yields in Poland and can cause a deficiency of available Ca in the surface levels [6]. This phenomenon is common on light soils with a low sorption complex, where significant amounts of calcium are lost through leaching. In contrast, there may be an accumulation of this nutrient in the subsurface horizons. These soils are also typically characterised by an acid reaction and low fertility of the surface horizons. As a result, the uptake of calcium is reduced in soils with a pH below 6.5, significantly reducing yields [7]. Recent studies have demonstrated a positive correlation between calcium (Ca) and soil organic carbon (SOC) content (in a range of soil types), because organo-mineral complexes play an important role in the retention and accumulation of SOC [8]. Previous scientific reports show that the content of these elements in the soil can be regulated by applying calcium fertilisers and by farmyard manure application [7,9]. It is assumed that approximately 40% of agriculturally used soils in Poland are deficient in available Ca [10]. Polish soils are generally low in this element because in temperate climates, CaCO₃ reacts with H₂CO₃ to form Ca(HCO₃)₂-bicarbonate, which, when dissolved in water, leaches easily into the soil profile [11]. Calcium leaching is also caused by soil acidification [11]. Mercik et al. [9] report that up to 150–300 kg CaO is lost annually from the arable layer of 1 ha, the highest of any nutrient.

The basic goal in this context is to find an answer to the question of the extent to which the long-term (more than 40 years) fertilisation of Albic Luvisols with farmyard manure and mineral nitrogen with different crop rotations influences the total content of calcium and organic carbon not only in the arable-soil layer, but in the whole soil profile. It is important to emphasise that due to such a significant surface contribution of Luvisols, it is on their sustainable use that Poland’s food security is based (Systematics of Soils of Poland [2]). However, the effect of fertilisation and crop rotation on the allocation of total forms of Ca and SOC within the soil profile is not well known yet. Therefore, the aim of this study was to provide a preliminary assessment of the dynamics of Ca and SOC movement in the soil profile of an Albic Luvisol after 40 years of mineral nitrogen fertilisation and manure application in two 4-year crop rotations, which differed primarily in terms of their organic matter management systems.

2. Materials and Methods

A long-term field experiment was carried out at the Grabów Experimental Station, belonging to the Institute of Soil Science and Plant Cultivation in Puławy, Poland, in the Mazowieckie province (Lat: 51°21′ N; Long: 21°40′ E) since 1979. The climate at the site is temperate, with mean annual rainfall of about 577 mm and a mean annual air temperature of 9.0 °C (based on the 40-year climatic normal from 1979 to 2023). The experiment included two 4-year crop rotations: crop rotation A (which depletes the soil of organic matter), where grain maize, winter wheat, spring barley, and silage maize were cultivated, and crop rotation B (which enriches the soil with organic matter), with the cultivation of maize, winter wheat plus mustard, spring barley with undersown grass–clover, and a grass–clover mixture. Details of the crop rotation are provided in Annex 1. Until 2008, potatoes were grown in both rotations, but were then replaced by grain maize in view of the rapid expansion of grain maize in Poland. Barley, maize, and wheat straw were removed from the field in both crop rotations. Mustard (Sinapis alba L.) for green manuring is sown in the third decade of August, shortly after disc harrowing of winter wheat stubble. At the beginning of November, green mustard biomass is disced, and about two weeks later incorporated into the soil by ploughing. The grass–clover sward is harvested in three to four cuts per year and removed. Grass–clover sod is disced and ploughed down in the autumn. Within each crop rotation, the application rates of farmyard manure (FYM) and inorganic N fertiliser were varied in a split-plot design replicated in four blocks per field. A detailed description of the experiment can be found in the work [12]. The agro-meteorological conditions are close to optimal with dry periods. In the years 2022–2023, the genetic profile of the podzolic soil from zero objects (M0, N0), i.e., unfertilised and fertilised with mineral nitrogen (N0–0, N1–50, N2–100, and N3–150 kg·N⁻¹) and farmyard manure (FYM0–0, FYM15–15, FYM25–25, FYM35–35,FYM45–45 t·ha⁻¹), from the A and B rotations, was uncovered in order to characterise the soil profiles of the podzolic soils studied and to determine the physico-chemical properties of the soils from the layer (0–30, 30–55, and 55–130 cm) depending on the rotation.

