The Application of Sewage Sludge-Derived Compost or Biochar as a Nature-Based Solution (NBS) for Healthier Soil

Abstract

The present study highlights the possibility of using sewage sludge-derived compost (SSC) or biochar (SSB) as valuable organic amendments. Such utilization of sewage sludge fulfills the principles of a carbon farming and nature-based solution strategy (NBS). This study focused on a detailed analysis of quantitative and qualitative changes in soil C compounds (total carbon—TC, total organic carbon—TOC, humic substances—CHS, labile carbon—LC, and water extractable organic carbon—WEOC), which resulted from the application of SSC or SSB; an assessment of variability in total and available forms of N and S as biogenic components that are integrally related to the organic matter of the amendments used in the experiment; and an indication of the possible relationships between C compounds and available nutrients. The experiment was conducted under greenhouse conditions with terra rosa soil amended with SSC or SSB at different application rates (25, 50, 75, 100% by mass). Soil samples were analyzed for the abovementioned parameters using appropriate analytical methods. Regardless of the organic amendment, the values of tested parameters increased with the applied dose, with the differences being significantly greater in relation to the contents determined for the control soil. In general, the application of SSC was more favorable than SSB, which was manifested by 12–49-fold higher TOC, 6–24-fold higher total N, and 10–41-fold higher total S levels. An exception was found for the content of available sulfur, which was significantly higher in the soil fertilized with biochar. In addition, SSC contributed more humic acid carbon (12.5–24.15 g∙kg⁻¹) and labile carbon (10.34–27.37 g∙kg⁻¹). On the other hand, SSB had a greater effect on fulvic acid carbon levels (2.18–2.75 g∙kg⁻¹), which were comparable to the levels of LC (3.44–6.86 g∙kg⁻¹) and WEOC (2.56–6.28 g∙kg⁻¹). The research results highlighted the validity of processing SS into compost or biochar for further use for agricultural/reclamation purposes. Despite their different impacts on the studied soil properties, both organic amendments are important for maintaining soil health and can play a significant role in carbon farming as NBS practices. The findings allow us to conclude that the strategy of increasing the amount of C through SSC or SSB fertilization is the advisable direction in sustainable soil management.

1. Introduction

Currently, with the increase in ecological awareness, the assumptions of the circular economy, expressed by the cycle of reducing, reusing, and recycling, are increasingly being implemented. This is of great significance in light of the large amount of global consumption, which effectively causes increased waste generation. The list of waste categories is wide and covers all areas of human activity [1]. With respect to the mass of organic waste, the rate of the increase in the last decade, which is currently estimated to be approximately 50% of the global volume of solid waste, is rapid [2]. Sewage sludge (SS) plays a significant role among wastes, especially biodegradable wastes. As a result of rapid urbanization and industrialization, the developed wastewater infrastructure and an increasing number of connected generators, the mass of SS generated in individual countries has been growing. This situation requires an individualized approach to this group of waste using rational and environmentally friendly methods. SS has a high content of organic matter, nitrogen and phosphorous, as well as additional macro- and micronutrients, which underlines its fertilization quality [3,4,5,6]. Despite its many advantages, untreated SS may pose a threat to the environment due to the release of hazardous substances, both organic and inorganic or pathogenic organisms [2]. Therefore, the proper management of SS is essential and continues to be a challenge for many countries [3,7]. It should be strongly emphasized that the sustainable disposal of SS is consistent with current trends in circular economy practices [3,8,9]. The potential use of SS in the form of compost or biochar is important for maintaining carbon resources in soils, which is of agricultural and environmental importance [3]. In practice, the following approach to managing SS is used: no recycling (landfill, storage, dumping at sea), recycling (land use, composting, anaerobic digestion), and energy recovery (incineration) [9]. The choice of management method depends on the technological and economic development of the country, as well as the public awareness of local residents. These listed factors mainly determine the actions taken for proper SS management. In the literature, a number of studies have confirmed the validity of using compost based on SS for agricultural or reclamation purposes, which is indicated by positive changes in soil and plant chemistry [10,11,12,13]. When using compost, attention is given to the introduction of a significant amount of organic matter, which improves sorption and buffering properties of the soil and stabilizes its pH by replenishing alkaline cations (Ca and Mg). The role and importance of introducing organic matter with organic fertilizers are well proven in the context of maintaining soil health and fertility. Moreover, the favorable qualitative parameters of humus compounds in various composts need to be stressed here, especially in the case of those made on the basis of SS as pointed out by Jakubus [14]. Despite many positive aspects related to the use of SSC, it should be noted that they decompose relatively quickly in the soil, which may be considered as disadvantageous. Depending on soil and climatic conditions, the release of nutrients as a result of the mineralization of organic matter occurs within 2–3 years after compost application. Jakubus and Graczyk [15] showed that, during an incubation experiment lasting 180 days, sewage sludge-derived compost used as an organic amendment to sandy soil caused a statistically significant increase in the contents of total organic carbon, total and mineral nitrogen, and available phosphorous. Nevertheless, the cited study confirmed that compost may act as a slower release fertilizer rather than mineral fertilizer. Regardless of the relatively fast decomposition rate of SSC, Agegnehu et al. [16] reported that only a small proportion of the applied organic compounds would remain stable in the soil over a long period of time, with most released back to the atmosphere as CO₂. Therefore, the use of more stable C compounds, such as biochar, is recommended as an alternative to traditional organic fertilizers. Soils enriched with biochar have received substantial and increasing attention during the last decade. This is due to the unique properties of this pyrolysis product. Biochar is a carbon-rich material, in which C is arranged into aromatic structures and, occasionally, into piles of graphite-like layers [17]. The physicochemical properties of biochar materials vary depending on the biochar feedstock and pyrolysis conditions. Nevertheless, the use of biochar as a soil amendment may improve the physiochemical and chemical properties of soils. Due to its porous structure, it improves soil porosity and aggregate stability, increases water retention capacity and soil aeration, while simultaneously reducing its bulk density [18,19]. In their review, Agegnehu et al. [16] indicated that, regardless of the substances used for biochar production, a common denominator can be found regarding the positive changes manifested as increased pH and cation exchange capacity, at decreased Al amounts or reductions in nitrogen mineral form losses. Zhao et al. [20] reported that biochar application to soils could indirectly increase the bioavailability of nutrients, such as nitrogen, phosphorus, and potassium, and consequently improve crop production. On the other hand, Jiang et al. [21] underlined an important role of biochar in improving soil quality and carbon sequestration. However, biochar application to soil has different effects on soil biota in comparison to the addition of fresh organic matter, e.g., in the form of compost. This is related to the high stability of biochar carbon and the general lack of its biologically available forms, in contrast to heterogeneous fresh organic matter.

