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Article

The Influence of Sewage Sludge Composts on the Enzymatic Activity of Reclaimed Post-Mining Soil

by
Magdalena Myszura-Dymek
and
Grażyna Żukowska
*
Faculty of Agrobioengineering, Institute of Soil Science and Environment Shaping, University of Life Sciences in Lublin, ul. St. Leszczynskiego 7, 20-069 Lublin, Poland
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(6), 4749; https://doi.org/10.3390/su15064749
Submission received: 10 February 2023 / Revised: 28 February 2023 / Accepted: 6 March 2023 / Published: 7 March 2023
(This article belongs to the Section Soil Conservation and Sustainability)
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Abstract

:
Mining leads to serious degradation of the ecological values of the landscape. After mining is completed, degraded areas should be reclamated in order to mitigate the destructive effects of mining activities. Effective reclamation aims to initiate soil-forming processes. The paper evaluates the effects of land reclamation in post-mining areas 12 to 14 years after the reclamation process. The assessment was based on a determination of the activity of selected enzymes. Municipal sewage sludge compost (SSC) and compost with a composition of 70% municipal sewage sludge + 30% fly ash (SSFAC) were used as an external source of organic matter in the reclamation. The activity of dehydrogenases, phosphatases, and urease was determined. The fertilization of reclaimed soil with compost caused a significant increase in the activity of the assessed enzymes. Significantly higher dehydrogenase activity was found in the soil treated with SSC. The soil treated with SSFAC was characterized by higher activity of phosphatase and urease. The one-time application of composts from sewage sludge and sludge with fly ash, and the introduction of a mixture of grasses, allow for a permanent reclamation effect. An additional ecological advantage of this reclamation model is waste management, which is part of the circular economy strategy.