After harvesting of the crops, the soil was sampled to determine changes in soil organic carbon. Soil samples were taken from the plough layer (0–30 cm) in September every year after the end of the rotation using a soil corer (30 mm internal diameter). The moist soil samples were sieved through a 2 mm sieve until analysis. Determinations of pH (pH in the KCl suspension (mol·dm⁻³) (PN-ISO 10390:199) and the content of soil organic carbon using the Tyurin method (PB 021-wyd.IV.28.08.2020) were performed by the certified chemical laboratory of the Institute of Soil Science and Plant Cultivation in Pulawy, Poland.

The following were determined in the soil profiles of both shifts from zero and fertilised objects: pH and Ca (royal water HCl/HNO₃ mixture; extraction of soil in a mixture of concentrated hydrochloric and nitric acid at a volume ratio of 3:1; subsequent determination by atomic emission spectrometry). For the purpose of this study, soil samples were collected in 2019 from two neighbouring crop rotation fields on which maize for silage (rotation A) and grass–clover mixture (rotation B) were grown according to the conventional crop management system.

Statistical Analysis

In this study, a multi-way Analysis of Variance (ANOVA) with a split-plot design was employed to evaluate the main effects of FYM application (FYM) and inorganic N fertilizer (N), as well as their interaction effects. The crop rotation was treated as a block to determine whether it significantly influenced the results. The statistical model for the two-way ANOVA, with N and FYM as the main factors and crop rotation as the block factor, is represented as follows:

Y_ijk = μ + α_i + β_j + (αβ)_ij + γ_k + ϵ_ijk

where

Y_ijk is the observed response variable for the i-th level of FYM, j-th level of N, and k-th crop rotation;

μ is the overall mean of the response variable;

α_i is the effect of the i-th level of FYM;

β_j is the effect of the j-th level of N;

(αβ)_ij is the interaction effect between FYM and N;

γ_k is the effect of the k-th crop rotation;

ϵ_ijk is the random error term, assumed to be normally distributed with a mean of zero and constant variance.

The null hypothesis (H₀) assumes no significant main or interaction effects of N and FYM or crop rotation (A and B). A significance level of α = 0.05 was used to assess statistical significance. The analysis was conducted separately for the soil layers at 0–30 cm, 30–60 cm, and 60–90 cm, with the set of dependent variables varying for each layer based on the specific soil properties measured at each depth.

Table 1. p-values from ANOVA results. Red font indicates a significant individual effect (ns—non-significant).

	Soil Depth (cm)
	A (0–30)			B (30–60)			C (60–90)
	pH	Ca	SOC	pH	Ca	Corg	pH	Ca	SOC
Crop rotation	<0.001	0.006	<0.001	<0.001	ns	ns	ns	ns	ns
N fertisation	<0.001	0.016	ns	<0.001	ns	ns	ns	0.006	ns
Farmayrd manure	<0.001	0.013	0.001	<0.001	0.047	ns	ns	ns	ns
N fertilisation x farmayrd manure	ns	ns	ns	ns	ns	ns	ns	ns	ns

Ca—calcium; SOC—soil organic carbon.

presents the p-values of the main and interaction effects of the factors analysed in this study.

Additionally, we performed a linear regression for all of the data and a Pearson correlation analysis in relation to the crop rotation in Luvisol profiles.

3. Results

3.1. Changes in Soil pH, Calcium, and Soil Organic Carbon

The ANOVA results showed a statistically significant effect of different crop rotations, N fertilisation, and farmyard manure application on changes in the soil pH, Ca content, and SOC of Luvisols, especially at a 0–30 cm depth. The interactions between N fertilisation and farmyard manure were insignificant (Table 1). Soil pH, Ca content, and SOC were significantly altered in individual horizons between crop rotations. The level of N fertilisation significantly changed soil pH and Ca content in the A (0–30)-horizons, soil pH in the B-horizons (30–60), and Ca content in the C (60–90)-horizons of the Luvisol. The FYM (farmyard manure) dose significantly affected changes in soil pH, Ca content, and SOC in the A-horizons and soil pH and Ca content in the B-horizons of the Luvisol.