Regardless of the differences between composts and biochars, it should be strongly emphasized that both substances are valuable and can fulfill certain functions in C farming [17,22]. Carbon farming is nothing more than the management of C reservoirs at the farm level in order to mitigate climate change. The principle of this approach is to reduce emissions of carbon dioxide (CO₂), methane (CH₄), and nitrogen compounds into the atmosphere. First, carbon farming involves the management of C content in the soil through various practices, including conservation cultivation, diversified crop structure, intercropping, the proper use of organic fertilizers, and balanced mineral fertilization. Generally, the mechanisms involved in carbon farming promote the restoration of the lost functions of the ecosystem and strengthen their service delivery. These practices not only enhance CO₂ absorption, but also provide additional soil conservation services such as improved fertility, reduced erosion, and increased productivity; generally, they lead to enhanced soil health and improved water quality [22,23]. According to Shahane and Shivay [24], the concept of soil health is very extensive and has many definitions, which results from the many functions that soil performs in the environment, as well as its properties. Commonly, soil health refers to soil quality and is defined as a vital living ecosystem that sustains plants, animals, and humans. Nowadays, soil health is emphasized in the context of planned actions within the European Green Deal [25]. One of the most important strategies required to achieve climate neutrality by 2050 is to strengthen the key element of soil health, i.e., soil organic matter, which regulates many functions including carbon storage. According to Montanarella and Panagos [26], sustainable soil management practices support the retention and enhancement of C stocks in soil and thus climate change mitigation. Cited authors recognized an important role of soils in environmental protection (the Biodiversity strategy) and climate change (the Climate Law). This was also confirmed by the research by Debele et al. [27], which showed that as many as 64% of all activities implemented under this program address environmental co-benefits and activities, e.g., improving biodiversity and carbon storage. Among other factors, maintaining soil health contributes to the sustainable management of soil resources to properly conduct the biogeochemical cycling of nutrients, enhance the microbial population, and maintain the optimal level of soil organic carbon. This latter factor is one of the most important criteria for soil health evaluation, so maintaining the correct level is the most important and can be achieved using organic amendments. The introduction of carbon farming practices is part of the nature-based solution (NBS) strategy. As it results from the meta-analysis of 547 scientific papers conducted by Debele et al. [27], the NBS concept is mainly popular and implemented in Europe, while the other parts of the world are unfortunately poorly involved in the activities of NBS tasks and assumptions. According to the European Commission [28], NBSs are solutions that refer to natural patterns, they are cost-effective, and simultaneously provide environmental, social, and economic benefits focusing on developing resilience of the environment. Keesstra et al. [29] pointed to the strengthening of environmental system services in relation to soil, among others. Those authors stated that soil solutions aim to enhance soil health and soil functions, through which local eco-system services will be maintained. This is important in relation to the Mediterranean-type ecosystems, which are heavily affected by intensive soil erosion processes due to erodible parent materials, variable and often unfavorable weather conditions, and the intensive use of natural resources.

According to the abovementioned literature, organic amendments have beneficial effects on soil fertility and health, which is primarily attributed to changes in soil reaction, sorption properties and a positive balance of nutrients. Much less research has focused on the impact on the quality and quantity of soil humus compounds, which are the most important elements of organic matter and, in addition to TOC, are crucial indicators of soil health. Humic substances perform critical functions in soil formation and physical, chemical, biological and environmental aspects [30], therefore, studies in this area are needed. According to Piccolo [31], humic substances are heterogeneous macromolecules that can be divided into three fractions: humins, humic acids (HAs) and fulvic acids (FAs). Regarding solubility, HAs and FAs are extractable under alkaline conditions and separable from humins, which represent the nonsoluble fraction. Approximately 65–70% of humic substances are composed only of fulvic and humic acids. The validity of undertaking research to assess the quality and quantity of carbon compounds introduced into the soil with SS-derived compost or biochar is emphasized by the fact that SS itself is an attractive waste characterized by a significant number of biogenic components, such as C, N, and S. Jakubus et al. [32] showed that fulvic acids predominate in the composition of SS-derived humic substances. Moreover, the cited authors detected considerable amounts of water-extracted organic carbon in SS. In the opinion of Jakubus et al. [32], these carbon compounds are responsible for the creation of a significant nutrient pool that serves as a main energy source for microbiological activity and a primary source of mineralizable N and S.

Usually humus is considered to be more stable in soil than in freshly applied organic matter in the form of organic fertilizers. The introduction of biochar or compost into soil may disrupt natural conditions, influencing soil health. How much and to what extent such an interaction can occur remains a significant and unanswered question that we would like to address and verify in this study. Therefore, its primary objective is to evaluate the effectiveness of sewage sludge-derived compost (SSC) and biochar (SSB) as organic amendments for improving soil health within the framework of carbon farming and nature-based solutions (NBS). Specifically, the study aims to 1. quantify the effects of different application rates of SSC and SSB on soil carbon, including total carbon (TC), labile carbon (LC), water-extractable organic carbon (WEOC), and humic substances carbon (CHS); 2. assess the impact of SSC and SSB on the total and available forms of nitrogen (N) and sulfur (S) as biogenic components integrally related to the organic matter of the amendments used in the experiment; and 3. investigate the relationships between soil carbon compounds and the available forms of N and S, to understand how SSC and SSB influence soil health. Based on these objectives, we hypothesize that 1. increasing the application rates of SSC and SSB will lead to significant increases in analyzed soil parameter amounts (TC, LC, WEOC, CHS, total, and available S, total and available N) compared to unamended soil. 2. the type of organic amendment (SSC vs. SSB) will differentially influence the abovementioned soil indicators with SSC having a more pronounced effect due to its higher nutrient content, and 3. the soil application of SSC or SSB will be in line with the principles of sustainable soil management framework.

2. Materials and Methods

2.1. Research Design

A greenhouse pot trial was conducted under controlled environment conditions using terra rossa (Rhodic Luvisol) [33] as an experimental soil and Chinese cabbage (Brassica rapa L. subsp. Pekinensis (Lour.) Hanelt.) as a test plant. Although the plants were used in the experiment, this research does not concern this aspect, focusing only on the soil and changes taking place there. Terra rossa prevails as a soil type in the Istrian region and contains a large fraction of kaolin clay with a small amount of vermiculite. The average clay content is 46.9%, 60.2%, and 75.9% for A, E, and Bt, respectively [34]. The soil samples used for the experiment were taken from the rhizosphere layer. After drying and sieving (2 mm), the soil was mixed in appropriate mass proportions with two organic amendments: compost from sewage sludge (CSS) and biochar from sewage sludge (BSS). Both compost and biochar were prepared using the same dewatered aerobically stabilized sewage sludge (SS) obtained from a local wastewater treatment plant. The SS was subjected to pyrolysis and composting to generate biochar and compost, respectively, which were used as organic amendments in this study. A detailed description of compost and biochar preparation was given by Černe et al. [35], and the necessary information is provided there. The initial mixture composed of SS and wheat straw (at a dose 40 kg of straw per 1 m³ of SS) was composted in the static pile and was completed after three months of the process. For the SS biochar, the Kon–Tiki system was employed [36].

