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Article

Synergistic Effects of Biochar and Algae-Transformed Organic Waste from the Dairy Industry on Soil Organic Matter and Soil Sorption Properties

by
Vladimír Šimanský
1,* and
Ján Horák
2
1
Institute of Agrochemistry and Soil Science, Faculty of Agrobiology and Food Resources, Slovak University of Agriculture, Trieda A. Hlinku 2, 949 76 Nitra, Slovakia
2
Institute of Landscape Engineering, Faculty of Horticulture and Landscape Engineering, Slovak University of Agriculture, Hospodárska 7, 949 76 Nitra, Slovakia
*
Author to whom correspondence should be addressed.
Land 2026, 15(5), 857; https://doi.org/10.3390/land15050857
Submission received: 31 March 2026 / Revised: 14 May 2026 / Accepted: 15 May 2026 / Published: 16 May 2026
(This article belongs to the Special Issue Feature Papers for “Land, Soil and Water” Section, 2nd Edition)
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Abstract

Biochar and algal extracts are promising organic soil amendments, but their synergistic effects on soil organic matter and the sorption complex are still insufficiently understood. Therefore, a 30-day laboratory incubation experiment was conducted using a Haplic Luvisol to evaluate the effects of biochar (S+B), an N-rich algal extract (S+AGN), and their combined application (S+AGN+B) in comparison with the untreated control soil (S). The results showed that biochar led to a substantial increase in soil organic carbon (Corg) by 49% in S+B and by 50% in S+AGN+B treatments compared to S. Labile carbon (CL) increased by 48% in S+B and by 40% in S+AGN+B. The algal extract alone did not significantly affect either CL or Corg. Non-labile carbon increased by 2.22 g kg−1 in S+B but slightly decreased in the combined treatment (−2.00 g kg−1), indicating different dynamics of stable carbon fractions when both amendments are applied simultaneously. The combined treatment S+AGN+B, however, had the strongest effect on soil sorption properties. Specifically, the sum of basic cations was the highest among all treatments (189 mmol(+)kg−1, i.e., +18–28 mmol(+)kg−1 compared to S, S+B, and S+AGN), while the cation exchange capacity (CEC) reached the highest values (198 mmol(+)kg−1, representing an increase of 7–27 mmol(+)kg−1 compared to the other treatments). The base saturation remained high across all treatments, and the highest value was observed in S+AGN+B (95.6%). PCA confirmed that the combined treatment produced the most pronounced shifts in the multivariate parameter space and demonstrated a synergistic effect exceeding the effects of the individual organic amendments. Overall, the results indicate that biochar is the dominant factor contributing to the accumulation of stable carbon and the improvement of CEC, whereas the algal extract enhances the accumulation of labile carbon fractions and synergistically promotes the saturation of the sorption complex. The combined application of biochar and algal N effectively increases soil organic matter and sorption capacity.