Both rotations showed increased Ca and SOC content with a higher rate of FYM in the A-horizons of Luvisols, but the A crop rotation demonstrated higher calcium levels. In contrast, the B rotation excelled in carbon storage (Figure 1). In the A-horizons of the Luvisol, nitrogen fertilisation alone had no significant effect on changes in content and SOC in either crop rotation. Only applying FYM at a dose of 35 t ha⁻¹ (at FYM350N0) significantly increased the pH by 1.4 units compared to the FYM0N3 treatment. Significant changes were observed between the FYM0N3 and FYM35N0 treatments regarding soil pH and Ca content (an increase of 1.3 pH units and 0.7 g kg⁻¹ of Ca in favour of FYM35N0) in the B crop rotation.

In the B-horizons of the Luvisols, the situation was slightly different compared to the A-horizons within both crop rotations (Figure 2). Apart from the statistically significant differences in soil pH treatments in both crop rotations and Ca in the A crop rotation, no significant changes were observed between the treatments across both crop rotations.

In the A crop rotations, there were significant differences for the N and FYM combinations in pH and Ca content. There were no significant differences for SOC. All dependent variables in the C-horizons of Luvisols in the B crop rotation were insignificantly different between the N and FYM combinations (Figure 3).

3.2. Relationships Between Soil Properties in Luvisols

The linear analysis of the soil properties, specifically pH, Ca content, and SOC, reveals noteworthy relationships that affect soil fertility and nutrient availability in Luvisols. Initially, a significant (p < 0.001) positive linear dependence between pH and Ca content was observed, indicating that as soil pH increases, the concentration of calcium tends to rise as well (Figure 4). Conversely, a significant negative linear relation was found between pH and SOC, suggesting that organic carbon tends to decrease slightly as pH increases, indicating a possible decline in soil acidity in Luvisols. Moreover, the analysis reveals a significant positive linear relation between calcium content. Conversely, a significant negative linear dependence between Ca and SOC suggests that Luvisols with greater Ca levels may exhibit slightly lower SOC.

However, when we assessed the correlation relationships more specifically in relation to the crop rotation in Luvisol profiles, we observed differences (

Table 2. The results of the Pearson correlation analysis for each soil layer and crop rotation are presented separately. Significant correlations are indicated in a red font.

	Crop Rotation	Soil Horizons	Soil Layer	pH	Ca	SOC
pH	A	A	0–30	1.00	0.40	0.06
Ca				0.40	1.00	0.23
SOC				0.06	0.23	1.00
pH		B	30–60	1.00	−0.06	−0.04
Ca				−0.06	1.00	0.21
SOC				−0.04	0.21	1.00
pH		C	60–90	1.00	0.74
Ca				0.74	1.00
SOC						1.00
pH	B	A	0–30	1.00	0.79	0.29
Ca				0.79	1.00	0.52
SOC				0.29	0.52	1.00
pH		B	30–60	1.00	0.06	−0.02
Ca				0.06	1.00	−0.07
SOC				−0.02	−0.07	1.00
pH		C	60–90	1.00	0.75
Ca				0.75	1.00
SOC						1.00

). In both cropping systems, a positive correlation was found between pH and calcium in the A-, B-, and C-horizons of Luvisols. However, in the poor-cropping system with organic matter (A cropping system), this correlation was weaker (yet still significant) compared to the enriched-cropping system with organic matter. In both cropping systems, pH positively correlated with Ca in the A-, B-, and C-horizons of Luvisols; however, a stronger relationship in the cropping system enriched with organic matter was observed. These results highlight the significant role of soil organic carbon, as evidenced by the noteworthy correlation between organic carbon (SOC) and Ca, specifically in the enriched-cropping system (B cropping system), which enhances soil organic matter.