The proportions, in which the organic substances were mixed with the soil were the same and constituted 0%, 25%, 50%, 75%, and 100% of the total mass of the organic amendments. According to these assumptions, the following nine treatments were prepared: C—control, C25—mixed soil with 25% SSC; C50—mixed soil with 50% SSC; C75—mixed soil with 75% SSC; C100—100% SSC; B25—mixed soil with 25% SSB; B50—mixed soil with 50% SSB; B75—mixed soil with 75% SSB; and B100—100% SSB. The prepared mixtures were placed in 3 L pots (height 12 cm × 20 cm). The pots were randomly arranged in four replicates to reduce the effect of nuisance factors. Relative humidity (35 to 70%) and both daytime (15 °C to 25 °C) and night time (8 °C to 12 °C) temperatures were controlled using environmental monitoring sensors (ONSET, Bourne, MA, USA).

The duration of the greenhouse experiment corresponded to the length of the plants’ growing season and lasted 3 months. After harvesting, the soil samples were collected and they were oven-dried at 60 °C (UF160, Memmert GmbH Co., Schwabach, Germany) for three days until they reached a constant weight and then sieved through a 2 mm mesh (LINKER Industrie-Technik GmbH, Kassel, Germany).

2.2. Methods

A Vario Max CNS elemental analyzer was used to determine total carbon (TC), total nitrogen (TN), and total sulfur (TS) contents in the soil, biochar, and compost samples. On the basis of TC, TN, and TS, respective ratios were calculated. In the soil samples, as well as the CSS and BSS samples, the total organic carbon (TOC) content was determined via wet combustion according to the Walkley–Black procedure [37], while the labile carbon (LC) content was determined via KMnO₄ oxidation [38]. According to the protocol of Ghani et al. [39] the contents of cold (CWEOC) and hot (HWEOC) water-extractable organic carbons were assayed with the final determination of organic carbon by wet combustion [37]. The water extractable organic carbon (WEOC) was the sum of the values assessed for CWEOC and HWEOC. Using the method elaborated by Kononova and Bielczikova [40], the humus fractionation of the experimental samples was performed. According to this procedure, humic substances were extracted with a mixture of 0.1 mol∙L⁻¹ Na₄P₂O₇ + 0.1 mol∙L⁻¹ NaOH solution. After the precipitation of humic acids (HAs) at pH 1.5, fulvic acids (FAs) were separated. The assessment of fulvic acid carbon (CFA) and the humic substance carbon (CHS) was performed in a reaction with 0.1 mol∙L⁻¹ KMnO₄ in H₂SO₄ medium. Based on the obtained results, the difference in quantity of the humic acid carbon (CHA) content was calculated. For the extracted fractions of humic substances, the optical density (Q_4/6) was determined at 465 and 665 nm. Additionally, for determined carbons of fulvic and humic acids, the polymerization degree (PD) values were calculated and the following equation was applied [41]: PD = ${\displaystyle \frac{CHA}{CFA}}$.

Readily available plant S (AS) in the samples was extracted with 2% CH₃COOH for 1 h [42]. The turbidimetry method with the precipitation of barium chloride was used for sulfur determination. The nutrient concentrations in the extracts were measured spectrophotometrically at 495 nm. Available nitrogen (AN) in the samples was determined, applying the Keeney and Nelson procedure [43] with steam distillation in a 2 mol∙L⁻¹ KCl solution. Then, the extracts were tritressed to the end point with 0.1 mol∙L⁻¹ HCl. This method provides the simultaneous determination of mineral nitrogen (available) as the sum of N-NH₄ and N-NO₃.

To provide details on the design of the experiment, it is presented in the form of a scheme in Figure 1.

2.3. Statistical Analysis

The one-way ANOVA was applied for the statistical elaboration of the obtained results, where each of the analyzed parameters was tested independently using the F test at the significance level α = 0.05. The null hypothesis stated that the mean values of the parameters are equal for each of the nine treatments, and it was tested against the alternative hypothesis stating that not all means are equal. In the case of the rejection of the null hypothesis, the least significant differences were calculated using the Tukey method at the significance level α = 0.05. Tukey’s analysis was performed to distinguish homogeneous groups among the nine treatments. Homogeneous groups are indicated by the same lowercase letters and show that the mean contents of the tested parameters did not differ significantly. In addition, for the analyzed parameters, Pearson’s correlation coefficients were calculated, and on the basis of these values, the determination coefficient was elaborated as follows: R² (%) = R²·100%. Considering mutual correlations between the studied variables, simple regressions models were developed according to equation: y = β₀ + β₁x, and it is interpreted as follows: if parameter x increases by one unit, parameter y increases (decreases) by β₁ units. The data were analyzed using software working in the Windows environment (STATOBL). All the data presented in this paper are averages of 4 replicates.

3. Results

3.1. Effects of SSC or SSB on Total C, N, and S Contents as Well as Available N and S Amounts

As shown by the data in

Table 1. Contents of essential nutrients in the experimental samples (g∙kg⁻¹ of d.m.).

Treatments	TN *	TS	TC	C:N	C:S	C:N:S
C25 *	7.13 d	0.85 e	63.61 d	8.9	74.8	10.5
C50	13.66 e	1.57 bc	142.56 c	10.4	90.8	6.6
C75	20.63 b	2.6 b	196.5 b	9.5	75.6	3.7
C100	27.95 a	3.28 a	262 a	9.4	79.9	2.9
B25	2.15 e	0.53 de	23.81 e	11.1	44.9	20.9
B50	4.16 de	0.96 de	40.92 de	9.8	42.6	10.2
B75	4.81 de	1.7 de	52.49 d	10.9	30.9	6.4
B100	5.25 de	1.95 bc	60.72 d	11.6	44.9	8.6
Control	1.21 f	0.08 f	5.34 f	4.4	66.8	55.2

* for an explanation of the abbreviations, see Section 2; different letters in the same column indicate that the means (n = 4) were significantly different.

and

Table 2. Available amounts of nitrogen and sulfur in the experimental samples (mg∙kg⁻¹ d.m.).

