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Article

Impact of Agricultural Land Use on Organic Carbon Content in the Surface Layer of Fluvisols in the Vistula River Floodplains, Poland

by
Miroslaw Kobierski
1,*,
Krystyna Kondratowicz-Maciejewska
1 and
Beata Labaz
2
1
Department of Biogeochemistry and Soil Science, Faculty of Agriculture and Biotechnology, Bydgoszcz University of Science and Technology, 6 Bernardynska Street, 85-029 Bydgoszcz, Poland
2
Institute of Soil Science, Plant Nutrition and Environmental Protection, Wroclaw University of Environmental and Life Sciences, Grunwaldzka 53, 50-357 Wroclaw, Poland
*
Author to whom correspondence should be addressed.
Agronomy 2025, 15(3), 628; https://doi.org/10.3390/agronomy15030628
Submission received: 30 January 2025 / Revised: 23 February 2025 / Accepted: 27 February 2025 / Published: 28 February 2025
(This article belongs to the Section Soil and Plant Nutrition)
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Versions Notes

Abstract

:
Floodplains with fluvisols in Poland are crucial areas for both agriculture and environmental relevance. The largest areas of fluvisols are located in the floodplains of the Vistula River and have been identified as significant reservoirs of organic carbon. Humic substances were determined using the following procedure: Cdec—carbon after decalcification, CHA+CFA—carbon of humic and fulvic acids (extracted with 0.5 M NaOH solution), CFA—carbon of fulvic acids (extracted with 2 M HCl solution), CHumin—proportion of carbon in humins. The extraction of soluble organic matter (DOC and DON) was also determined. In the surface layer of grasslands, significantly higher mean contents of total organic carbon (TOC) and total nitrogen (Nt) were found compared with arable soils. In fluvisols used as grasslands, compared to the arable soils, significantly higher contents of Cdec, CHA, CFA, Chumin, DOC, DON, and C-stock were observed. The study results indicate that the agricultural use of environmentally valuable lands, such as floodplains, affected the stock of organic carbon and the properties of the humic substances. Grasslands stored significantly more SOC (10.9 kg m−2) than arable soils (6.7 kg m−2), emphasizing their role as organic carbon resevoirs. Agricultural practices such as limiting plowing and introducing grasslands can support carbon sequestration. Therefore, the role of fluvisols in floodplains in carbon sequestration should be emphasized in climate change mitigation strategies.