1. Introduction

The declining quality of soils due to degradation, agricultural intensification, and climate change creates an urgent need to implement new, environmentally sustainable approaches for restoring soil functions. One promising strategy is the application of organic soil amendments, including biochar, algal biomass, and various modified liquid organic wastes derived from algae-based valorization processes in the food industry [1].
Biochar is a stable, carbon-rich material produced by the pyrolysis of biomass [2]. Experimental and field studies have demonstrated that its application improves soil fertility and promotes the formation of stable soil organic matter [3], while simultaneously increasing soil porosity [4], sorption capacity [5], nutrient retention [6], and water-holding capacity [7]. Numerous laboratory experiments and synthesis studies confirm the positive effects of biochar on microbial activity [8,9], whereas review papers and comparative studies report consistent improvements in overall soil quality [2,9,10]. Biochar also plays a key role in the long-term stabilization of soil carbon and can serve as a sorbent for toxic metals or organic pollutants, thereby contributing to the revitalization of degraded soil systems [11]. Although biochar is an important tool for sustainable and regenerative soil management, its effects are highly variable and strongly dependent on the specific properties of the biochar and local soil–climate conditions. For this reason, several region-specific experimental studies conducted under Central European conditions emphasize the need for considerate, site-specific approaches to biochar application [12,13,14]. Biochar should thus be viewed as a component of a comprehensive soil management strategy rather than a universal solution, requiring long-term experimental verification for practical implementation.
Alongside biochar, interest is also increasing in other organic soil amendments, including algae and algal biomass [1], as well as various modifications of organic inputs derived from the secondary valorization of food-processing waste by algae. Algae and their soil-applied products may enhance soil organic matter (SOM), support microbial interactions, and improve nutrient availability. When applied directly or as components of biostimulants [15], microalgal products have been shown in greenhouse and field studies to modify soil processes, regenerate degraded soils, and increase crop yields [16,17]. Physiologically active compounds derived from algae stimulate root development, plant hormones, and photosynthesis. Some species simultaneously reduce salinity, bind toxic substances, and enhance plant stress tolerance [18]. In addition, experimental studies demonstrate that algae-derived exopolymers promote soil aggregation, leading to increased porosity, water retention, and overall soil structural stability [19]. They further contribute to the increased availability of N, P, K, Mg, Fe, and other elements [16] and, thanks to their capacity to fix atmospheric nitrogen, may partially substitute the application of mineral nitrogen fertilizers [15]. Beyond its agronomic effects, the algal extract used in this study represents a product of secondary valorization of dairy-processing wastewater. Algae-based systems enable the recovery of residual nitrogen and organic compounds from process water and their conversion into functional inputs for soil management, thereby reducing the environmental burden associated with agro-industrial effluents and closing nutrient cycles [15,16,19]. From a circular bioeconomy perspective, the integration of algal wastewater treatment with agricultural soil amendment strategies supports sustainable nutrient recycling and resource-efficient waste management [18,19]. The combined application of algal extracts and biochar therefore extends the relevance of this study beyond soil processes alone, linking soil quality improvement with environmentally responsible resource management under Central European conditions.
Since biochar is predominantly a stable carbon source and often characterized by a high C:N ratio, its application may temporarily immobilize soil nitrogen. The high carbon content stimulates microbial activity, which in turn reduces the amount of plant-available mineral nitrogen [20]. For this reason, production methods and application strategies frequently aim to enrich biochar with nitrogen by mixing it with composts, organic fertilizers, algal biomass, or their liquid organic extracts derived after algal bioconversion. Research indicates that combining algae and biochar can significantly alter soil sorption properties. Recent laboratory and pot experiments indicate that the combined application of algae-derived products and biochar can significantly alter soil sorption properties [21]. The combination of biochar and algae thus represents a promising ecological tool for optimizing soil properties, as biochar provides a stable carbon matrix with high sorption capacity, while algae supply bioactive substances, modify soil biology, and influence nutrient dynamics.
Despite the growing number of studies, a comprehensive understanding of the interactions between these two types of organic amendments across different soil environments is lacking. More research is needed to determine their effects on the formation of stable organic matter and the mechanisms underlying their combined effects on soil sorption properties. Therefore, the combined use of biochar and algal organic extracts represents a promising research direction with the potential to develop effective and environmentally sound strategies for restoring soil quality. This study also addresses a knowledge gap in understanding the mechanisms underlying the synergistic regulation of soil organic matter and the soil sorption complex following combined biochar and algal extract application under typical soils in Central Europe. The aim of this study was therefore to (i.) comprehensively evaluate the effects of biochar, algal extract, and their combination on changes in soil organic matter (SOM) and soil sorption properties, and (ii.) determine the relationships between SOM and soil sorption depending on the tested organic additives. It should be noted that short-term incubation studies are essential for understanding initial mechanistic responses, which form the basis for interpreting longer-term soil processes. We hypothesized that (H1) the application of biochar alone and in combination with the algal extract would significantly increase soil organic carbon content and both labile and non-labile carbon fractions; (H2) treatments containing biochar would significantly improve soil sorption properties, particularly cation exchange capacity and base saturation; and (H3) the combined application of biochar and algal extract would result in a synergistic effect exceeding the effects of the individual amendments.

2. Materials and Methods

2.1. Soil Characteristics

The soil used for establishing the laboratory experiment originated from the research station of the Slovak University of Agriculture in Nitra, located in Malanta. The soil profile was classified on the basis of its morphogenetic characteristics as a Haplic Luvisol [22]. Soil was collected from the A-horizon, homogenized, air-dried, and subjected to initial soil analyses. Prior to the experiment, the soil contained 151.3 g kg−1 clay, 488.3 g kg−1 silt, and 360.4 g kg−1 sand. The soil contained 15.0 g kg−1 Corg and 1.67 g kg−1 CL, and its pH was slightly acidic (6.20); the cation exchange capacity (CEC) was 159 mmol(+)kg−1, and the sorption complex was saturated at 91.6%. Soil analyses prior to the establishment of the experiment were performed using standard methodological procedures, which are strictly described in Hrivňaková et al. [23].

2.2. Characteristics of the Tested Organic Amendments

The biochar used in this laboratory experiment was produced by pyrolysis at 550 °C for 30 min in a Pyreg reactor (Pyreg GmbH, Dörth, Germany) from paper fiber sludge and cereal husks (mixed at a 1:1 ratio; supplied by Sonnenerde, Riedlingsdorf, Austria). The resulting product contained, on average, 531 g kg−1 total organic carbon, 14 g kg−1 total nitrogen with a C:N ratio of 38:1, 57 g kg−1 Ca, 3.9 g kg−1 Mg, 15 g kg−1 K, and 0.77 g kg−1 Na. The particle size ranged from 1 to 5 mm, with a specific surface area of 21.7 m2 g−1 and an ash content of 38.3%. The biochar had an average pH of 8.80.
The algal extract, used either alone or combined with biochar, was supplied in liquid form. The Scenedesmus obliquus extract was prepared using a wastewater-to-algae ratio of 1:10. The wastewater, originating from dairy production (process water, i.e., without sanitary wastewater), was added 3 days prior to extraction. The purification process was carried out at approximately 21 °C under a 12/12 h light/dark cycle. The algal solution contained 4.32 mg L−1 NO3− and 0.442 mg L−1 NH4+, with a pH of 8.72. The chemical characterization of the algal extract was limited to inorganic nitrogen forms (NO3− and NH4+) and pH. Total organic carbon (TOC) dissolved organic carbon (DOC), and total nitrogen (TN) were not quantified in the present study. Therefore, the algal extract was not treated as a quantified source of organic carbon input, but rather as a functional biostimulant affecting microbial and biochemical soil processes.