1. Introduction

Land use and/or the modification of the natural landscape by humans has resulted in a significant transformation of the terrain. These changes can contribute to disrupting the balance of ecosystems including the soil environment, leading to its acidification and nutrient depletion or leaching, i.e., loss of soil organic matter (SOM) [1].
Mining is an economic sector that exerts particularly high pressure on the environment. Negative externalities caused by the mining industry include the removal of native vegetation, loss of biodiversity, changes in water relations, soil and water pollution, generating large amounts of waste, and occupation and degradation of large areas of soil [2]. Of particular note is the devastation of soils, which are the supporting matrix of terrestrial ecosystems [3].
Today, growing concerns about the environmental impact of mining highlight the importance of reclamation in the study of mined soils [4]. Furthermore, land reclamation strategy is a key aspect of degraded area management [5]. The economic use of degraded areas requires the restoration of healthy soils on them [6,7,8]. The main objective of rehabilitating post-mining landscapes is to reconstruct the soil, increase the soil nutrient and organic matter content, and restore the biological activity of the topsoil, which is necessary for the development of soil-forming processes [9].
A common environmental practice for the remediation of degraded soils is the use of organic waste [10,11,12] and fertilizers produced from these wastes [13,14,15,16,17].
A form of waste that can be used for this purpose is municipal sewage sludge, the use of which for agricultural, non-agricultural, and soil remediation purposes is contingent on the identification of its physical, chemical, and microbiological properties [18,19].
Sewage sludge contains nutrients and trace elements necessary for plant growth [20] and organic matter that can act as a soil conditioner [21].
However, sewage sludge cannot be applied in a way that is harmful to the soil and, consequently, to the environment [22,23]. In its raw state, sewage sludge contains high concentrations of readily decomposable organic matter, heavy metals [24], pharmaceuticals [25], microplastic contamination [26] and pathogens, therefore, their direct disposal or introduction into the soil can cause serious risks such as groundwater and soil contamination, as well as odors [27].
In order to reduce the negative impact of sewage sludge on the environment, stabilization is carried out by methane fermentation [28] and adding calcium carbonate or waste with high calcium content, such as fly ash from power plants [29,30]. One factor limiting the use of sewage sludge for fertilization and soil remediation is the potential threat from pathogenic microorganisms, which is why sewage sludge must be subjected to hygienizing processes. One effective way to hygienize sewage sludge is to compost it [31,32]. Composting leads to a safe and stable bioproduct that can be used as an organic fertilizer and the high temperatures achieved during this process eliminate pathogens [33].
The quality of sludge compost depends on the properties of the materials used and the conditions under which the composting process takes place. Due to the high moisture content, sludge for composting should be mixed with dry materials. The materials used as fillers (BA) during the composting of sewage treatment sludge include organic fractions of municipal solid waste [34,35], sawdust, wood chips, and other agricultural bio-waste and mineral materials such as zeolites [36], bentonite [37], pumice [38], and fly ash [39].
Fly ash resulting from the combustion of coal in thermal power plants is considered a problematic solid waste [40] needing disposal. Numerous studies indicate the wide potential of fly ash in enhancing soil productivity and remediation of degraded land [41,42,43,44,45], and the fertilizer potential of fly ash has been reported in the literature on acid mine spoil reclamation [46,47]. Fly ash is a source of almost all macro- and micro-nutrients [48] and its addition increases the content of essential plant nutrients in the soil [49].
Studies also indicate that fly ash can be used to optimize the composting process of organic wastes [50,51], including sewage sludge, and improve the fertilizing properties of the resulting composts [52,53,54,55]. The addition of fly ash reduces the content of soluble P in compost, which reduces the risk of eutrophication after the application of SS in the soil [53]. Moreover, FA can increase the nutrient content of nutrient-poor composts.
In many countries, legal regulations impose an obligation to recultivate post-mining areas [56]. However, there are no regulations regarding the assessment of the effectiveness of the performed reclamation procedures.
Indicators based on the physical, chemical, and biological properties of soils are used to assess the effects of fertilization and reclamation. Although soil biological properties are considered more difficult to measure, predict, and quantify, they are potentially more sensitive to changes in the soil environment [57,58].
Soil enzymes are natural mediators and act as catalysts in many important soil processes such as the decomposition of organic matter released into the soil during plant vegetation, the reactions of soil humus formation and decomposition, the release and availability of mineral substances to plants, the fixation of molecular nitrogen, and the movement of carbon, nitrogen, and other basic elements of the biochemical cycle [59]. Soil enzymes respond to changes in soil management more rapidly than other variables and can, therefore, be useful as early indicators of biological change [60].
Based on enzyme activity, the biochemical potential of the soil can be estimated, and thus the capacity of the soil to carry out a range of processes important for ecosystem functioning and resilience can be measured [61]. Therefore, enzyme activity tests have been proposed as an integrated measure of soil quality [62,63]. Numerous authors have indicated that enzyme activity reflects soil productivity and is considered a sensitive indicator of soil fertility and the quality of reclaimed land [59,64,65]. The usefulness of enzymatic activity for assessing the remediation effects of degraded soils was confirmed by Araujo et al. [66], Zhang et al. [67], Kabiri et al. [68], and Russel et al. [69].
Studies on the effect of sewage sludge compost on the properties of arable and reclaimed soils are found in the literature, but there are few studies on sludge-ash composts. In addition, the studies most often cover a period of 2–3 years. This study evaluated the impact of two composts (from 100% sewage sludge and 70% sewage sludge + 30% fly ash), 12, 13, and 14 years after their introduction to degraded post-mining soil, on the effectiveness of reclamation.
The aim of this study was to evaluate the subsequent effects of sewage sludge composts and sewage sludge with fly ash composts on the enzymatic activity of reclaimed post-mining soil. Differences in dehydrogenase, phosphatase, and urease activities were assessed 12, 13, and 14 years after reclamation.