1. Introduction

Soil organic carbon (SOC) is an important part of the global carbon cycle, which is affected by land management, land-use changes, and climate change [1,2,3]. Soil is a significant component of the terrestrial ecosystem. It plays a vital role in regulating atmospheric CO2 concentrations through the natural transformation of carbon compounds into humus. Additionally, it contributes to climate change mitigation by enhancing carbon sequestration [4,5,6]. Changes in SOC stocks can significantly affect the concentration of carbon dioxide in the atmosphere and even contribute to climate change [7,8]. Agricultural management practices that promote an increase in SOC contribute to improving soil quality and enhance the soil’s ability to sequester carbon, which is essential for mitigating global warming [9,10,11,12,13]. However, the effectiveness of carbon sequestration depends on various factors, including climate conditions, soil type, and land use [14]. The intensification of agricultural management through increased soil tillage, simplified crop rotation, and insufficient or the lack of application of organic fertilizer decreases C-stock in arable soils [15,16,17]. It is well known that conventional tillage techniques negatively impact SOC content, while simplified crop rotations and inadequate organic fertilization have a detrimental effect on carbon stocks. Conventional tillage practices can lead to a significant reduction in SOC and an increase in CO2 emissions due to the disruption of soil aggregates and the heightened exposure of organic matter to microbial decomposition [18,19,20,21,22]. Increasing carbon stocks in agricultural soils enhances their physical, chemical, and biological properties by reducing bulk density, improving soil aggregate structure, boosting water retention, increasing nutrient availability, and promoting microbial activity [23]. The distribution of particle sizes, TOC content, cation type, and climate conditions can affect the formation of stable aggregate structures [24,25]. More than half of TOC is found in complexes with the clay fraction, which can protect organic matter from microbiological degradation in agricultural soils [26,27]. Clay content influences the variation in total organic carbon resources, affecting TOC stability through the formation of persistent soil aggregates [28]. Bulk density (BD) is considered a key indicator of soil health, as it influences soil porosity, infiltration, water-holding capacity, rooting depth, plant nutrient availability, and the activity of soil microorganisms [29]. Proper measurement of BD plays a crucial role in determining SOC stocks. In the European Union (EU), fixed depth, BD, and SOC content are commonly used for SOC stock calculations. This method has been widely applied in scientific research comparing SOC stocks between treatments or over time periods. Nevertheless, many authors recommend using the equivalent soil mass (ESM) method to account for variations in BD when calculating SOC stocks. The ESM method enhances the interpretation of study results, especially when multiple BD measurements are taken at different depths and time intervals [30,31,32,33].
Agricultural management practices have the potential to serve as an effective strategy for mitigating climate change through carbon sequestration in soils [34]. Key strategies that enhance carbon sequestration and contribute to climate mitigation include conservation tillage systems, manure management, returning crop residue to the field, cover cropping, management of grasslands, and agroforestry [35,36,37]. Bossio et al. [37] indicated that soil carbon represents 25% of the potential of natural climate solutions, with 60% attributed to the restoration of depleted stocks and 40% to the protection of existing carbon reserves in soils. For agriculture and grasslands, soil carbon constitutes 47% of the climate mitigation potential. Arable soils, which cover approximately 35% of the global land area, contain relatively huge reservoirs of organic carbon. Consequently, C-stock is undergoing changes due to human-induced land surface transformations and intensive agricultural land use. Variations in the TOC stored in soils can have a significant impact on atmospheric CO2 concentrations [38,39]. The atmosphere contains two to three times less carbon than the soil in the form of SOC, which represents an important carbon pool within the environment [40,41]. Soil represents the largest terrestrial carbon reservoir, with substantial amounts stored in the surface layer of soil profiles [42,43]. The soils (surface and subsurface layers) store 74% of the global terrestrial C pool [44]. The main source of CO2 flux at the land-atmosphere interface is the decomposition of SOC. The main factor determining the rate of this process is temperature, which is important in the context of global warming [45,46]. Plant growth and crop yields are primarily influenced by temperature and precipitation, both of which also play a crucial role in regulating the decomposition of SOC [47,48].
In the EU, a low SOC content (<10 g kg−1) in arable soils is a significant factor limiting the resilience of soil to adverse environmental effects [31,49]. In this context, the preservation of humus-rich soils, such as the fluvisols under study, is important. Schiefer et al. [50] showed that, in Central Europe, the increase of SOC stocks in soil is strongly influenced by the state of the soil development. Modern agriculture should be based on the sustainable management of non-renewable natural resources, including soil, to ensure long-term environmental and agricultural viability. Yield losses due to soil erosion represent a natural risk to agriculture. Panagos et al. [51] estimated that severe soil erosion results in an annual crop yield loss of 0.43% across 12 million hectares of agricultural land in the EU. Biodiversity-based agriculture is a form of farming that aims to provide food security during uncontrollable climate change [52]. The organic carbon content of the world’s soil has decreased, requiring more effective agronomic strategies to improve carbon storage. Simplified tillage practices with effective land use systems (such as no-till) and enhanced fertility programs that enable carbon sequestration should be promoted in accordance with sustainable agriculture [53]. These practices include the implementation of diversified crop rotations, particularly with grasses and legumes, as well as the application of natural and organic fertilizers [54,55]. Since the conversion of a natural ecosystem to an agroecosystem, soil organic carbon stocks have decreased by an average of 30–55% [56]. The balance between greenhouse gas emissions and carbon retention ultimately determines the extent to which soil can contribute to climate change mitigation [57]. This balance is influenced by the soil properties, cultivation, climate, and fertilization. According to Hansen et al. [58], arable land has the potential to sequester at least 10% of the 8–10 Gt/yr of CO2 emitted annually.
Carbon sequestration strategies can help restore agricultural land productivity by recovering more than half of the original content of organic carbon [23]. This is particularly relevant in natural habitat areas, such as floodplains. The Vistula River’s riverbank erodes almost yearly during floods. Water erosion is a natural geological process for the formation of fertile alluvial soils that have been accelerated by human activity, leading to significant environmental consequences. Fluvial deposits reflect a diversity of mechanisms responsible for sediment transport and deposition in distal floodplain areas. The fluvial activity of rivers can have a significant impact on the properties of fluvisols due to their continuous interaction with the river dynamics [59]. The ecosystem process of sediment deposition in floodplains affects the physical, chemical, and biological properties of fluvisols, which are characterized by their relatively high SOC content [60,61,62]. The floodplains of the lower Vistula River are a protected area within the Vistula Landscape Park and are used as semi-natural and improved grasslands as well as arable lands.
The aim of this study was to investigate the C-stock and properties of humic substances in cultivated fluvisols used as grassland and arable soils. An additional objective of the study was to assess the content of humic substances and evaluate their role as indicators of changes in SOC resulting from the different agricultural uses of fluvisols. Humic substances, which include humins, fulvic acids, and humic acids, typically account for approximately 40–70% of SOC and play a crucial role in global carbon sequestration [63,64,65,66]. The structural complexity of these substances enhances their ability to bind with minerals, which is essential for the stabilization of organic carbon [60,67]. Humic substances contribute to carbon sequestration by promoting the formation of water-stable soil aggregates, which protect organic carbon from microbial decomposition [68]. This microbial activity is essential for the humification process, in which organic matter is transformed into humic substances [69]. This research presents the results of the SOC content and the properties of humic substances in fluvisols found in the lower Vistula River floodplains. The SOC resources were studied in relation to the agricultural use of floodplain soils, which have been identified as environmental carbon reservoirs.