2.3. Establishment and Management of the Laboratory Experiment

The laboratory experiment was conducted in an incubation room where temperature (23–25 °C) and moisture conditions could be controlled throughout the 30-day incubation period. To maintain stable moisture levels, soil in each container was watered with distilled water to keep moisture content within 50–60% of the soil’s water-holding capacity (WHC). The experimental setup followed the scheme presented in Table 1. The laboratory experiment was established as a completely randomized design with four treatments (S, S+B, S+AGN, and S+AGN+B), each represented by five independent replicate containers (n = 5 per treatment). The containers were randomly assigned to treatments at the beginning of the experiment and were not mixed during the incubation period.
The experiment was conducted in containers with a volume of 1.08 dm3, having a reduced upper diameter of 80 mm and a bottom support plate with a diameter of 97.5 mm. Silicone mesh and filter paper were placed at the bottom of each container. Each container was filled with 713.6 g of soil (1.95% moisture content), which had been previously homogenized and sieved through a <5 mm mesh. In the biochar treatment (B), biochar was applied at a rate of 1% (w/w, dry matter basis), corresponding approximately to a field application rate of about 20 t ha−1 when assuming incorporation into the top 0–20 cm soil layer with a bulk density of 1.3 g cm−3. This conversion is provided for practical reference, while acknowledging that laboratory container conditions do not fully represent field-scale heterogeneity. In treatments with the algal extract (AGN), the N-rich extract obtained from algae and wastewater was applied at a rate of 142.5 mL L−1. The chemical characterization of the algal extract was limited to inorganic nitrogen forms (NO3− and NH4+) and pH. Total carbon, dissolved organic carbon, and total nitrogen were not quantified in the present study, and the extract was therefore treated primarily as a functional biostimulant input rather than as a quantified source of organic carbon. Subsequently, the moisture content in all soil samples was adjusted to 22.3% (i.e., 60% WHC). In the AGN treatments, an additional irrigation dose of approximately 30 mL was applied in the third week of the experiment to restore soil moisture to 22.3%. Throughout the experiment, soil moisture in the containers was maintained within the range of 18.9% (lower limit) to 22.3% WHC. Whenever the moisture dropped below 18.9%, the soil was rewatered to the target moisture level of 22.3%.

2.4. Soil Sampling and Analysis

Soil sampling was destructive; therefore, separate container sets were used for each sampling date to avoid disturbing the incubation process. The first soil sampling (Sampling 1) was conducted on the second day after the establishment of the experiment, and the second soil sampling (Sampling 2) was performed at the end of the 30-day incubation period. At each sampling date, soil from each replicate container was collected individually, air-dried, sieved to fine earth, and subsequently used for the determination of soil organic matter parameters and soil sorption properties. The content of organic carbon (Corg) was determined oxidometrically [24], and the content of labile carbon (CL) was determined using the method of Loginow [25]. Based on Corg and CL, the content of non-labile carbon (CNL) was calculated according to Equation (1), and carbon lability (L) was calculated according to Blair et al. [26], as shown in Equation (2).
CNL = Corg − CL
L = CL/CNL
From the parameters of the soil sorption complex, hydrolytic acidity (Ha) was determined according to Kappen using titration in a 1 mol L−1 CH3COONa extract, and the sum of exchangeable basic cations (SBC) was determined by titration in a 1 mol L−1 HCl extract [23]. The SBC represents the total concentration of exchangeable Ca2+, Mg2+, K+, and Na+. Based on the values of Ha and SBC, the cation exchange capacity (CEC) and the degree of base saturation (Bs) were calculated using Equations (3) and (4), respectively.
CEC = Ha + SBC
Bs = SBC/CEC · 100

2.5. Statistical Analysis

Statistical analyses were conducted using Statistica™ version 13.1 (Dell Inc., Round Rock, TX, USA). The means of soil characteristics were compared across treatments using one-way analysis of variance (ANOVA). The homogeneous groups were distinguished by the Tukey test for α = 0.05. A Principal Component Analysis (PCA) was performed to identify relationships between soil characteristics and different treatments. All computations were carried out at the α = 0.05 significance level.