2. Materials and Methods

2.1. Experimental Site

The experiment was set up on the site of the “Jeziórko” sulphur mine (Poland, 50°34′22″ N 21°40′45″ E), located in the municipality of Grębów, in the north-western part of the Podkarpackie Voivodeship, in the district of Tarnobrzeg.
At the “Jeziórko” sulphur mine, sulphur had been extracted by underground smelting (Frasch method) since 1967. Between 1967 and 2001, over 74 million Mg of sulphur was extracted. This figure represents more than 58% of the total amount of mineral extracted in the Polish sulphur deposit [70].
The investigated soils showed typical post-industrial character and, according to the WRB (Word Reference Base for Soil Resources) systematics, they are classified as technogenic soils (Technosols) [71]. Technosols include a variety of extremely heterogeneous soils in urban, industrial, communication, mining, and military areas. Technosols are strongly influenced by human activities and man-made materials, with their properties dominated by their technical origin [72]. Their evolution significantly depends on the duration of pedogenesis as well as on the properties of the post-mining soil and vegetation cover [73].
A model plot experiment was established on soilless ground with a granulometric composition of weak loamy sand [13] (Figure 1a). The soil was characterized by strong acidity, poor sorption properties, and low organic carbon and nitrogen content (Table 1) [47].
Treatment was carried out to neutralize the acidic reactions, for which purpose slaked lime was applied at a rate of 100 Mg·ha−1 (Figure 1b). Each variant of the experiment was carried out on three plots with an area of 15 m2 in three repetitions. In total, each of the variants was carried out on an area of 45 m2. In the experiment, compost made from municipal sewage sludge (SSC) and compost composed of 70% municipal sewage sludge + 30% fly ash (SSFAC) were used to fertilize the soil. The composts were applied once at a rate of 180 Mg·ha−1 [74]. The control site (CS) was lime-only soil. Biological reclamation in this area consisted of greening the area with a grass mixture: Meadow fescue (Festuca pratensis)—41.2%, Red fescue (Festuca rubra)—19.2%, Meadow clover (Trifolium pratense)—6%, Ryegrass (Lolium multiflorum)—12.4%, Perennial ryegrass (Lolium perenne)—14.7%, and Cocksfoot (Dactylis glomerata)—6.5% (Figure 1c).

2.2. Soil Sampling

Soil samples for the study were taken on four dates: Term I (2008y)—before plant sowing (beginning of the experiment) composts were applied after 2 weeks, Term II (2020y)—after 12 years of vegetation, Term III (2021y)—after 13 years of vegetation, and Term IV (2022y)—after 14 years of vegetation.
During the sampling period, the vegetation cover was 100%.
Soil samples were taken from a depth of 0 to 20 cm (humus horizon) according to the rules defined in ISO 18400 [75], from five points distributed over the surface of each plot. The analysed sample was the average of the five samples.

2.3. Organic Carbon Content, Total Nitrogen, and C/N Ratio

Total organic carbon (TOC) was determined in dry soil samples by combustion using a TOC-VCSH apparatus [76] with an SSM-5000A module (Shimadzu Corp.; Kyoto, Japan). The total nitrogen (TN) content was determined by the modified Kjeldahl method using a Kjeltech TM 8100 distillation unit (Foss; Hillerød, Denmark). The C/N ratio was calculated from the ratio of total organic carbon (TOC) and total nitrogen (TN). All determinations were performed in three parallel replications.

2.4. Soil Enzyme Activity

The activities of three soil enzymes, i.e., dehydrogenases, neutral phosphatase, and urease were determined in three replicates. The activity of dehydrogenases (Adh) was determined by the Thalmann method [77] using a 1% solution of 2,3,5-triphenyl tetrazolium chloride (TTC) as the substrate. Phosphatase activity (Aph) was determined according to Tabatabai and Bremner [77], using a 0.8% p- nitrophenyl disodium phosphate solution as the substrate in a pH 7.0 buffer. Urease activity (AU) was determined according to Zantua and Bremner [77], using a 2.5% urea solution as the substrate. The reagents used to mark enzymes came from Sigma-Aldrich, the USA. Enzyme activities were determined using a CECIL CE 2011 spectrophotometer (Cecil Instruments; Cambridge, England) at the following wavelengths: λ = 485 nm for dehydrogenases, λ = 410 nm for phosphatase, and λ = 410 nm for urease. Soil samples for enzymatic analyses were collected and stored according to the rules defined in the Polish PN-ISO 1998 standard [78]. All determinations were performed in three parallel repetitions.

2.5. Statistical Analyses

All analyses were performed in three parallel replicates and presented as the mean of these replicates. Results were statistically analysed using STATISTICA 13.1 with ANOVA models and Tukey’s multiple post-hoc tests (HSD) at a significance level of α = 0.05.
The impact of two factors (reclamation variant and term) on the measurable feature and the interaction effect, i.e., the interaction of the analyzed factors, were assessed. For each factor, the degrees of freedom, the sum of squares of variance (SS), the mean square of variance (MS), and the value of the test statistic (F) were determined. Assuming the null hypothesis is true, it follows a Snedecor’s F distribution with the number of degrees of freedom (df) corresponding to the number of degrees of freedom of the factor analysed and the error (random factor).
Two methods of numerical taxonomy (cluster analysis) were used to determine groups of reclamation variants characterized by the similarity of enzyme activity/content (dehydrogenase, phosphatase, and urease) and indicator values (TOC, TN, and C/N) [79]:
  • Ward’s method—determination of the number of clusters (distance measure—Euclidean) based on visual analysis of the graph of distance changes in successive agglomeration phases (significant increase in agglomeration distance in the graph) and the linkage tree (Ward’s dendrogram);
  • The k-means method—to determine the composition of the clusters. An analysis of variance was also conducted to determine the significance of cluster variation.
Principal component analysis (PCA) was used to interpret the relationships between the variables studied and to look for relationships between reclamation methods and the soil properties studied.