Treatments	AN *	AS
C25 *	89.24 d	25.35 e
C50	174.99 c	46.18 e
C75	319.68 b	56.19 e
C100	398.4 a	67.73 e
B25	40.83 ef	407.88 d
B50	58.33 e	711.22 c
B75	60.66 e	1056.93 b
B100	70.83 e	1389.76 a
Control	31.5 f	8.22 e

* for an explanation of abbreviations, see Section 2; different letters in the same column—indicate that the means (n = 4) were significantly different.

, the application of SSC or SSB had a statistically significant impact on the values of the soil parameters tested. The total amounts of N, C, and S in the SSC were 262, 27.95, and 3.28 g∙kg⁻¹, respectively, and in relation to the levels of the elements in SSB (TC—60.72, TN—5.25, TS—1.95 g∙kg⁻¹) and in the control soil (TC—5.34 g∙kg⁻¹, TN—1.21 g∙kg⁻¹, TS—0.08 g∙kg⁻¹), they were significantly higher (Table 1). In general, with increasing application doses of SSC or SSB, a higher content of a given nutrient was obtained, and the effect was more marked in the case of SSC. Increasing doses of SSB did not significantly affect the amounts of total N, C, or S, although compared to the amount of nutrients in the control soil, they were much greater. The values of the C:N, C:S, and C:N:S ratios determined for soil samples fertilized with SSC or SSB were clearly different from those found under control conditions. C:N values were higher and, regardless of the organic additive used, ranged from 9.4 (C100) to 11.6 (B100) compared to 4.4 (control) (Table 1). The C:S and C:N:S values were clearly greater, and in the control soil, they were 66.8 and 55.2, respectively (Table 1). Compared to the C:S value in the control soil, the use of SSC resulted in an increase in the value of the discussed parameter (74.8–90.8) and a decrease in the value of SSB (30.9–44.9). An opposite situation was noted in relation to the C:N:S value, where increasing doses of SSC and SSB contributed to a significant reduction in the value of this parameter (2.9–10.5) for the soil fertilized with both amendments (6.4–20.9) (Table 1).

Unlike the total contents of N and S, their available amounts provide direct information on their availability for plants, which is more valuable for practice. Therefore, the influence of SSC and SSB fertilization on the quantitative variability of N and S mineral forms in the experimental soil was analyzed. The data presented in Table 2 confirm the directions of quantitative changes shown for total amounts, because with the increase in the dose of the organic amendments used, the amounts of nutrients increased, although not in every case, the differences were statistically significant. Compared to the amount of AN in the control soil (31.5 mg∙kg⁻¹), sewage sludge-derived compost fertilization resulted in a 3–13-fold greater AN level in the soil. In the case of sewage sludge-derived biochar application, the increase in the described form of nitrogen was only 1.5–2.0 times greater. Such trends are not reflected in the percentage of AN in the total amount of nitrogen, as presented in Figure 2, because increasing doses of SSB contributed to a decrease in the percentage of nitrogen in the mineral form. Moreover, the control soil was characterized by the highest percentage (2.6%) compared to the treatments fertilized with either SSC (from 1.2 to 1.5%) or SSB (from 1.3 to 1.4%).

In turn, increasing doses of SSB significantly and clearly increased the amount of AS (from 407.88 to 1389.76 mg∙kg⁻¹) compared to the level of the nutrient in the control soil (8.22 mg·kg⁻¹). The SSC treatments resulted in 3–8-fold greater AS levels in the soil than the control treatment did; however, this difference was not statistically significant (Table 2). This effect was consistent with the percentage of AS in its total amount, which ranged from 2.1 to 3.0. A greater percentage of AS in the total sulfur amount was detected for the soil fertilized with SSB, and in the present study, as in the case of AN, with an increase in the SSB dose, the percentage decreased from 77% (B25) to 71% (B100) compared to 10% for AS in the control soil (Figure 2).

3.2. Effects of SSC or SSB on Various C Connections

Additionally, the quantitative changes in soil C in different combinations were governed by the type of organic amendments used and their dose (

Table 3. Carbon amounts in various soil connections (g∙kg⁻¹ d.m.), optical density (Q_4/6), and polymerization degree (PD) of humic substances in the experimental samples.

Treatments	TOC *	CHS	WEOC	LC	Q4/6	PD
C25 *	49.91 d	19.47 c	10.82 c	10.34 c	5.3	1.8
C50	102.35 c	33.56 b	46.05 b	15.45 b	5.6	1.4
C75	145.76 b	42.61 a	88.92 a	23.77 a	5.4	1.2
C100	206.58 a	45.25 a	109.46 a	27.37 a	5.3	1.1
B25	12.40 e	6.4 e	2.56 e	3.44 de	3.8	0.5
B50	21.48 e	10.4 de	4.20 e	4.93 de	3.8	0.3
B75	28.63 e	12.6 d	5.22 e	5.53 de	3.9	0.2
B100	30.92 e	14.35 d	6.28 e	6.86 c	3.8	0.2
Control	4.37 f	2.94 f	1.05 f	2 e	3.6	0.6

* for an explanation of abbreviations, see Section 2; different letters in the same column indicate that the means (n = 4) were significantly different.

, Figure 3). Generally, with increasing SSC or SSB doses, the values of the tested parameters increased adequately, differing significantly from the levels determined under the control conditions. The impact of SSC on the variability of the C amounts compared to SSB was greater, resulting in significantly higher amounts of TOC, CHS, CFA, CHA, WEOC, and LC. In relation to those in the control soil, the values of the above-mentioned parameters in the soil with the addition of SSC were significantly higher, with the largest difference in the case of WEOC (10.82–109.46 g∙kg⁻¹ compared to 1.05 g∙kg⁻¹ for the control soil), and the smallest for LC (10.34–27.37 g∙kg⁻¹ compared to 2.0 g∙kg⁻¹ for the control soil). The effect of SSB on the tested parameters was also significant, although to a lesser extent (Table 3, Figure 3). In relation to the TOC, CHS, CFA, CHA, WEOC, and LC contents found in the control conditions, the soil enriched with SSB was characterized by 1.5 to 7 times higher contents, with the smallest difference concerning LC (3.44–6.86 g∙kg⁻¹ compared to 2.0 g∙kg⁻¹ for the control), and the largest for TOC (12.40 to 30.92 g∙kg⁻¹ compared to 4.37 g∙kg⁻¹ for the control) (Table 3). As indicated by the data in Figure 3, the amounts of CFA and CHA increased with increasing doses of both SSC and SSB. However, the values were significantly greater for the compost treatment, ranging from 6.97 to 21.1 g∙kg⁻¹ for CFA and from 19.5 to 45.19 g∙kg⁻¹ for CHA, which was 7 to 16 times higher than the levels found in the control soil. Biochar addition also increased the amounts of CFA and CHA in comparison to the parameter values found in the control soil, although not as much as in the case of compost. The amounts of CHA ranged from 2.18 to 2.78 g∙kg⁻¹ and CFA from 4.22 to 11.62 g∙kg⁻¹ in the SSB treatments (Figure 3).