3. Results

3.1. Soil Organic Matter

During the first sampling, soil organic carbon content (Corg) ranged from 15.4 to 24.3 g kg−1, with the highest increases observed in the S+AGN+B and S+B treatments. In the second sampling, the trend partly shifted, with the highest Corg recorded in S+B, followed by S+AGN+B (Table 2). The difference between the two sampling dates showed a positive increment in S+B (+2.52 g kg−1), a very small change in S+AGN, and a decrease in S+AGN+B (−1.90 g kg−1). Overall mean values clearly demonstrated that Corg was highest in the treatments containing biochar (23.1–23.3 g kg−1), whereas the control soil and the soil amended solely with the N algal extract showed no significant differences (15.5–15.6 g kg−1). Overall, both the application of biochar and its combined application with the algal N extract had a statistically significant effect on increasing soil organic carbon content. In S+B and S+AGN+B, Corg increased by 49% and 50%, respectively, compared to the control (S).
During the first sampling, labile carbon (CL) increases in the treatments followed this order: S < S+AGN < S+B < S+AGN+B (Table 2). The highest value (2.54 g kg−1) was observed in the combined S+AGN+B treatment, indicating a strong synergistic effect arising from the interaction of biochar and the algal extract. A similar trend was observed in the second sampling: biochar and the combined N algal extract + biochar treatment exhibited CL values 48% and 40% higher, respectively, relative to the control. CL slightly increased in the biochar treatment (+0.29 g kg−1 in S+B), whereas in S+AGN it even slightly decreased (−0.13 g kg−1) between the first and second sampling. These findings suggest differences in the dynamics of organic matter mineralization resulting from the applied organic inputs. Average CL values confirmed significant increases in the treatments with biochar (2.65 g kg−1) and with the combined addition of biochar and the N algal extract (2.59 g kg−1), while the control and the soil amended with the N algal extract alone remained at lower and nearly identical levels (1.77–1.80 g kg−1).
The non-labile carbon content (CNL) followed the same pattern as Corg (Table 2). The highest CNL values were found in S+B and S+AGN+B in both sampling dates. The between-sampling difference was largest in the biochar treatment (+2.22 g kg−1), and the combination of biochar with the N algal extract resulted in a decrease (−2.0 g kg−1).
Despite the observed CNL decrease between the second and first sampling in the S+AGN+B treatment, mean CNL values clearly demonstrate that the combined application of biochar and algal extract resulted in the same level of non-labile carbon as biochar alone. This indicates that the observed between-sampling variation reflects short-term redistribution of carbon fractions during incubation rather than a reduction in the stable carbon pool. Based on mean CNL values, it is evident that biochar was the dominant source of stable, non-labile carbon fractions, whereas the soil alone and the soil amended with the N algal extract showed no effect on CNL (13.8 g kg−1).
Carbon lability (L) showed a different pattern from the other evaluated soil organic matter parameters (Table 2). In the first sampling, the highest L value was observed in S+AGN (0.138), suggesting a higher proportion of active carbon in labile form due to the application of the N algal extract. In the second sampling, however, the control (S) showed the highest active carbon content, while carbon lability in the other treatments ranged between 0.124 and 0.130. Opposing trends were also observed between the two sampling dates: carbon lability increased strongly (+0.018) in the control soil and slightly (+0.014) in S+AGN+B, biochar alone showed no effect (0.000), and the N algal extract even decreased carbon lability (−0.015). Overall, mean L values did not change significantly over the experimental period (Table 2).

3.2. Soil Sorption Capacity

During the first sampling, hydrolytic acidity (Ha) ranged from 5.78 to 8.19 mmol(+)kg−1, with the highest values recorded in the S+B treatment (Table 3). In the second sampling, increases in Ha were observed across all treatments. The difference between the second and first sampling confirmed that the changes within individual treatments were relatively dynamic; however, no significant differences were detected in the overall mean values.
During the first sampling, the values of the sum of basic cations (SBC) varied between 161 and 191 mmol(+)kg−1 (Table 3). The highest SBC value was determined in S+AGN+B, which also differed significantly from the other treatments. The highest SBC value (187 mmol(+)kg−1) was again recorded in this treatment during the second sampling, while the lowest values were determined in S+AGN. The difference between sampling events showed a slight increase in most treatments; however, in S+AGN+B, a decrease in SBC (−4.00 mmol(+)kg−1) was observed between the second and first sampling. Overall, the SBC content was statistically significantly higher in the S+AGN+B compared to other treatments.
The values of cation exchange capacity (CEC) followed the same trend as SBC (Table 3). CEC increase in the second sampling was most pronounced in the control (14.1 mmol(+)kg−1) and gradually decreased in the order S+B > S+AGN > S+AGN+B, but the difference was practically negligible (0.17 mmol(+)kg−1). Mean CEC values clearly separated the treatments into three statistically distinct groups, with biochar and the N algal extract showing the highest CEC values.
In this experiment, the soil sorption complex was saturated above 90% in all treatments. During the first sampling, base saturation (Bs) ranged from 95.2% to 96.7%, with the highest values found in treatments containing the N algal extract (Table 3). In the second sampling, a decreasing trend in Bs values was observed across all treatments, most noticeably in S+AGN (−3.40%). Overall, the highest base saturation of the sorption complex was recorded in S+AGN+B.

3.3. Relationships Between Soil Organic Matter Parameters and Soil Sorption Properties

The first two principal components accounted for most of the variability in the dataset of determined soil properties. PC1, which explained 47.0% of the total variance, was most strongly influenced by indicators of SOM and sorption characteristics (Figure 1). The strongest loadings were associated with Corg, CNL, and CL, indicating that these parameters primarily determined differences among samples along PC1. PC2, accounting for an additional 25.7% of the variability, mainly reflected a contrast between the degree of base saturation, which had a strong positive influence, and Ha, which exerted an opposite effect. Carbon lability (L) also contributed to PC2, albeit to a lesser extent. Together, PC1 and PC2 explained 72.8% of the total variance, representing a highly effective reduction in the dataset’s complexity.
Statistical testing confirmed that the differences among treatments were significant already at the level of PCA scores. For PC1, a very strong differentiation among treatments was detected (F = 60.61; p < 0.001), while PC2 showed a moderate but still statistically significant effect (F = 4.87; p = 0.0061). The combination of biochar and N algal extract (S+AGN+B) caused the most pronounced shift in the PCA space, indicating its strongest influence on SOM, sorption properties, and base saturation. Biochar alone (S+B) primarily improved SOM parameters and sorption capacity but had a weaker effect on the base saturation. The control (S) showed no substantial changes, and the application of the N algal extract alone (S+AGN) affected mainly the less stable organic fractions rather than the soil sorption properties.