2. Materials and Methods

2.1. Study Area and Sampling

The study area is in the Grudziadz Basin mesoregion (northern Poland), showing little variation in topography (Figure 1). Fifteen locations were selected on the floodplains. The soils of the study area were classified as fluvisols according to the World Reference Base (WRB) for Soil Resources [70] and correspond with alluvial soil orders of the Polish Soil Classification [71]. The climate in the Grudziadz Basin mesoregion is moderate, with an average annual temperature of 7.9 °C and a long-term average annual precipitation of 510 mm. The cultivated fields were fertilized with similar doses of NPK fertilizers, with an average of 135 kg per hectare: 70 kg of nitrogen (N), 25 kg of phosphorus (P), and 40 kg of potassium (K). A conventional cultivation system was used in the arable soils.
The grasslands were dominated by grass species with a high production potential, including fabaceous plants, and they contribute to agricultural production through livestock grazing (Figure 2). The arable soils are under crop rotation, including wheat, corn, and sugar beet, using conventional tillage with plowing to a depth of 25–30 cm. Soil samples were collected in April 2014. Until 2022, analyses were conducted after the samples had been stored under standard cold-storage conditions. The soil samples were collected in the floodplains from the surface layer (0–30 cm) of nine grassland soils and six arable fields and transported to the laboratory; for analysis, the samples were air-dried and passed through a sieve for the separation and determination of coarse fractions higher than 2.0 mm. Soil particles smaller than 2 mm in diameter were used for analysis of the physical and chemical properties.

2.2. Soil Sample Analyses and Laboratory Methods

Particle size distribution was determined by sieving and sedimentation using the Cassagrande’s areometric method modified by Prószyński [72]. The method described is based on Stoke’s Law, in which the free and continuous sedimentation of particles depends on their diameter, the density of liquids, temperature, and acceleration due to gravity. In this aerometric method, the density of the suspended particles was measured at specified time intervals from the beginning of sedimentation. The results of this analysis are obtained in weight percentages. The bulk density (BD) is the weight of dry soil (solids) divided by the total soil volume. The method of BD measurement was to collect the volume of soil (100 cm3) using a metal ring pressed into the soil. The ring is carefully excavated, trimmed level at the top and bottom, and dried for 2 days at 105 °C. Bulk density was expressed in megagrams per cubic meter (Mg m−3). Particle density was determined by the picnometric method using three replications. The values of pH were measured potentiometrically in a 1:2.5 (volume fraction) suspension of soil in a 1 mol L−1 potassium chloride solution (pH in KCl) using an Elmetron CP-551 pH Meter. The contents of total organic carbon (TOC) and total nitrogen (TN) were determined using a Vario Max CN analyzer (Elementar Analysensysteme GmbH, Hanau, Germany). The Scheibler method was used to estimate the calcium carbonate content from the volume of emitted carbon after the reaction with 10% HCl [73]. All measurements were conducted in three replications, and high-purity water (Milli-Q water) was used throughout. The fractionation of the soil’s organic matter was carried out with an NaOH solution after previous decalcification. The organic carbon fractions were determined according to the standard Schnitzer [74,75] method, as follows:
  • Cdec—carbon after decalcification (24 h) was achieved with 0.05 M HCl (1:10 w/v). After centrifugation, the residue was washed with distilled water until neutral.
  • CHA+FA—carbon of the humic and fulvic acids after extraction (24 h) with a 0.5 M NaOH solution (with occasional mixing, followed by centrifugation).
  • CFA—carbon of the fulvic acids (the resulting alkaline extract was acidified (for 24 h) with a 2 M HCl solution to pH = 2) after the precipitation of humic acids (CHA) through centrifugation.
The content of CHA was calculated from the following difference: CHA = CHA+FA − CFA.
  • Chumin—carbon of humin was calculated as the difference between the content of TOC and the C content in the respective fractions.
The extraction of soluble organic matter (DOC and DON) was also made in a solution of 0.004 M CaCl2 for 1 h at a ratio of the soil to extraction solvent of 1:10 (w/v). The organic carbon content in the individual fractions was determined using a Vario Max CN analyzer produced by Elementar (Germany). The contents of TOC and TN are expressed in g kg−1 of d.w. of soil but the DOC and DON contents are expressed in mg kg−1 of the d.w. of the soil sample.
The stock of soil organic carbon (C-stock) was determined in kg m−2, using the following formula:
C-stock = c·BD·t·(1 − θ%),
where c is the carbon content (g kg−1), BD is the bulk density (Mg m−3), t is the thickness of the soil horizon/layer (m), and θ is the % particle content larger than 2 mm [76].
The total porosity (TP) of the soil was calculated using the following formula, based on the particle and bulk density values:
total porosity = (PD − BD)/PD
where PD is the particle density (Mg m−3) and BD is the bulk density (Mg m−3). The water and air in the soil existed in the voids, thus a description of the soil porosity is a next step in understanding good turf management in grasslands.