The value of the Q_4/6 parameter determined for the soil fertilized with SSB was the same (3.8) regardless of the dose of the amendments used, and it was comparable to the Q_4/6 value found in the control soil (3.6). Under the conditions where the soil was enriched with SSC, the Q_4/6 parameter had higher values: 5.3–5.6 (Table 3). The degree of polymerization decreased as the dose of SSC or SSB increased. The lowest PD values were detected for SSB treatments (0.2 for B75 and B100). The highest values of PD were found in the case of SSC application—1.8 (C25). Compared to those of the control soil, the PD values calculated for soil fertilized with SSC were higher, while those for soil amended with SSB were lower (Table 3).

The data in Table 3 show that the amounts of LC and WEOC were comparable regardless of the SSB dosage. Irrespective of the SSC dose used, very similar levels of LC and CHA were detected. Despite such quantitative similarities, the relationships between the parameters were not statistically confirmed (

Table 4. Correlation coefficient values.

	WEOC *	CHS	LC	AN	AS
	Treatments with SSC doses
TOC	0.997 **	0.904 **	0.905 **	0.846 **	0.816 **
WEOC	-	0.927 **	0.915 **	0.876 **	0.847 **
CHS	n.s.	-	0.904 **	0.952 **	0.930 **
LC	n.s.	n.s.	-	0.886 **	0.899 **
	Treatments with SSB doses
TOC	0.984 **	0.838 **	n.s.	n.s.	0.856 **
WEOC	-	0.834 **	n.s.	n.s.	0.913 **
CHS	n.s.	-	n.s.	n.s.	0.800 **
LC	n.s.	n.s.	-	n.s.	0.709 **

* for an explanation of abbreviations, see Section 2, n.s.—not significant, ** indicates a significant α = 0.01.

).

As indicated by the data in Figure 4, the percentage of CHA in CHS was dominant (53.3–64.3%) in the soil fertilized with SSC, while SSB contributed to a higher percentage of CFA in CHS (65.9–81.0%). It should be noted that, in both cases, with increasing doses of organic additives, there was an increase in the percentage of CFA and a decrease in the percentage of CHA.

The percentage shares of CHS, WEOC, and LC in the TOC content are also interesting (Figure 5) because they clearly show the influence of the additives used. In contrast to the trend shown earlier (see Table 3), increasing doses of SSC and SSB resulted in a successive decrease in the percentage of CHS and LC in the TOC. The lowest percentages of LC were determined to range from 13.2 (C100) to 20.7% (C25) or from 19.3 (B75) to 27.7% (B25). The amendments used significantly reduced the share of LC in the TOC because, under control conditions, this form of C represented 45.8% of the TOC (Figure 5). The influence of SSB fertilization caused a higher percentage of CHS (43.9–51.6%) than SSC application (21.9–39.1%), which was still lower than that of the control (67.3%). The percentage of WEOC regardless of the SSB dose was at a comparable level (18.2–20.6%). However, higher SSC doses resulted in a higher percentage of WEOC (61% in the case of C75 and 53% in the case of C100). Generally, the percentage of WEOC (21.7–61.0%) was greatest in the soil fertilized with SSC. This was the opposite phenomenon to that of the control soil (24%) and the soil enriched with SSB (18.2–20.6%), where the lowest percentage shares of such soil C connections were recorded (Figure 5).

The fertilization effects of SSC and SSB were also assessed through the mutual dependencies between the analyzed parameters, and the results are presented as simple correlation coefficients (Table 4) and linear regression estimators (

Table 5. Estimators of linear regression parameters for pairs of variables.

Treatments	Y	X	β 0	β 1
Treatments with SSC doses	WEOC *	TOC	−5.668	0.559
	CHS	TOC	17.348	0.139
	LC	TOC	20.083	0.037
	AN	TOC	0.104	0.037
	AS	TOC	0.053	0.0005
	AS	WEOC	0.053	0.0001
	AN	WEOC	0.115	0.0028
	LC	WEOC	20.43	0.066
	CHS	WEOC	18.49	0.253
	LC	CSH	−50.199	3.425
	AN	CSH	10.291	81.164
	AS	CSH	−57.737	1468.3
	AS	LC	1.07	374.15
	AN	LC	18.589	19.935
Treatments with SSB doses	WEOC	TOC	−1.688	0.365
	CHS	TOC	14.894	0.418
	AS	TOC	−0.266	0.078
	AS	WEOC	0.051	0.224
	CHS	WEOC	12.85	1.118
	AS	CHS	12.48	4.375
	AS	LC	6.607	0.654

* for an explanation of abbreviations, see Section 2.

). The data showed more relationships in soils enriched with SSC than in those enriched with SSB. When analyzing the impact of SSC on the soil, the significant impact of the TOC content needs to be stressed, which determined the amount of WEOC in 99%, the amount of CHS and LC in 82%, the amount of AN in 89%, and AS in 67% (Table 4). The impact of TOC on the amounts of WEOC or CHS in SSC treatments was highlighted by linear regression parameters (Table 5). Assuming that there is an increase in TOC of 1 g, it can be expected that the amount of WEOC or CHS will increase by 0.559 g and 0.139 g, respectively. The impact of SSC on the soil was also expressed by the interaction between WEOC and CHS (R²∙100% = 86%), LC (R²∙100% = 84%), AN (R²∙100% = 77%), and AS (R²∙100% = 72%). A similarly strong effect was found for the amounts of CHS, which determined the amounts of LC, AN, and AS by 82, 91, and 86%, respectively (Table 4). As indicated by the data in Table 5, when the amount of CHS increased by 1 g, the amount of LC increased by 3.425 g, the amount of AN increased by 81.164 mg, and the amount of AS increased by 1468.3 mg. According to data in Table 4, the available amounts of N and S were 78% and 81%, respectively, as determined by LC contents. This was expressed by linear regression equations indicating an increase in AS by 374.15 mg and AN by 19.935 mg, assuming that the LC content increased by 1 g (Table 5). For soil fertilized with SSB, only some relationships were statistically confirmed (Table 4). The significant impact of the tested forms of soil C on AS needs to be stressed here. The amounts of TOC, WEOC, CHS, and LC determined the amounts of available sulfur by 73, 83, 64, and 50%, respectively. Moreover, the TOC contents correlated with WEOC (r = 0.984 **) and CHS (r = 0.838 **) contents (Table 4). Assuming that TOC, WEOC, CHS, and LC would increase by 1 g, it can be expected that the AS content in soil fertilized with SSB would increase by 0.078, 0.224, 4.375, and 0.654 mg, respectively (Table 5). Notably, the relationship between WEOC and CHS (r = 0.834 **), which was also described in the regression, indicated an increase in CHS in the soil of 1.118 g, assuming that the WEOC would increase by 1 g (Table 5).