4. Discussion

4.1. Effect of Organic Amendments on Changes in Soil Organic Matter

Although a 30-day incubation does not capture long-term carbon stabilization processes, it is sufficient to assess early-stage interactions between freshly added organic amendments and the soil sorption complex. Short-term incubations are widely used to detect synergistic effects, shifts in labile and stable carbon pools, and initial reorganization of exchange sites, which subsequently control longer-term trajectories of soil chemical properties. Therefore, the present study intentionally focuses on the mechanistic early responses rather than on long-term carbon persistence. The contents of Corg and CNL in this study were significantly increased as a result of the application of biochar alone, as well as due to the synergistic effect of biochar combined with the N algal extract (Table 2; Figure 1). Biochar typically enhances soil organic carbon stocks, primarily because pyrogenic carbon is highly stable and has a low mineralization rate. Global meta-analyses consistently report typical increases in soil organic C of 27–29% following biochar application, with even greater effects when biochar is combined with other organic fertilizers [4,27]. Chemically, biochar is enriched in aromatic, highly condensed structures that are resistant to decomposition. Nearly 97% of C in biochar belongs to the recalcitrant pool, with average turnover times ranging from several hundred to several thousand years [2], which explains why both Corg and CNL sustainably increased following biochar application (Table 2). Additional biochar properties, such as increased porosity, enhanced cation exchange capacity, sorption, and improved soil aggregation, further reduce the availability of organic compounds for microbial decomposition [4,28,29]. The results of this study suggest that most of the increase in Corg and CNL originated directly from pyrogenic carbon and from its protective effects on native soil organic carbon (i.e., negative priming). Biochar reduces the mineralization of both native and newly added C, increases microbial carbon use efficiency, and enhances the accumulation of microbial necromass within biochar pores. At the same time, carbon stabilization in aggregates is improved via bonding of aromatic C to fine fractions/clays (increasing macro-aggregate formation), which shifts carbon into less accessible pools [3,30,31]. The increase in CL observed in the S+B and S+AGN+B treatments may be related to the fact that biochar provides microhabitats and reactive surfaces that support the formation and retention of more labile C fractions. In addition, carbon originating from biochar can dynamically interact with SOM (adsorption/desorption), thereby enhancing the turnover of labile carbon pools in soil [32]. The application of the N algal extract in the S+AGN treatment had relatively minor effects on Corg and CNL, although carbon lability increased during the first sampling. Because total and dissolved organic carbon contents of the algal extract were not determined, its contribution to soil carbon pools cannot be interpreted quantitatively. A limitation of the present study is the absence of quantified TOC, DOC, and TN values of the algal extract. This prevents a direct assessment of its contribution as a carbon source. Therefore, the observed effects of the algal extract are interpreted primarily as indirect, process-driven mechanisms (e.g., stimulation of microbial activity, transient shifts in labile carbon dynamics), rather than as a direct input of measurable organic carbon. Similar interpretations of algae-derived inputs as primarily biostimulatory and process-modifying agents rather than direct carbon sources have been reported in previous studies [15,16,17,33]. This distinction is essential, as it suggests that the role of the algal extract in the combined treatment is not to supply stable carbon, but to modify biogeochemical processes that enhance the functional efficiency of the biochar matrix. Consequently, the synergistic effects observed in S+AGN+B are understood as resulting from the interaction between a stable carbon carrier (biochar) and a biologically active modifier (algal extract), rather than from additive carbon inputs. This conceptual framework is consistent with studies demonstrating that combinations of biochar with other organic inputs enhance soil functional properties primarily through interaction effects rather than simple additive contributions [4]. The combination of biochar with the N algal extract may yield synergistic effects, as evidenced by this study’s results (Table 2). The short-term decrease in non-labile carbon (CNL) in the S+AGN+B treatment (−2.00 g kg−1) is likely associated with a transient priming effect induced by reactive organic inputs and mineral N linked to the algal extract. While biochar alone tends to suppress the mineralization of native soil organic carbon [2,4], its combination with biologically active inputs may stimulate microbial activity and accelerate the turnover of previously stabilized carbon pools. This results in a temporary redistribution of carbon from non-labile to more labile forms or its loss via mineralization, which is consistent with short-term interactions between biochar and reactive organic inputs [32]. The relatively wide particle size distribution of the applied biochar (1–5 mm) likely contributed to heterogeneous microscale conditions within the soil matrix. Larger particles provide stable intra-pore microhabitats with reduced microbial accessibility, whereas smaller particles increase surface contact with soil aggregates, enhancing short-term interactions with labile organic fractions. Such structural heterogeneity can partly explain the observed differences in labile and non-labile carbon dynamics during the 30-day incubation. However, it should be emphasized that longer incubation periods would likely lead to further stabilization of carbon fractions and more pronounced changes in the sorption complex. In particular, processes such as biochar aging, organo-mineral associations, and microbial reprocessing of organic inputs may significantly modify both C dynamics and CEC over time. Therefore, the trends observed in this study represent early-stage responses that cannot be directly extrapolated to long-term field conditions.