2.3. Statistical Analyses

A statistical analysis was conducted based on the results of each of the individual parameters. The tables present the mean values from three replications. The post hoc test, Tukey HSD (Honest Significant Difference), and a one-way ANOVA variance analysis were used to compare the mean values of the parameters. Pearson’s correlation was also analyzed at a significance level of p < 0.05. The calculations were made using Statistica 13.0 (StatSoft, Inc., Tulsa, OK, USA).

3. Results

3.1. Basic Parameters of Soil Samples Collected from Arable Soils and Grasslands

The texture of the soil samples was sandy loam and silt loam [64]. There were no significant differences in the average contents of sand, silt, and clay in the grassland soil compared to the arable soil: 44.8% and 39.5% (p = 0.56), 38.2% and 43.2% (p = 0.43), and 17.0% and 17.3% (p = 0.93), respectively (Table 1).
The cumulative C density and mass results indicate a significant difference depending on land use, despite the studied soils having a similar grain size composition (Table 1). The particle density (PD) values ranged from 2.54 to 2.63 Mg m−3 for arable soils and from 2.52 to 2.60 Mg m−3 for the soil from grasslands. Most of the soil samples contained small amounts of CaCO3, and the pH was neutral and alkaline. In grasslands, the average pH value (6.78) was significantly lower (p = 0.03) in comparison with the arable soils. The soil samples did not exhibit salinity (EC ranging from 0.215 to 0.363 mS cm−1 in arable soils and from 0.252 to 0.512 mS cm−1 in the soil from grasslands). The pH values measured in the soil samples collected from the arable soils ranged from 6.79 to 7.46, with an average pH of 7.23 (Table 1). The average BD value in the surface layer of the arable soils was 1.52 Mg m−3, which was significantly higher than in the soil from the grasslands (1.34 Mg m−3) (Figure 3a). The average C-stock content was significantly higher (p = 0.006) in the soil of the studied grasslands (10.9 kg m−2) compared to the arable soils (6.7 kg m−2) (Figure 3b).

3.2. Properties of SOC and Humic Substances in the Surface Layer

The surface layer of the grasslands had significantly higher average TOC and TN levels compared to the topsoil of the arable soils (Figure 4a,b).
The TOC contents in grasslands and arable soils were 27.3 g kg−1 and 14.7 g kg−1 (p = 0.003), respectively, while TN contents were 2.6 g kg−1 and 1.5 g kg−1 (p = 0.002). The mean value of the C/N ratio in the surface layer of the arable soils (C/N = 9.4) was lower than in soil from the grasslands (C/N = 10.4) (Figure 5a). The average CHA/CFA ratio values calculated for the soil material of the surface layer were 1.64 in the arable soils and 1.61 in soil from the grasslands (Figure 5b). There were statistically significant differences between the average Cdec content in the surface layer of grassland soils (1.69 g kg−1) and arable soils (1.15 g kg−1), but the arable soils had a significantly higher percentage share of Cdec in the TOC pool (Table 2).
The average content of Chumin (6.72 g kg−1) in the arable soil samples was significantly lower in comparison to the grassland soils (11.81 g kg−1). The content of CHA (Table 2) in the surface layer of the arable soils ranged from 2.36 g kg−1 to 6.75 g kg−1 (mean 4.17 g kg−1), while the content of CHA in the grasslands soil was between 5.61 g kg−1 and 12.59 g kg−1 (mean 8.41 g kg−1). Samples taken from arable soils had a significantly lower average CFA content (2.66 g kg−1) compared to soil from the grasslands (5.35 g kg−1).
The average content of Chumin accounted for almost half of the pool of organic carbon compounds (46.1% in arable soils and 44.0% in grassland soil), and the ratio of humic to fulvic acid carbon content was similar. The percentage share of carbon in humic and fulvic acids in the TOC pool was lower in arable soils compared to in soil from the grasslands, but the differences were not statistically significant. The type of land use had a significant effect on the BD values, as the bulk density of arable land was significantly higher compared to grassland. Land-use changes resulted in a significant reduction in the TOC content of the surface layers. The total content of TOC in grassland soils was significantly higher than that in arable soils.