3. Results

3.1. TOC, TN, and C/N Content

Soil organic matter (SOM) is generally considered to be the single most important property affecting soil quality and function. It plays a key role in soil productivity, influencing almost all physical, chemical, and biological properties. Successful land reclamation depends on restoring the surface horizon with sufficient soil organic matter to maintain productivity [80]. SOM is one of the most important factors influencing enzymatic activity in soil [81].
In the year of setting up the experiment (I term), the TOC content of the control soil was 2.64 g·kg−1 (Table 2). There was little variation in the TOC content on the subsequent test dates, i.e., 12, 13, and 14 years after the establishment of the experiment. A significantly higher TOC content in the soil of this variant was found in the 14th year of the experiment (3.18 g·kg−1) (Table 2).
Such small fluctuations of organic carbon content in the control soil, observed in successive study dates, indicate that in the degraded soil in the area of the sulphur mine “Jeziórko”, the content of organic matter is stable, and its increase requires an inflow of external organic matter.
The increase in TOC content was significantly influenced by the introduction of the composts into the soil (p = 0.000 > α = 0.05) (Table 3). In the soil fertilized with sewage sludge compost (S + SSC), the TOC content was 13.69 g·kg−1 on the I study date and in the soil with the addition of sewage sludge compost and fly ash S + SSFAC, it was 13.21 g·kg−1; furthermore, these contents were significantly higher than in the soil of the control object. A decrease in the TOC content of the soil fertilized with the evaluated composts was observed at successive test dates (Table 2). Fourteen years after the introduction of the composts, the TOC content was still significantly higher than in CS. In addition, the soil of the S + SSFAC variant had a significantly higher TOC content than the soil of the S + SSC variant.
The sludge-based composts used in the field experiment to remediate soil degraded by strong acidification significantly increased the total nitrogen content (Table 2). The variability factors assessed had a significant effect on the TN content, including variance (p = 0.000 > α = 0.05), test date (p = 0.000 < α = 0.05), and interactions between the factors assessed (Table 4).
In the soil of the control site (CS), the TN content after the first growing season was 0.20 g·kg−1, and the same TN content was found in the soil of this site in the 12th growing season. A significant increase in the content of this nutrient (up to 0.33 g·kg−1) was recorded in the subsequent test dates (Table 2).
Soil enzyme activity is significantly correlated with the availability of soil nutrients, including nitrogen [59,82,83]. In soil fertilized with the evaluated composts, the TN content was significantly higher than in the CS at all evaluated dates. In the soil of the S + SSC variant, the TN content was not statistically significantly different in terms I, II, and III. A reduction in the TN content (to 0.84 g·kg−1) was observed in Term IV. The S + SSFAC variant showed a large reduction in the TN content after Term I. The TN content in terms II and III was not statistically significantly different (Table 2).
The changes found in the total nitrogen content were proportional to the changes in the organic matter content. In the control soil (CS), the C/N ratio ranged from 13.2 in the I term to 9.63 in the II term. In the soil fertilized with the composts, the C/N ratio was close to the value in the control soil (Table 2).
Ward’s cluster analysis based on TOC, TN, and C/N, considered to be indicators of soil quality, confirmed that the quality of soils produced on the reclaimed site is influenced by the type of reclamation materials used (Figure 2). The diagram shows two clusters. The first cluster contained one case (CS) and the second cluster contained two cases (S + SSC, S + SSFAC). In cluster two, the variants S + SSC and S + SSFAC were very similar to each other. The analysis of variance indicated that only the TOC content significantly differentiated the clusters obtained by the k-means method (p < 0.05) (Table 5).