4. Discussion

4.1. Changes in Soil pH, Calcium, and Soil Organic Carbon in Soil Profiles of Luvisols

The soil properties measured include Ca (g/kg) and Corg (%). In the A crop rotation, calcium content ranges from 0.85 g/kg to 4.02 g/kg, with a notably high value in the FYM35N0 treatment, likely due to the higher manure application (Figure 1) (Table 2). The current literature indicates that Ca plays a minor role in SOC accumulation in acidic soil environments [8]. The percentage of organic carbon (Corg) varies slightly, from 0.51% to 0.70%, indicating different levels of organic matter in the soil, but is slightly higher in some treatments with increased organic application (e.g., FYM35) (Figure 1) (Table 2). Both rotations show increased nutrient availability with higher organic matter, but the A crop rotation demonstrates higher calcium levels, while the B rotation excels in carbon storage. Ca may contribute to the preservation of C with a specific biochemical composition due to its preferential association at depths up to 70 cm. As shown in the literature [13], in carbonate-free soil, Ca can mediate SOC protection through physicochemical mechanisms, including the precipitation of dissolved organic matter from the soil solution. The content of total calcium, irrespective of the profile studied, increased with depth in all soil profiles. The highest calcium contents were found in the BtC-horizons, with the highest calcium contents in the profiles from crop rotations A and B without mineral nitrogen fertilisation and crop rotation B with Nmin fertilisation—150 kg/ha (Figure 2). Similar results were obtained in [14,15]. Slightly lower Ca contents were found at the other sites. In general, it can be assumed that the Ca contents in the BtC-horizons were almost twice as high as in the surface horizons of the soil profiles studied (Figure 2). Similar results were obtained by [12]. The leaching of calcium and magnesium is also caused by soil acidification [13,16,17].

Moreover, other researchers have noted that more intensive soil cultivation and base cation content can improve soil organic carbon content [17,18]. The same trend was also observed in our study. This study showed that the Corg content was highest in the surface levels of Ap, in rotation B. The highest Corg content of 0.76% was recorded in the profile from rotation B with a fertilisation level of Nmin = 100 kg/ha, while the lowest Corg content (0.54%) was recorded in the profile from rotation A without Nmin fertilisation. The differences in Corg content between sites were not statistically different. The high organic carbon concentration in the topsoils in crop rotation B suggests the correct practices for maintaining or even increasing organic matter content in the soil [18]. The same trend was also observed in our study.

The literature review shows that the association between Ca and SOC has been under-studied in acidic soils, as Ca is assumed to be less important in acidic soil ecosystems [8]. Rasmussen et al. [19] and Solly et al. [20] proved the correlation between effective cation exchange capacity (CEC) and SOC decreases at a soil pH of <5.5. In a depleted cropping system devoid of organic matter, the application of nitrogen (N) did not have any significant effects on the pH changes in the Ap and Et horizons of Luvisols. The application of 100 kg N/ha raised the pH from 5.4 to 5.8, while a 150 kg N/ha application resulted in an increase to 5.7 in the BtC-horizons of Luvisols. The nitrogen sourced from industrial fertilisers can induce acidification within the application zone, followed by the leaching of basic cations into deeper soil profile layers [21,22]. The process of soil leaching due to water movement leads not only to illuviation, typical for Luvisols, but also to degradation through the leaching of easily soluble salts such as calcium (Ca). This phenomenon can further impact pH levels throughout the soil profile. Additionally, plant roots may contribute to pH alteration through their exudates, particularly within the rhizosphere, where the concentration of nitrogen from industrial fertilisers is higher. Conversely, in an enriched-cropping system that included organic matter, the situation was markedly different. In the A-horizons of Luvisols, the application of 100 and 150 kg N/ha significantly increased the pH by 22% and 24%, respectively, compared to the unamended control. In the Et horizons of Luvisols, the application of 100 kg N/ha significantly raised the pH by 0.7 and 0.4 pH units relative to the unamended control and the treatment with 150 kg N/ha, Figure 3). This response may be associated with the root exudation from a grass–clover mixture at this depth. These crops likely exhibit a more pronounced response to lower nitrogen application rates, which may subsequently influence pH changes within the rhizosphere. Furthermore, root exudates from leguminous crops, such as the grass–clover mixture utilised in this study, can influence soil pH by promoting microbial activity and nutrient availability, leading to improved nutrient uptake and better crop yields [23]. Overall, the effect of different cropping systems on pH alterations was observed to interact with the application of both nitrogen rates in the A-horizons of Luvisols, with a more pronounced effect noted following lower nitrogen application. Greater efficacy of nitrogen fertilisation in the A-horizons of Luvisols was identified in the organic-matter-enriching cropping system compared to the organic-matter-depleting system. The presence of more organic matter also implies enhanced buffering capacity of the soil, which mitigates drastic pH fluctuations, thereby reducing the extent of pH decline [24]. It has been reported that organic matter enhances the soil’s cation exchange capacity and mitigates the adverse consequences of acidification [25,26]. The pH values in the illuvial levels were not significantly different. This may indicate a more dynamic process of washing deep into the profiles of alkaline cations in this rotation.