3.3. Statistical Anaysis Results

According to Pearson’s correlation analysis, a significant negative correlation was observed between BD and TOC, C-stock, Nt, DOC, and DON (Table 3). Most humic substances exhibited strong positive correlations with one another. A low negative correlation was noted between the C-stock and sand content (r = −0.41), whereas a significant positive correlation was found between the C-stock and TP (r = 0.69). C-stock was also significantly positively correlated with the humic substances, TOC, Nt, DOC, and DON. Among the humic compounds, CHA (r = 0.98), CFA (r = 0.97), and Chumin (r = 0.97) exhibited the strongest positive correlations with C-stock, confirming their function in enhancing chemical interactions with fine mineral particles. We noted a positive correlation between C-stock and the contents of clay (r = 0.46) and silt (r = 0.31). Additionally, the humic substances were negatively correlated with sand content, as follows: CHA (r = −0.40), CFA (r = −0.45), and Chumin (r = −0.21).

4. Discussion

4.1. Description of Basic Parameters of Soil Samples Collected from Floodplains

In Poland, the geomorphological description of soil particles with a diameter of 2–0.05 mm includes mainly quartz, with a density of 2.65 Mg m−3, with a small addition of other primary minerals (e.g., feldspars). In addition, this group of particles contains small amounts of organic remains. Therefore, a decrease in the diameter of the granulometric fraction is accompanied by an increase in the content of SOC [77,78]. Bulk density, as well as particle density (PD) and the total soil porosity (TP), are considered key indicators of soil health as they affect soil’s porosity, infiltration, and water-holding capacity and have a strong impact on the exchange of energy and matter between the atmosphere and the pedosphere. They also have a strong impact on plant rooting depth, plant nutrient availability, and the activity of soil microorganisms, which influence many soil processes, including the humification of organic matter [79]. The study by Bormann and Klaassen [80] indicated that the physical characteristics of soil and soil infiltration depend on both land use and soil type. In addition, some studies have emphasized that agricultural land contributes to the global carbon cycle both by emitting CO2 (humus loss through mineralization) and by absorbing CO2 from the atmosphere when plants that produce biomass through photosynthesis have transferred it to the soil. As a result, organic compounds are decomposed into humic substances in the humification process [81,82]. This is particularly important in the case of fluvisols, as they are characterized by high fertility and productivity with respect to other soils; thus, most vegetable production on fluvisols generally succeeds [83]. In the floodplain ecosystem, C-stock is a natural sink of atmospheric CO2 and is protected against microbial processes through the formation of organo-mineral complexes in water-stable soil aggregates. However, on arable land, fluvisols are particularly threatened by the deterioration of soil aggregate stability and the reduction of SOC due to the increased surface transport of soil particles during flooding.
This study indicated that the type of land use had a significant effect on the BD values at a soil depth of 0–30 cm. The average bulk density values in the study soils were higher than those reported for the topsoil of croplands and grasslands across the EU, as described by Panagos et al. [84]. However, they were comparable to data reported by Borek [83] for fluvisols in the Odra River valley, Poland, as well as for soils in the lower reaches of the Vltava River, Czechia [85], which have similar soil textures and the SOC contents. The quantity and quality of soil organic matter are closely related to the soil’s physical properties, which confirms the multifunctional role of SOC and its reliability as an indicator of soil quality [86]. As the SOC content decreases, the soil BD may increase and the TP decreases, which has a direct effect on soil infiltration [87,88]. The study confirmed that grasslands had a significantly higher average C-stock than croplands as a result of the higher SOC content, despite lower soil BD values. Moreover, the average C-stock in grasslands had a higher average TOC content compared to the fluvisols in Central and Eastern Europe (8.9 kg m−2 at a 0–30 cm soil depth) presented by Batjes [38]. In arable soils, the increased BD and decreased TP values were related to the loss of organic matter and soil compaction due to tillage practices [89]. The results of this study are in line with the research of Gajić [29] who reported that the total SOC content was significantly higher in permanent grasslands than in arable soils at a depth of 10 cm. Conant et al. [90] described that the organic carbon content in the soil surface layer at 0–30 cm increased by approximately 20 Mg C/ha for 20–40 years after arable land is converted to grassland. Forty percent of the initial SOC resources were lost in 25 years because of the opposite practice, which involved turning permanent pasture into arable land [91,92].