4.2. Effect of Organic Amendments on Changes in Soil Sorption Properties

Although increases in Ha were observed across all treatments in both sampling events, the increase was most pronounced in S+B. However, mean values did not indicate significant differences among treatments (Table 3), suggesting that the soil’s buffering capacity mitigated the effects of the applied organic inputs [34]. The highest SBC values were observed in both sampling events in the S+AGN+B, whereas the S+AGN treatment showed lower SBC values (Table 3). This indicates that the combination of the N algal extract and biochar enhances the accumulation and retention of basic cations more effectively than either amendment applied alone. Although individual exchangeable Ca2+, Mg2+, and K+ concentrations were not quantified separately in the present study, the significantly higher sum of basic cations (SBC) observed in the combined S+AGN+B treatment indicates a reorganization of the soil sorption complex rather than a simple additive input of cations. This interpretation is supported by the high Ca and K contents of the applied biochar, which may modify cation binding environments and the selectivity of exchange sites. Under short-term incubation conditions, such processes are expected to preferentially affect cation partitioning within the exchange complex rather than substantially alter total base saturation. However, because individual Ca:Mg:K ratios were not measured directly, these changes are interpreted as mechanistic indications rather than quantitative shifts. Future studies should explicitly quantify individual exchangeable base cations to verify changes in Ca:Mg:K proportions and to better resolve the mechanisms underlying sorption complex reorganization following combined biochar and algal extract application. The elevated SBC in this treatment likely results from the combined contribution of ash content, functional groups present in the biochar [4,35], and the addition of the N algal extract. These may increase the number of sorption sites, alter Ca:Mg:K ratios, and stabilize basic cations within the sorption complex. The biochar-specific surface area (21.7 m2 g−1) provides a substantial density of reactive surfaces that directly contribute to the development of charge-related sorption sites. During short-term incubation, the oxidation and proton dissociation of oxygen-containing functional groups on these surfaces increase negative charge density, which enhances CEC. The pronounced increase in CEC observed particularly in the S+AGN+B treatment indicates that surface-mediated processes, rather than bulk ash effects alone, were the dominant driver of nutrient retention. An additional factor contributing to the increase in CEC is the shift in soil pH induced by both amendments. The biochar (8.80) and algal extract (8.72), both with alkaline pH, increase soil pH compared to the initial condition (6.20), which promotes the deprotonation of functional groups on soil colloids and organic matter. This leads to an increase in negative surface charge and enhances cation exchange capacity, particularly in soils with variable charge components. Therefore, part of the observed CEC increase may result from the activation of pH-dependent charge sites in addition to the formation of new sorption surfaces [2], which reinforces the synergistic increase in CEC observed in the combined treatment. CEC was significantly higher in S+AGN+B compared with the other treatments (Table 3). The combined application of both organic inputs increased the number of negative charges available for exchangeable cations (functional groups formed during biochar aging, oxidation processes, and linkages with organic ligands), which is further enhanced by the presence of the N algal extract. In the literature, increases in CEC following biochar application are widely documented, though with an important caveat: the magnitude of the effect depends strongly on the type of biochar, its dose, and the soil’s intrinsic properties [4,36,37]. The present study clearly demonstrates a synergistic interaction between biochar and other organic inputs, particularly regarding the chemistry of the sorption complex. An interesting detail in this study was the short-term difference between the second and first sampling: in S+AGN+B, the difference in CEC values was minimal (0.17 mmol(+)kg−1). This suggests that this combined treatment stabilized CEC values over time. Base saturation values were also highest in S+AGN+B and differed significantly from the S+B treatment. In the literature, increases in Bs after biochar application are commonly attributed to its liming effect, ash contribution, and increased CEC, while combinations of biochar with other organic inputs (compost, digestate, residues, or, as in this study, N algal extract) tend to be more effective than biochar alone or biochar combined with mineral fertilizers. The increase in base saturation results from chemical reorganization processes within the sorption complex [4,38]. It is important to note that the present results were obtained under plant-free laboratory conditions. In soils with active vegetation, plant–soil–microbial interactions would likely modify the observed dynamics. Root exudation can stimulate microbial mineralization of organic carbon, while plant uptake influences nitrogen availability and nutrient cycling. Therefore, the presence of plants could enhance the turnover of labile carbon fractions and potentially alter the balance between mineralization and stabilization processes observed in this study.