4. Discussion

4.1. The Importance of SSC and SSB in Sustainable Soil Management

As a major C sink, soils play an important role in mitigating greenhouse gas emissions, which is a direct reference to the ambitious European target of a climate neutral EU by 2050 [26]. Simultaneously, soil is a fragile resource that needs to be carefully managed and safeguarded for future generations; consequently, one of the main goals of the “A Soil Deal for Europe” mission is to conserve soil organic C stocks [25]. Therefore, soils need to be managed sustainably, with particular attention being paid to the maintenance and enhancement of C resources. The most common and cost-effective method of maintaining the proper C balance in the soil is the use of organic fertilizers (manure, slurry), but in many regions, these fertilizers are unavailable. An alternative to these methods is the compost or biochar produced, for example, from waste such as SS, which is characterized by significant amounts of C and N compounds [3,5,32,44]. Singh et al. [22] emphasized the significant role of biochar, which not only improves soil health, but also enhances the carbon sink of soil by adding nutrients in the form resistant to degradation. Such activities also refer to good carbon farming practices. Moreover, carbon farming facilitates sequestering carbon in long-term storage forms promoting soil health improvement and agricultural output in the framework of nature-based solutions. This research focused on strengthening environmental services through the organic amendment application to improve/repair/protect soil. According to Keesstra et al. [29], soil protection is the third ecosystem service and most NBSs are soil-based solutions.

Regardless of the fact that the purpose and practical use of compost and biochar is the same, it should be emphasized that both substances differ chemically. The chemical composition of composts and biochar depend on the substrates, from which they are made. Even if we assume that we have the same substrate in the form of SS, they also differ significantly depending on the origin and method of wastewater treatment. This finding may be confirmed by the study of Jakubus [14], who analyzed SS-derived composts. In relation to the presented data cited, the author determined comparable amounts of TN, smaller amounts of TS, and two times greater contents of TOC and CHS for SS-derived composts. Additionally, the DP and Q_4/6 values were significantly greater than those of the compost used in these studies. A similar situation applies to biochars, because they can be produced from various raw materials under different pyrolysis conditions, causing differentiated physicochemical properties to affect the stability of biochar influenced on its durability in soil [2,22,45,46]. Zoghlami et al. [46] reported that biochar from SS produced at a low temperature (260 °C) had greater TN and OM contents than biochar produced at 420 °C or 610 °C. Ayaz et al. [47] for biochar with SS reported the same level of TC and a smaller amount of TN in relation to the data presented in this study. Regardless of the fact that the chemical composition of both SSC and SSB will differ from the literature data, the effect of both organic amendments on soil conditioning is much more important. This is especially significant when we focus on the effects of fertilization on the changes in soil properties. Research has particularly emphasized the positive impact of compost and biochar on the levels of total organic carbon (TOC) and nitrogen (TN) and the amounts of available macronutrients [2,13,16,48]. These studies also confirmed the above and indicated benefits resulting from the use of SSC or SSB, which is expressed by the increasing values of the parameters tested.

4.2. Quantitative Changes of Various C Connections

Considering that TOC is one of the soil health indicators, it is important to assess its amount in the soil. According to the values reported by Shahane and Shivay [24], the control soil was characterized by low TOC (<5 g·kg⁻¹), and under the influence of SSC or SSB doses, it contained high amounts of TOC (>7.5 g·kg⁻¹). Much less research has focused on assessing the qualitative parameters of humus compounds in compost or biochar as well as soil after compost or biochar application. From a practical point of view, this is very important, since various C compounds that are components of organic matter constitute a common key element of soil health, because they affect the chemical, biological, and physical properties of soil and thus determine fertility and functionality of the soil ecosystem. Humic substances are the most recalcitrant and highly polymerized complex mixtures of molecules of various sizes and shapes. The fractionation of humic substances is based on their solubility in acids or alkali, while their chemical characteristics do not differ substantially [31]. CHAs, which improve soil buffering and sorption properties and available water capacity, play a special role. In our study, the applied doses of both SSC and SSB contributed to a significant increase in the amount of CHAs compared to the CHA contents in the control soil. However, higher doses of SSC or SSB resulted in an increase in the percentage of CFAs with a simultaneous reduction in the percentage share of CHAs, and this phenomenon was more evident in the case of SSB. This effect of SSB may be the result of pyrolysis conditions, because according to Rodrigues et al. [45], high-temperature biochars (>400 °C) are characterized by relatively low molar ratios of H to organic C and exhibit higher stability than materials processed at low temperatures, resulting in higher molar ratios.