4.2. Properties of SOC and Humic Substances in the Surface Layer

The key role of soil organic matter is determined by the properties of the humic compounds, which are an important component of the global carbon cycle. Since they represent a significant part of the resources of SOC [93], they are involved in all the processes that occur in the soil and affect its physical, chemical, and biological properties [94]. In the analyzed arable soils, the humic parameters deteriorated significantly and lower contents of Cdec, CHA, CFA, Chumin, DOC, DON, and C-stock were observed compared to soil from the grasslands. Similar results were reported by Debska et al. [95]. The organic carbon content in cultivated soils has decreased, necessitating more effective agronomic strategies to enhance carbon storage. Humic substances (humic and fulvic acids, humins) exhibited the strongest positive correlations with C-stock, confirming their role in enhancing chemical interactions with fine particles. This is further supported by the observed positive correlation between the C-stock and the clay content and its negative correlation with the sand fraction content in the analyzed fluvisols. The formation of clay-humic complexes further enhances soil carbon stabilization [96]. Moreover, the implementation of conservation tillage and cover crops may improve organic matter accumulation and promote the formation of humic substances. Dudek et al. [97] confirms the usefulness of humic and fulvic fractions in determining the rate of degradation of SOC-rich soils.

4.3. Recommendations for Improving Carbon Sequestration

In order to prevent a decrease in SOC content, it is crucial to adopt more effective agronomic strategies that promote the accumulation of organic matter, such as sustainable agricultural practices. These agrotechnical practices include reduced or no tillage, mulching of the soil surface with crop residues, the use of catch and rotation crops with grasses and legumes, and the application of natural and organic fertilizers [54,98]. The soil quality of fluvisols, which are defined by the alluvial deposits and SOC content, is influenced by various agricultural practices. Crop cultivation and livestock grazing can significantly affect the SOC levels in fluvisols [22,99]. Tillage systems, such as no-till practices, can help retain soil organic carbon, whereas conventional tillage often leads to its depletion. Jończak et al. [100] noted that soil-forming processes influenced by agricultural practices significantly impact the organic carbon content in fluvisols. Farmers who implement carbon farming practices—such as increasing carbon inputs through soil amendments, cover crops, residue management, and crop rotation—can enhance both carbon stocks and soil health. Consequently, the annual carbon sequestration rates for different management practices range from 100 to 1000 kg C ha−1 [101]. The management of agricultural practices has a crucial impact on SOC, particularly in fluvisols, which are common in floodplain valleys. These soils are highly fertile but also susceptible to degradation, making them particularly sensitive to changes in agricultural management. Implementing appropriate practices can significantly enhance the SOC content, which is essential for soil health and its capacity for carbon sequestration [102].
In recent decades, the area of grasslands in Poland has decreased. This trend is particularly concerning, as grasslands play a crucial role in maintaining biodiversity, as well as climatic, protective, and landscape functions [103,104]. Global grassland loss over the past century has resulted from their conversion into arable land for animal feed crop production [105]. Implementing agricultural practices that enhance biodiversity within various farming systems across the European Union (EU) is essential for the sustainable restoration of ecosystems on agricultural land, especially in the context of the global climate crisis [106]. Semi-natural habitats and grasslands that have been degraded by agricultural activities must be preserved or restored to support biodiversity [107]. Moreover, grasslands and extensive grazing management have been demonstrated to contribute to climate change mitigation by controlling erosion, regulating carbon dioxide flux, and carbon enhancing storage [108]. One of the most effective methods to decrease greenhouse gas emissions is returning arable land to natural and semi-natural grasslands, as well as rewetting drained meadows and peatlands through the restoration of hydrological conditions, even on a small scale [109]. This is particularly important for fluvisols located in landscape parks such as the Vistula River floodplains.