3.2. Soil Enzymatic Activity

Soil enzymes are an effective means of assessing soil quality due to their high sensitivity and rapid reaction to changes in the soil environment [84].
The results of this study show clear changes in the enzymatic activity of post-mining soil reclaimed with the evaluated composts (Table 6). In the soil reclaimed with S + SSC and S + SSFAC, the activity of the analysed enzymes was significantly higher than in the control soil (CS).
Dehydrogenase activity (Adh) is one of the most important soil enzyme assays, as it determines the correct sequence of all pathways in soil biogeochemical cycles [85]. Dehydrogenase activity can be used to assess soil quality, determine the effect of soil use on soil quality, and evaluate the regeneration of degraded soils [64].
The variant (F = 9891.58, p = 0.000 > α = 0.05) and test date (F = 8450.52, p = 0.000 < α = 0.05) had a significant effect on Adh. A significant interaction was also found between the factors tested (Table 7).
In the soil of all variants, at the first test date, dehydrogenase activity did not differ and was approximately 0.25 cm3 H2∙kg−1∙d−1 (Table 6, Figure 1b). In the soil of the control object (CS), there was a significant increase in Adh in the second term (0.33 cm3 H2∙kg−1∙d−1) and in terms III and IV there was a reduction in its content to a level not significantly different from the first term (Table 6, Figure 1b). The obtained results indicated that in the soil of the S + SSC variant, the activity of Adh in each term assumed the highest values in all test terms. In the soil of the S + SSFAC variant, Adh was significantly higher than in CS and significantly lower than in S + SSC (Table 6, Figure 3). The most favorable conditions stimulating dehydrogenase activity in soil fertilized with the evaluated composts occurred in the 13th and 14th years of the study.
Soil phosphatase is a biochemical indicator of soil quality, and its activity plays a key role in the productivity of terrestrial ecosystems [86].
The reclamation variant (p = 0.000 > α = 0.05) and the timing of the study had (p = 0.000 < α = 0.05) a significant effect on Aph, and a significant interaction was found between the factors studied (Table 8).
The Aph of the soil of the control site (CS) took the lowest values in each term and ranged from 19.34 to 25.37 mmol PNP·kg−1·h−1 in the first and last test term, respectively (Table 8). Compared to CS, the fertilization of the reclaimed soil with the composts (S + SSC and S + SSFAC) increased phosphatase activity. Higher values were recorded in the S + SSC variant and were the highest in S + SSFAC (Table 8, Figure 4). In the soil of the S + SSC variant, Aph on the I date was 36.27 mmol PNP·kg−1·h−1; it decreased significantly in the 12th and 13th year of the study, and significantly increased again on the next date. In the soil of the S + SSFAC variant, a significantly lower Aph was recorded only in the III term of the experiment.
Urease is a urea-degrading enzyme and is widely regarded as a good indicator of nitrogen (N) mineralization [87].
The fertilization of the reclaimed soil with the composts significantly increased the activity of the enzyme in question (Table 9). All variants differed significantly in AU activity on all dates. The soil fertilized with S + SSFAC had the highest urease activity. A decrease in AU was recorded on the last test date in each variant (Table 6, Figure 5).
Ward’s cluster analysis (Figure 6), based on the activity of the assessed enzymes, confirmed that soil enzyme activity in soils produced on reclaimed land was higher in soils enriched with the assessed composts.
Ward’s dendrogram (Table 10, Figure 3) indicates two clusters—cluster one (CS) and cluster two (S + SSC and S + SSFAC).
Principal component analysis (PCA) has been widely used to identify the most sensitive factor explaining significant differences between different types of use [88].
Figure 7 shows the results of the principal component analysis (PCA). Factors 1 and 2, extracted during the analysis, explain a total of 100% of the variance in the analysed properties of the studied soils. Factor 1 explains 85.22% of the variability of the analysed properties (total variance) and is strongly correlated with all indicators except C/N. Factor 2 explains 14.78% of the variability of the studied traits and is most strongly correlated with C/N.
AU and Aph and TOC and TN are positively correlated with each other. So, if AU increases, Aph also increases, which is followed by an increase in TOC and an associated increase in TN. No negatively correlated indicators were found among those studied.
Considering both factors, it can be seen that the CS variant was characterized by the lowest values of all indices, while the S + SSC and S + SSFAC variants had the highest values, of which S + SSC was characterized by higher values of C/N, Adh, and TOC than the S + SSFAC variant. Additionally, the S + SSFAC variant was characterized by higher AU, Aph, and TN values than the S + SSC variant.