4.3. Relationships Between Soil Organic Matter and Soil Sorption in Response to Organic Amendments

The PCA results clearly indicated that the greatest variability among treatments was determined by SOM indicators (Figure 1). Organic matter represents the primary factor influencing the physico-chemical properties of soils, including buffering capacity and cation exchange capacity. In particular, humic substances, key constituents of SOM, strongly affect sorption interactions and cation binding, thereby directly determining soil sorption potential [39,40]. It is therefore not surprising that these parameters contributed most strongly to PC1. The strong separation of the S+AGN+B treatment in the PCA indicates a mechanistic synergy rather than a simple additive response. Biochar primarily contributes stable carbon and persistent exchange surfaces [2,14,28], whereas the algal extract supplies reactive organic compounds and stimulates biological processes [15,16,17]. Their combination, therefore, enhances both carbon dynamics and the reorganization of the soil sorption complex, which explains the pronounced multivariate response observed in the PCA. The combined application of biochar and the N-rich algal extract (S+AGN+B) produced the most pronounced shift in the PCA space, demonstrating a synergistic effect on both organic matter and the soil’s sorption characteristics. The mechanisms underlying this effect are that biochar substantially increases soil sorption capacity [5] due to its high content of stable aromatic carbon, porous structure, and the presence of surface functional groups that enhance CEC and the binding of basic cations [6]. In addition, the N-rich algal extract provides reactive organic matter fractions that enrich the soil with labile carbon and expand the spectrum of organic functional groups capable of participating in sorption processes [41]. As a result of their combination, both stable (biochar-derived) and labile (algal extract-derived) organic fractions are reinforced simultaneously. This dual effect explains why the S+AGN+B treatment showed the strongest separation in the PCA. The S+B treatment also increased organic matter and sorption capacity, but to a lesser extent. This aligns with findings in the literature [6], which show that biochar’s effectiveness is often amplified through interactions with other organic sources that supply reactive organic groups and promote its integration into the soil sorption complex. Conversely, the S+AGN treatment resulted in the greatest increase in labile carbon fractions (Table 2), but only minimally affected sorption parameters (Table 3). This is because labile organic carbon lacks the ability to bind cations as stably as humified SOM fractions. This distinction between labile and stable carbon forms in the context of soil processes is also confirmed by recent syntheses on organic amendments [41]. The statistical significance of differences in PCA scores (both PC1 and PC2) therefore genuinely reflects that the organic amendments fundamentally modified only those properties that were the subject of evaluation, namely, SOM parameters and soil sorption capacity. These results are fully consistent with the mechanisms described in the literature regarding the role of SOM as the principal regulator of soil sorption and soil chemical behavior [6,39,40].

5. Conclusions

The results demonstrate that biochar is the dominant factor controlling the accumulation of stable soil organic carbon and the development of the soil sorption complex, whereas the algal extract primarily acts as a biostimulant, influencing labile carbon dynamics rather than contributing directly to stable carbon pools. The combined application of biochar and algal extract produced a clear synergistic effect, leading to the highest values of cation exchange capacity, sum of basic cations, and base saturation, while simultaneously enhancing both labile and non-labile carbon fractions. In contrast, the algal extract applied alone had only short-term effects on labile carbon and did not significantly modify sorption properties, confirming that it cannot substitute for structural carbon inputs.
Overall, the study highlights that improvements in soil organic matter and sorption capacity are maximized when stable carbon inputs are combined with reactive, biologically active compounds that enhance their functional integration. These findings represent short-term mechanistic responses, and further long-term or field-scale studies are required to confirm the persistence of the observed effects under dynamic soil–plant conditions.

Author Contributions

Conceptualization, V.Š. and J.H.; methodology, V.Š. and J.H.; software, V.Š. and J.H.; validation, V.Š. and J.H.; formal analysis, V.Š. and J.H.; investigation, V.Š. and J.H.; resources, V.Š.; data curation, V.Š.; writing—original draft preparation, V.Š.; writing—review and editing, V.Š. and J.H.; visualization, V.Š. and J.H.; supervision, V.Š. and J.H.; project administration, J.H.; funding acquisition, J.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the EU NextGenerationEU through the Recovery and Resilience Plan for Slovakia under the project No. 09I01-03-V04-00075/2025/VA.

Data Availability Statement

The data sets generated and analyzed during the current study are available from the authors upon a reasonable request.

Acknowledgments

The authors express their gratitude to the editor and the reviewers for their constructive comments.