5. Conclusions

The SOC dynamics were examined in the context of degradation processes associated with the agricultural use of floodplain soils. Grasslands are considered an effective management system for enhancing soil structure, as they mitigate compaction and increase soil porosity. The soil from the semi-natural and improved grasslands contained significantly higher levels of Cdec, CHA, CFA, and CHumin compared to the arable soils of the studied floodplains. The agricultural use of fluvisols significantly influenced the C-stock and properties of the SOC. Grasslands stored significantly more SOC (10.9 kg m−2) than arable soils (6.7 kg m−2, p = 0.006), emphasizing their role as organic carbon reservoirs. The results of this study indicate that reducing the intensive use of fluvisols as arable soils is beneficial for long-term carbon storage in the surface layer, as observed in semi-natural and improved grasslands. Maintaining a balance between intense land use and the ability of the soil to store carbon will be essential for farmers cultivating fluvisols in floodplains. Agricultural practices such as limiting plowing, applying organic fertilizers, implementing cover crops, and introducing grasslands can support carbon sequestration. Increasing the area where fluvisols remain in their natural habitat or as natural grasslands can help prevent the excessive mineralization of SOC. Where arable farming is necessary, implementing minimum tillage techniques and minimizing the frequency of plowing should be considered. The results clearly indicated that the stock of organic carbon and properties of humic substances depend on the type of agricultural use in ecologically valuable areas, such as floodplains. Therefore, in the context of global climate change mitigation, carbon sequestration in semi-natural and improved grasslands should receive increased attention and protection. The assessment of humic substances can serve as a useful indicator of changes related to the agricultural use of fluvisols in the context of carbon sequestration. As essential components of sustainable soil management, they contribute to the stabilization of SOC, the improvement of soil structure, and the stimulation of microbial activity. Further research is needed to investigate the dynamics of humic substances in different land-use systems to optimize their role in carbon sequestration. In future research, we recommend increasing the sample size, which enables a more accurate assessment of the impact of the agricultural use of fluvisols on SOC stock. This will affect the power analysis and statistical significance of the results and increase the generalizability and clarity of the findings.

Author Contributions

Conceptualization, M.K.; methodology, M.K., K.K.-M. and B.L.; formal analysis, M.K., K.K.-M. and B.L.; investigation, M.K. and B.L.; resources, M.K. and B.L.; data curation, M.K., K.K.-M. and B.L.; writing—original draft preparation, M.K. and B.L.; writing—review and editing, M.K. and B.L.; visualization, M.K. All authors have read and agreed to the published version of the manuscript.

Funding

The research was made as part of 2716/B/P01/2011/40 research project, financed by the National Centre of Science (NCN) in Poland.