4. Discussion

The research was carried out on soils degraded by borehole sulphur mining at the Jeziórko Mine. As a result of the mining activities, these soils were strongly acidified [89]. They are also characterized by low organic matter and nutrient content, and are often devoid of plant cover, resulting in a reduced or total lack of biological activity [16].
The reclamation of soils devastated by mining activities aims to restore biologically active soil capable of performing ecosystem services. As indicated by studies presented by Baran et al. [90] and Joniec et al. [15,16], wastes such as sewage sludge, tailings lime, and rockwool have a positive effect on a number of properties of soils degraded by the sulphur mining industry. The significant impact of sewage sludge on biological life in the soil environment is related to the strong positive effects of this type of waste on organic matter, nutrient content, soil porosity, bulk density, aggregate structure, and water-holding capacity [21]. Soil quality parameters indicate that the application of sewage sludge results in significant improvements in organic matter and nutrient content of modified technosols, as reported in various works [47,91,92].
Another waste with significant fertilizer potential is fly ash. Options for the use of coal fly ash include the reclamation of wasteland to increase nutrient content and the revitalization of degraded land [93].
Studies indicate that ash and municipal sewage sludge, due to their high nutrient content, can be used jointly as fertilizers in biomass production or the biological restoration of degraded land [45,94,95].
The introduction of sewage sludge or ash into the soil carries a risk of environmental pollution [27,96]. As studies show, this risk can be reduced by composting the waste [36]. Samars et al. [97], based on physicochemical parameters, indicated that the addition of FA to SS could be considered as an alternative sludge-stabilizing agent; hence, in the present study, compost from sewage sludge and sewage sludge with ash was used to rehabilitate a devastated post-mining soil.
The effect of land reclamation in mining areas can be assessed by open (vegetation, biodiversity) and invisible (soil properties, microorganisms) methods [98]. In our study, the evaluation was based on a determination of the activity of selected enzymes (Adh, Apf, and AU) 12, 13, and 14 years after the reclamation process.
Enzyme activity reflects the advancement of soil-forming processes [64], illustrates the trends of pedogenic processes in post-mining soils, and enables the assessment of the effectiveness of reclamation treatments being applied [62]. The literature shows that enzymatic activity has been repeatedly used to study the state of soil environments affected by different degrees of anthropopression [16,99,100,101,102].
Enzyme activity is correlated with soil organic matter content, as the latter plays a key role as a precursor for enzyme synthesis (increases soil microbial biomass, which is the source of enzymes) and in the physical stabilization of enzymes [103].
The results showed that the composts applied to fertilize the reclaimed soil significantly increased the TOC content, compared to that in the CS. On the first study date, SSC-fertilized soil had a significantly higher TOC content than SSFAC. At the end of the study, the TOC content was significantly higher in soil treated with SSFAC. The results obtained confirm the thesis known from the literature that, in soil reclamation, the introduction of external organic matter and its quality are important for organic matter accumulation processes [104]. With a dose of 180 Mg·ha−1 SSC, more TOC was introduced into the soil than with SSFAC. The higher TOC content found 14 years after reclamation in the S + SSFAC variant indicates that the organic matter of this compost was more stable, which is confirmed in studies by other authors, indicating the stabilizing effect of fly ash on the organic matter of sewage sludge and other organic wastes [97].
In the soil fertilized with the composts, the content of TN was significantly higher than in CS on all study dates. The observed changes in the TN content under the influence of the applied composts are consistent with the research results [105].
The results showed significant differences in dehydrogenase, phosphatase, and urease activities between the evaluated variants as well as the test dates.
Dehydrogenases are considered to be indicators of total microbial activity in the soil, as they are found exclusively in living cells where they catalyse oxidoreductive processes [106]. Adh indicates the presence of physiologically active microorganisms [107]. Adh is strongly associated with carbon cycles and soil organic matter (SOM) [107], and is also associated with the activity of other soil enzymes, e.g., catalase and β-glucosidase, and the presence of nitrogen. Adh plays an important role in the biological oxidation processes in soil [93]. Soil dehydrogenases have been widely studied and, in relation to other soil parameters, have been found to be a reliable, sensitive, and useful indicator of changes in soil quality [108].