Conflicts of Interest

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

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Land 15 00857 g001
Figure 1. Principal Component Analysis (PCA) biplot illustrating relationships between soil organic matter parameters (Corg, CL, CNL, L) and soil sorption properties (Ha, SBC, CEC, Bs) across experimental treatments. PC1 and PC2 explain 72.8% of the total variance.
Figure 1. Principal Component Analysis (PCA) biplot illustrating relationships between soil organic matter parameters (Corg, CL, CNL, L) and soil sorption properties (Ha, SBC, CEC, Bs) across experimental treatments. PC1 and PC2 explain 72.8% of the total variance.
Land 15 00857 g001
Table 1. Treatments of laboratory experiment.
Table 1. Treatments of laboratory experiment.
TreatmentsAbbreviation
Soil (control)S
Soil + biocharS+B
Soil + algae N extractS+AGN
Soil + algae N extract + biocharS+AGN+B
Table 2. Effect of organic soil amendments on changes in soil organic matter parameters (mean values, standard deviations).
Table 2. Effect of organic soil amendments on changes in soil organic matter parameters (mean values, standard deviations).
Parameter SoilSoil+BSoil+AGNSoil+AGN+B
Corg
(g kg−1)		Sampling 115.5 ± 0.20 a21.8 ± 0.40 b15.4 ± 0.46 a24.3 ± 1.15 c
Sampling 215.5 ± 0.44 a24.3 ± 0.52 c15.8 ± 0.14 a22.4 ± 0.33 b
Difference: 2-10.062.520.44−1.90
Mean15.5 ± 0.36 A23.1 ± 1.41 B15.6 ± 0.43 A23.3 ± 1.34 B
CL
(g kg−1)		Sampling 11.65 ± 0.07 a2.51 ± 0.06 b1.87 ± 0.06 b2.54 ± 0.12 c
Sampling 21.89 ± 0.09 a2.80 ± 0.12 b1.74 ± 0.09 a2.64 ± 0.11 b
Difference: 2-10.230.29−0.130.11
Mean1.77 ± 0.15 A2.65 ± 0.18 B1.80 ± 0.10 A2.59 ± 0.13 B
CNL
(g kg−1)		Sampling 113.8 ± 0.24 a19.3 ± 0.39 b13.5 ± 0.48 a21.7 ± 1.08 c
Sampling 213.7 ± 0.39 a21.5 ± 0.48 c14.1 ± 0.19 a19.7 ± 0.38 b
Difference: 2-1−0.182.220.56−2.00
Mean13.8 ± 0.36 A20.4 ± 1.26 B13.8 ± 0.48 A20.7 ± 1.35 B
LSampling 10.120 ± 0.01 ab0.130 ± 0.00 bc0.138 ± 0.01 c0.117 ± 0.00 a
Sampling 20.138 ± 0.01 b0.130 ± 0.00 ab0.124 ± 0.01 a0.130 ± 0.00 ab
Difference: 2-10.0180.000−0.0150.014
Mean0.129 ± 0.01 A0.130 ± 0.01 A0.131 ± 0.01 A0.124 ± 0.01 A
Note: Corg—soil organic carbon, CL—labile carbon, CNL—non-labile carbon, L—carbon lability. Different lowercase letters (a, b, c) indicate statistically significant differences among treatments within each sampling date, whereas uppercase letters (A, B) indicate differences among mean values across the entire experiment (Tukey test, p < 0.05).
Table 3. Effect of organic soil amendments on changes in soil sorption parameters (mean values, standard deviations).
Table 3. Effect of organic soil amendments on changes in soil sorption parameters (mean values, standard deviations).
Parameter SoilSoil+BSoil+AGNSoil+AGN+B
Ha
(mmol(+)kg−1)		Sampling 17.00 ± 0.00 ab8.19 ± 0.12 b5.78 ± 0.00 a6.58 ± 1.70 ab
Sampling 211.7 ± 0.73 a14.1 ± 0.80 b11.9 ± 1.09 a10.7 ± 0.42 a
Difference: 2-14.665.916.144.17
Mean9.33 ± 2.51 A11.2 ± 3.17 a8.85 ± 3.34 a8.67 ± 2.56 a
SBC
(mmol(+)kg−1)		Sampling 1171 ± 6.05 a164 ± 7.09 a161 ± 2.68 a191 ± 7.25 b
Sampling 2180 ± 4.75 bc171 ± 3.38 ab163 ± 2.71 a187 ± 7.55 c
Difference: 2-19.406.202.20−4.00
Mean176 ± 7.58 B168 ± 6.70 AB162 ± 3.08 A189 ± 8.08 C
CEC
(mmol(+)kg−1)		Sampling 1178 ± 6.05 a173 ± 6.70 ab167 ± 2.68 a197 ± 8.02 b
Sampling 2192 ± 4.87 bc185 ± 3.42 ab175 ± 2.68 a198 ± 7.31 c
Difference: 2-114.112.18.340.17
Mean185 ± 9.40 B179 ± 8.66 AB171 ± 5.21 A198 ± 8.08 C
Bs
(%)		Sampling 196.1 ± 0.14 ab95.2 ± 0.21 a96.6 ± 0.10 b96.7 ± 0.80 b
Sampling 293.9 ± 0.37 bc92.4 ± 0.43 a93.2 ± 0.52 ab94.6 ± 0.36 c
Difference: 2-1−2.14−2.88−3.40−2.13
Mean95.0 ± 1.17 AB93.8 ± 1.56 A94.9 ± 1.85 AB95.6 ± 1.30 B
Note: Ha—hydrolytic acidity, SBC—sum of basic cations, CEC—cation exchange capacity, Bs—base saturation. Different lowercase letters (a, b, c) indicate statistically significant differences among treatments within each sampling date, whereas uppercase letters (A, B, C) indicate differences among mean values across the entire experiment (Tukey test, p < 0.05).
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Šimanský, V.; Horák, J. Synergistic Effects of Biochar and Algae-Transformed Organic Waste from the Dairy Industry on Soil Organic Matter and Soil Sorption Properties. Land 2026, 15, 857. https://doi.org/10.3390/land15050857

AMA Style

Šimanský V, Horák J. Synergistic Effects of Biochar and Algae-Transformed Organic Waste from the Dairy Industry on Soil Organic Matter and Soil Sorption Properties. Land. 2026; 15(5):857. https://doi.org/10.3390/land15050857

Chicago/Turabian Style

Šimanský, Vladimír, and Ján Horák. 2026. "Synergistic Effects of Biochar and Algae-Transformed Organic Waste from the Dairy Industry on Soil Organic Matter and Soil Sorption Properties" Land 15, no. 5: 857. https://doi.org/10.3390/land15050857

APA Style

Šimanský, V., & Horák, J. (2026). Synergistic Effects of Biochar and Algae-Transformed Organic Waste from the Dairy Industry on Soil Organic Matter and Soil Sorption Properties. Land, 15(5), 857. https://doi.org/10.3390/land15050857

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