An increase in the amount of CHAs in soil occurring with increasing doses of biochar was also demonstrated in studies by Mierzwa-Hersztek et al. [49], Šimansky et al. [50], Šrank and Šimansky [51] and You et al. [52]. However, most of the cited authors did not confirm these changes statistically, indicating only a tendency toward gains. Interestingly, increasing doses of biochar reduced the percentages of CHAs and CFAs [49,51], which is partially consistent with the data obtained in this experiment. Additionally, no effect of the applied SSB dose on Q_4/6 values with a simultaneous decrease in DP was found in this study. This was confirmed by the findings reported by Mierzwa-Hersztek et al. [49] or Šrank and Šimansky [51]. The increase in TC and TOC in the soil that occurred under the influence of biochar dose was also noted by Šrank and Šimansky [51] and You et al. [52], although those authors did not confirm this statistically. Jing et al. [53] showed that, despite the addition of biochar, the TOC content increased, and during the vegetation period, a gradual reduction in the TOC content was observed. Jiang et al. [21] also reported a significant, positive correlation between the rate of biochar application and the highest content of TOC in soil. Simultaneously, the cited authors found a reduction in water-soluble carbon compounds with increasing biochar doses. In their opinion, this indicates that biochar application can improve soil quality, as manifested in the greater C pool. In turn, Cybulak et al. [54] determined that biochar had a moderate effect on soil characteristics, leading to gains in the amount of organic matter, including increases in the contents of CHSs and TOC. Those authors stated that the addition of biochar into the soil caused the growth of structures characterized by low molecular weight and a low degree of HS humification. This observation is consistent with the results from our study, where more CFA was found under the conditions, in which SSB was applied. This was also supported by the low values of the degree of polymerization (DP), an indicator showing the relative speed of HA and FA transformation. For SSB and soil enriched with it, the DP value did not exceed 1.0, indicating a low degree of polymerization and humification of humic compounds. Another interpretation is provided by the analysis of optical density. The Q_4/6 ratio is negatively related to the aromatic polycondensation degree and the molecular weight of humic substances. High Q_4/6 values (above 5.0) indicate the presence of low-molecular-weight aromatic molecules, such as fulvic–like compounds, a more labile fraction, characteristic of young, relatively unstable organic materials. On the other hand, low Q_4/6 values (below 5.0) indicate high contents of large-molecular-weight molecules, such as humic–like compounds, representing a more stable fraction, usually present in well-matured organic materials [14]. In SSB-amended soils, the Q_4/6 values were lower (3.8–3.9) than those in the soils treated with SSC (5.3–5.6), but still comparable to the values determined for the unamended control soil (3.6). Therefore, it is difficult to clearly determine the quality and polymerization of the carbon compounds under the given experimental conditions. Regardless of such an observation, one should be aware of the slow rate of transformation of humic compounds supplied from SSB, which may fundamentally change their nature and which will be noticeable in longer term research.

Apart from general information related to humic compounds, limited data are available for more in-depth studies on the WEOC or LC. Jakubus and Michalak-Oparowska [55] emphasized that these forms of C represent a load of easily mineralized C compounds, which play a key role in the organic matter transformation of organic amendments in soil. Because they are considered to be components of the labile and the most active fraction of organic substances, the sensitive measures of subtle changes in organic matter are possible. This statement is confirmed by the literature on the subject and the present research. According to Corvasce et al. [56], water-extractable organic matter (WEOM) is identified with the mobile and easily soluble fraction of organic matter. In turn, Lv et al. [57] claimed that water-extractable organic matter is the most active fraction of organic waste and is subject to change. In the opinion of the cited authors, the fraction of these compounds may directly reflect the organic matter transformation process and thus directly determine, among other factors, the changes in the WEOC content.

Increasing doses of organic amendments resulted in an increment of WEOC or LC amounts, although this was not statistically confirmed in each treatment (especially in the case of SSB treatments). In general, the amounts of WEOC (2.56–6.28 g∙kg⁻¹) and LC (3.44–6.86 g∙kg⁻¹) in the soil fertilized with SSB were comparable, which may indicate a relatively small amount of easily mineralizable C. Additionally, Šrank and Šimansky [51] reported that only the LC content tended to increase, although it was not statistically significant. On the other hand, Cross and Sohi [58] observed the rapid utilization of labile carbon from biochar as a result of its soil application, which, according to the cited authors, had a stabilizing effect on the native amounts of LC in the soil. Significantly greater amounts of LC and WEOC were detected in the soil amended with SSC; however, the values were not comparable except for the dose of treatment C25. The SSC and the soil enriched with it were characterized by a significantly greater level of WEOC (10.82–108. 46 g∙kg⁻¹) and LC (10.34–27.37 g∙kg⁻¹), which indicates a considerable potential of easily mineralized C, N, and S resources introduced with SSC. The importance of SSC in shaping the amounts and interdependencies between C compounds is confirmed by statistical data. Referring to the determination coefficients, it was found that, under the conditions of the SSC used, the amount of TOC strongly determined the amount of WEOC (99%), CHS, and LC (82%). In the case of SSB, TOC amounts determined WEOC and CHS contents in 97% and 70%, respectively.

4.3. Relationships Between Applied Organic Amendments and Available Amounts of N and S

Nitrogen and sulfur are essential macronutrients required in large amounts by plants. It is associated with their role in the co-creation of important organic compounds such as the carbohydrates, proteins, and lipids of plant cells. Nitrogen as a constituent of proteins, nucleic acids, chlorophyll, coenzymes, phytohormones, and secondary metabolites plays a central role in plant metabolism. Nitrogen is taken up in a nitrate or ammonium form and is assimilated into amino acids in plant roots or shoots. Sulfur is taken up as a sulfate and assimilated into S-containing amino acids such as cysteine. This compound is a precursor of S-containing enzymes and coenzymes, as well as secondary compounds such as phytochelatins (detoxification of metals), or allicins and glucosinolates (feeding deterrents) [59]. Due to the mineral forms, in which both nutrients are taken, it is important to assess not only their total contents, but also their available amounts. Additionally, the available amounts of nutrients are treated as key soil health parameters, which emphasize their importance not only in plant nutrition, but also in monitoring and assessing soil fertility [60].

Our study revealed that the application of organic amendments in different ways influenced available amounts of N and S in the soil. The amount of AN was greater in the soil fertilized with SSC and the content of AS was higher in the soil enriched with SSB. According to the values given by Shahane and Shivary [24], available nitrogen is considered a soil health indicator, and values below 280 kg∙ha⁻¹ are low and insufficient. The control soil in this experiment was characterized by a low AN resource. The increase in the amount of AN to a satisfactory level, which indicates the health of the soil, was recorded only under the conditions of the applied SSC and at a dose higher than 25% by mass, which emphasizes the fertilizing nature of SSC in relation to the nutrient in a form directly available to plants. Abd Elsalam et al. [11] also noted a positive effect of the SS-derived compost on the mineral and total N levels in the soil, although this effect was more pronounced for N-NO₃. In turn, Ukalska-Jaruga et al. [61] reported a stronger fertilizing effect of exogenous organic matter in the case of N-NH₄, while the SS-based compost had the weakest effect. The differential release of mineral N from SS-derived compost should be attributed to the process of transforming N into a more stabilized residual form, which is more difficult to decompose [62].

Generally, the impact of biochar on mineral forms of nitrogen is not clear, as evidenced by literature data. The authors [63,64] pointed to the reduction of N-NO₃ and N-NH₄ and, consequently, the a poor fertilizing effect of biochar. In turn, Jing et al. [53] reported a decrease in N-NH₄ with increasing N-NO₃ as an effect of biochar fertilization. In this study, an increase in the amount of AN in the soil under the influence of biochar doses was found, but this has not been statistically confirmed. This may indicate a slow release of available nitrogen forms from SSB and an expected long-term fertilizing effect. A probable explanation for the poor fertilization effect of SSB in the experimental conditions should be sought in nitrogen transformation that occurs during the SS pyrolysis process. According to Zoghlami et al. [46], as the pyrolysis temperature increases (above 30 °C), nitrogen is transformed into a heterocyclic aromatic form with more stable structures. Of course, in this regard, the possible volatilization of NH₃, N₂O emissions, and nitrogen leaching are reduced, but at the same time, we must bear in mind that the amount of available forms of the nutrient for plants also decreases. This possibility was also indicated by Ayaz et al. [47].