4.2. Relationships Between Soil Properties in Luvisols

A strong positive linear relation between pH and Ca content was observed in the soil profiles of Luvisols (Figure 1). This finding is in line with studies suggesting that higher pH levels can enhance calcium availability in soils [27,28,29]. On the other hand, a negative linear trend (no significant) was found between pH and SOC, suggesting that organic carbon slightly decreases as pH increases, possibly indicating a reduction in soil acidity. This observation is consistent with literature indicating that high pH soils directly affect organic matter decomposition rates, thereby influencing organic carbon content [29]. Conversely, a negative linear non-significant trend between Ca and SOC suggests that soils with higher calcium levels may have slightly lower SOC. Rowley et al. [30], in their study, discussed the role of calcium in soil organic carbon dynamics. The study highlights that increased calcium prevalence is associated with variations in soil organic carbon content, often showing a negative correlation in certain soil types.

The findings of this study underscore the critical role of soil organic carbon in influencing soil pH and calcium levels. The stronger correlation observed in the enriched-cropping system with organic matter suggests that organic amendments can enhance soil properties, leading to improved nutrient availability and overall soil health [31,32]. Furthermore, the stabilisation of organic matter through the presence of calcium in the A-horizon may contribute to maintaining soil structure and fertility, which is essential for sustainable agricultural practices [33]. These results are consistent with previous studies that have demonstrated the beneficial effects of incorporating organic matter into agricultural practices. Organic amendments can significantly improve soil quality indicators, including pH and nutrient content. Therefore, implementing practices that promote organic matter retention could be crucial for enhancing soil functionality and crop productivity. The accumulation of organic carbon in this cropping system is also a result of the stabilisation of organic matter through higher calcium content in the A-horizon. In conclusion, the relationship between pH, calcium, and organic matter in different cropping systems highlights the importance of soil management practices aimed at increasing organic carbon content. This not only promotes better nutrient dynamics, but also enhances the resilience of agricultural soils to environmental stressors.

5. Conclusions

Our findings indicate that farmyard manure application significantly influences soil pH, calcium content, and SOC, particularly in the A-horizons (0–30 cm). A higher dose of FYM (35 t ha⁻¹) increased pH and calcium levels (an increase of 1.3 pH units and 0.7 g kg⁻¹ of Ca in favour of FYM35N0) in the B crop rotation). Additionally, the differences in correlation strength between the two cropping systems underline the importance of organic matter management in maintaining soil quality. The enriched-cropping system (rotation B) demonstrated a stronger positive relationship between pH and Ca, implying that organic amendments promote calcium retention. The total Ca content was similar in both rotations; however, the correlation analysis revealed a stronger relationship between SOC and Ca in the enriched-cropping system (rotation B) compared to the poor-organic-matter system (rotation A). In the 0–30 cm soil layer, the correlation coefficient between SOC and Ca was r = 0.52 in rotation B, while in rotation A, it was lower at r = 0.23. In deeper layers, this relationship weakened or became non-significant.

These findings underscore the need for long-term sustainable nutrient management strategies in Luvisols to balance soil acidity, calcium mobility, and organic carbon sequestration. Effective organic matter enrichment, particularly through farmyard manure application, can improve soil structure and nutrient cycling, mitigating nutrient leaching risks while enhancing soil resilience under changing climatic conditions. Future research should focus on the interactions between calcium and organic carbon stabilisation processes to optimise soil management practices for long-term agricultural sustainability.