Data Availability Statement

The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author/s.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. Schemes of the study location. Localization of the areas and soil profiles under investigation. Schematic maps of Europe (A) and Poland (B). Study location—Grudziadz Basin, Lower Vistula River (C).
Figure 1. Schemes of the study location. Localization of the areas and soil profiles under investigation. Schematic maps of Europe (A) and Poland (B). Study location—Grudziadz Basin, Lower Vistula River (C).
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Figure 2. Arable soil after a flood episode in spring (A); grassland in summer (B).
Figure 2. Arable soil after a flood episode in spring (A); grassland in summer (B).
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Figure 3. Soil parameters and the significance levels (ANOVA, Tukey test). Description: (a) bulk density, (b) stock of TOC.
Figure 3. Soil parameters and the significance levels (ANOVA, Tukey test). Description: (a) bulk density, (b) stock of TOC.
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Figure 4. Soil parameters and the significance levels (ANOVA, Tukey test). Description: (a) total organic carbon, (b) total nitrogen.
Figure 4. Soil parameters and the significance levels (ANOVA, Tukey test). Description: (a) total organic carbon, (b) total nitrogen.
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Figure 5. Average values of ratios and the significance levels (ANOVA, Tukey test). Description: (a) carbon to nitrogen content, (b) carbon content of humic acids to fulvic acids.
Figure 5. Average values of ratios and the significance levels (ANOVA, Tukey test). Description: (a) carbon to nitrogen content, (b) carbon content of humic acids to fulvic acids.
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Table 1. The basic parameters of the soil samples (0–30 cm).
Table 1. The basic parameters of the soil samples (0–30 cm).
ParametersArable SoilsGrasslandsSignificance Level
Sand (%)
2.0–0.05 mm		39.5 ± 14.344.8 ± 16.00.56
Silt (%)
0.05–0.002		43.2 ± 10.538.2 ± 17.00.43
Clay (%)
<0.002 mm		17.3 ± 7.417.0 ± 5.80.93
Cumulative C
density (Mg C ha−1)		66.8 ± 20.5104.1 ± 29.10.0006 *
Cumulative mass (kg m−2)445.5 ± 20.9383.2 ± 18.20.0003 *
PD (Mg m−3)2.58 ± 0.0352.55 ± 0.0300.18
TP (m3 m−3)0.41 ± 0.220.47 ± 0.020.0003 *
pH in 1M KCl7.23 ± 0.246.78 ± 0.350.03 *
CaCO3 (g kg−1)8.32 ± 5,76.80 ± 4.020.59
EC (mS cm−1)0.284 ± 0.060.333 ± 0.080.28
PD—particle density; TP—total porosity; EC—electrical conductivity; ±standard deviation; *—statistically significant (one-way ANOVA variance analysis, post hoc test—Tukey HSD).
Table 2. Characteristics of the humic substances.
Table 2. Characteristics of the humic substances.
ParametersArable SoilsGrasslandsSignificance Level
Cdec (g kg−1)1.15 ± 0.221.69 ± 0.350.005 *
CHA (g kg−1)4.17 ± 1.538.41 ± 2.560.003 *
CFA (g kg−1)2.66 ± 1.255.35 ± 1.820.008 *
Chumin (g kg−1)6.72 ± 1.9611.81 ± 2.140.0006 *
Cdec in TOC (%)8.27 ± 2.066.31 ± 0.970.04 *
CHA in TOC (%) 28.2 ± 2.2130.5 ± 2.360.11
CFA in TOC (%) 17.5 ± 3.0819.3 ± 2.840.31
Chumin in TOC (%)46.1 ± 3.6544.0 ± 3.830.34
DOC (g kg−1)0.51 ± 0.221.15 ± 0.480.009 *
DON (mg kg−1)40.2 ± 17.068.5 ± 15.90.006 *
DOC in TOC (%)3.4 ± 0.584.09 ± 0.940.17
DON in TOC (%)2.95 ± 0.352.77 ± 0.420.44
Carbon content in various substances: Cdec—after decalcification; CHA—humic acids; CFA—fulvic acids; Chumin—humins; DOC—dissolved organic carbon; DON—dissolved organic nitrogen; *—statistical significance (one-way ANOVA variance analysis, post hoc test—Tukey HSD).
Table 3. Pearson’s correlation coefficients (r) between the physical and chemical properties and the texture; n = 15.
Table 3. Pearson’s correlation coefficients (r) between the physical and chemical properties and the texture; n = 15.
SiltClayCdecaCHACFACHuminDOCDONBDTPC-stockTOCNt
sand−0.94 *−0.82 *−0.08−0.40−0.45−0.21−0.27−0.18−0.120.22−0.41−0.33−0.38
silt 0.57 *0.050.320.390.140.230.080.11−0.190.310.270.28
clay 0.100.420.430.260.270.310.11−0.200.460.350.43
Cdeca 0.88 *0.85 *0.92 *0.87 *0.83 *−0.86 *0.81 *0.87 *0.91 *0.84 *
CHA 0.97 *0.95 *0.94 *0.83 *−0.80 *0.73 *0.98 *0.99 *0.98 *
CFA 0.91 *0.94 *0.76 *−0.77 *0.69 *0.97 *0.97 *0.96 *
CHumin 0.91 *0.86 *−0.87 *0.80 *0.97 *0.98 *0.95 *
DOC 0.73 *−0.79 *0.71 *0.93 *0.95 *0.92 *
DON −0.78 *0.74 *0.81 *0.84 *0.79 *
BD −0.99 *−0.78 *−0.84 *−0.79 *
TP 0.69 *0.76 *0.70 *
C-stock 0.99 *0.99 *
TOC 0.98 *
Carbon in various substances: Cdec—after decalcification; CHA—humic acids; CFA—fulvic acids; Chumin—humins; TOC—total organic carbon; Nt—total nitrogen DOC—dissolved organic carbon; DON—dissolved organic nitrogen; TP—total porosity; *—significant correlation coefficient, p < 0.05.
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Kobierski, M.; Kondratowicz-Maciejewska, K.; Labaz, B. Impact of Agricultural Land Use on Organic Carbon Content in the Surface Layer of Fluvisols in the Vistula River Floodplains, Poland. Agronomy 2025, 15, 628. https://doi.org/10.3390/agronomy15030628

AMA Style

Kobierski M, Kondratowicz-Maciejewska K, Labaz B. Impact of Agricultural Land Use on Organic Carbon Content in the Surface Layer of Fluvisols in the Vistula River Floodplains, Poland. Agronomy. 2025; 15(3):628. https://doi.org/10.3390/agronomy15030628

Chicago/Turabian Style

Kobierski, Miroslaw, Krystyna Kondratowicz-Maciejewska, and Beata Labaz. 2025. "Impact of Agricultural Land Use on Organic Carbon Content in the Surface Layer of Fluvisols in the Vistula River Floodplains, Poland" Agronomy 15, no. 3: 628. https://doi.org/10.3390/agronomy15030628

APA Style

Kobierski, M., Kondratowicz-Maciejewska, K., & Labaz, B. (2025). Impact of Agricultural Land Use on Organic Carbon Content in the Surface Layer of Fluvisols in the Vistula River Floodplains, Poland. Agronomy, 15(3), 628. https://doi.org/10.3390/agronomy15030628

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