Although the SSC was richer in total forms of S, it provided smaller amounts of available forms of the nutrient compared to SSB. This is due to the changes that S undergoes during the composting process. Jakubus and Graczyk [62] showed that, during the composting process, the amount of available S changes significantly, resulting in its smaller amounts. This could be attributed to the humification process, which plays an important function in reducing the available amounts of nutrient and simultaneously increasing its stable connections. According to Bardhod et al. [65], at the end of the composting process, during maturation phases, simple S compounds (inorganic and organic) are incorporated into solid complexes. Moreover, Gu et al. [66] indicated that during the thermophilic phase, high microbial activity and temperature lead to rapid mineralization of simple organic compounds into simple inorganic sulfur compounds such as H₂S, CH₃SH, and SO₂, which may also be subject to losses due to emissions, and further reduce the amount of available S. Therefore, it is necessary to emphasize the importance of biochar in the potential supplementation of the available form of S, a nutrient that is often omitted in standard fertilization, even though it plays a fundamental role in plant nutrition. The results indicated that SSB significantly increased the amount of AS in the soil even though the total amount of S in the biochar was lower than that in the compost. Zhao et al. [20] indicated that biochar may be a potential source of sulfur. However, the S availability from biochar also depends on pyrolysis conditions. The cited authors showed that the availability of S from biochar decreased with increasing temperature. Low-temperature biochars contain more sulfate, which is immediately and readily available to plants. This is due to the easy degradation of organosulfurs found in proteins. The decomposition temperatures of sulfur-bearing amino acids such as cysteine and methionine are 178 and 283 °C, respectively, which suggests that the initial sulfur release is predominantly caused by the decomposition of organosulfur [20].

Our study confirmed significant relationships between AN and AS and the individual C compounds of biochar and compost, which were stronger in the case of the latter organic amendment. This indicates a potential source of available forms of N and S, although with different degrees of release from such complexes. This is also evidenced by the determined relationships, which indicate close relationships not only between the forms of C, but also between mineral N and S. A greater number of these dependencies was demonstrated in the conditions of soil fertilized with SSC. The regression equations showed that, more durable compounds resistant to decomposition, such as humic substances, play a greater role in shaping the level of available forms of N and S. The theoretical impact of LC was weak, although it should still be considered important in determining the bioavailability of the abovementioned nutrients. Generally, the increases in LC or CSH can be attributed to the greater mobilization of S or N in the soil from SSC rather than from SSB. This indicates the different susceptibility of both organic substances to decomposition under soil conditions, and, consequently, a different impact on the amounts of available N and S.

4.4. Challenges and Future Research Recommendations

It is necessary to be aware that the resulting mass of SS needs to be rationally managed, so as not to lose a valuable source of biogenic components, and at the same time, not negatively affect the environment. The direct use of SS is only permitted in some countries and only under strict conditions. Therefore, the choice of the method of managing SS through composting or pyrolysis becomes a natural path, although not applicable under all conditions. Here, it is necessary to consider the local infrastructure, processing possibilities, acceptance of the local community and possibilities of selling the product. While composting SS is becoming common and is positively perceived by farmers, the process of pyrolysis is little known on a wide scale, and biochar itself is also rarely used in practice. Economic issues and the availability of an appropriate commercial-scale installation certainly play a role here. This is one of the challenges faced by local authorities and the world of science. The transfer of information from academics/researchers to farmers/other members of the agricultural industrial complex is mandatory. Expanding knowledge should also concern the importance and role of organic additives (SSC, SSB) in maintaining soil health, and thus sustainable soil management. Awareness of the importance of proper environmental management in the light of the European Green Deal is not always widespread among farmers and decision makers, which should be viewed as a negative phenomenon. The obligations that farmers bear, although considered locally, have global significance, which particularly concerns progress in sustainable soil use and management. It should be recognized that this is a complex process that requires interdisciplinary cooperation and support. In view of the above, the following recommendations can be made:

• To follow the life cycle assessment (LCA) of SSC and SSB in different soil and climate conditions and with different crops;
• To develop universal indicators for soil health that reflect the effect of sustainable soil management practices in a realistic and meaningful way;
• To compare the effect of carbon farming in relation to SSC vs. SSB with clear recommendations in relation to specific soil conditions.

5. Conclusions

The applied sewage sludge-derived compost and biochar differed significantly in terms of the tested parameters, with SSC having a more favorable chemical composition for agricultural practice, appropriately modifying the chemical properties of the fertilized soil. SSB significantly affected the level of available S in soil. In turn, SSC more strongly determined the amounts of available N in the soil. Regardless of the differences between the organic amendments used in the study, their increasing doses contributed to an adequate and gradual increase in the values of the parameters tested in the soil and were significantly greater than the levels measured in the control soil. This also confirms the importance of SSC and SSB in maintaining a positive C balance in the soil, because significant changes have been documented in the quantity and quality of various organic C combinations. Despite the weak polymerization of humic substances (dominance of FAs), SSB and the soil fertilized with it were characterized by a higher percentage of humic substances in TOC, which is potentially expressed by the introduction of permanent C compounds into the soil. The SSC and the soil enriched with it were characterized by a significantly dominant share of WEOC, which is closely connected to the fast decomposition of easily mineralizable carbon compounds and an enhanced microbial activity of the soil, as well as the release of N from readily mineralizable combinations. A strong influence of SSC was observed with regard to TOC, CHS, LC, and WEOC, as well as the nutrient integrally bound to the organic matter of SSC, i.e., N. Considering that the abovementioned C compounds are characterized by different susceptibility, it can theoretically be assumed that they may play an important function in both the mineralization and humification of SSC in the soil. It should be noted that the amounts of persistent C compounds introduced with SSC and SSB doses can considerably improve the sorption and buffering properties of the soil. Such a possibility is connected with maintaining soil health, as well as the role of both substances in carbon farming and NBS practices. This last highlighted aspect indicates directions for further long-term research on the transformation of SSC and SSB in soil, especially when we consider strengthening soil C resources as an element of sustainable soil management under the European Green Deal framework, particularly related to environmental protection and mitigation of climate change.