The results showed that on the first test date, Adh in CS-, SSC-, and SSFAC- amended soil dremained similar and was approximately 0.25 cm3 H2∙kg−1∙d−1. On subsequent test dates, Adh significantly increased in compost-amended soil, with a significantly higher Adh in soil treated with SSC, compared to the soil of the S + SSFAC object. Adh is strongly associated with carbon cycling and soil organic matter (SOM) [94], which, with respect to the results obtained in our study, was confirmed by PCA analysis.
The observed changes in Adh under the influence of applied composts are confirmed by studies evaluating Adh under the influence of sewage sludge composts applied to arable soils [109,110].
Phosphatases are active soil enzymes that are sensitive to changes in the soil environment and, as indicated by Krämer et al. [111], phosphatase activity can be a good indicator of organic phosphorus mineralization potential and soil biological activity. The results showed that fertilization of the post-mining soil with composts (SSC and SSFAC) increased the phosphatase activity. The Aph of the soil of the control object took the lowest values in each term and ranged from 19.34 to 25.37 mmol PNP·kg−1·h−1 in the first and last terms of the study, respectively. Compared to the CS, fertilization of the reclaimed soil with composts (S + SSC and S + SSFAC) significantly increased Aph. The soil of the S + SSFAC variant was characterized by a significantly higher Aph within the soil fertilized with the composts.
Phosphatases play an important role in the conversion of organic P into inorganic forms that are available to plants [112]. Global studies have shown that soil TN content is a good predictor of phosphatase activity. A high TN content supports high phosphatase activity, as phosphatase synthesis requires a high amount of N [113,114,115]. The results obtained are consistent with this view. The PCA analysis showed a significant positive correlation between Aph and TN content.
The direction of changes in Aph on successive dates was influenced by the type of compost used. The soil of the S + SSC variant showed a significant decrease in Aph in terms II and III and an increase in Term IV. The soil of the S + SSFAC variant showed a significant increase in Aph in Term II, a decrease in the next term, and then an increase in Term IV. Similar sweeps of Aph were recorded by Kaur et al. [110] when the soil was incubated with the addition of sewage sludge and compost. The increase in Aph in the soil evaluated with the composts in Term I was probably partly due to the direct contribution of phosphatase activity from the composts and the stimulation of phosphatase production by the microflora [116]. The observed reduction in Aph may have been due to the fact that quantities of C and N were introduced into the soil along with the composts, which increased microbial activity and thus P demand and phosphatase release. With time, the soil C content decreases, resulting in lower microbial activity, a lower P demand, and thus lower phosphatase activity [117].
Of the many soil enzymes, urease (urea amidohydrolase, EC 3.5.1.5), closely related to metabolism, biological cycling, and the bioavailability of nitrogen [87,118], is a key enzyme. The results showed that the composts had a significant effect on increasing AU in the remediated soil. Within the composts evaluated, the soil of the S + SSFAC site had higher AU than the soil of the S + SSC variant. The PCA analysis showed that AU was significantly positively correlated with TN and TOC content, which is consistent with the results of Vaheda et al. [119]. Urease is an extracellular enzyme that is made stable by forming complexes with organic and mineral colloids [120,121]. High soil UA is believed to be a direct indicator of improved soil fertility, helping to increase nitrogen uptake by plants [87,122].

5. Conclusions

The obtained results showed that sewage sludge composts and sewage sludge with the addition of fly ash have a beneficial subsequent effect on the enzymatic activity of the reclaimed post-mining soil.
Compared to the control soil, fertilization with compost significantly increased the activity of the assessed enzymes. The extent of increasing enzymatic activity in soils formed on reclaimed land was significantly dependent on the composition of the applied compost. Within the assessed composts, significantly higher dehydrogenase activity was found in the soil to which compost made from sewage sludge was applied, whereas the soil on which sewage sludge and fly ash compost was applied was characterized by higher phosphatase and urease activity.
All the analysed enzymes, i.e., dehydrogenases, phosphatase, and urease, were sensitive indicators of long-term changes caused by the application of the composts to degraded post-mining soil; therefore, the activity of these enzymes can be used in monitoring and assessing the effects of reclamation of degraded soils.
Enzymatic activity, its changes in the analysed period of research, and the content of total organic carbon and total nitrogen indicate that the soil fertilized with sewage sludge and ash compost was characterized by significantly better properties. Our research proves that a one-time application of composts made of sewage sludge and sludge with the addition of ash, as well as the introduction of plant cover—a mixture of grasses, which allows for a lasting reclamation effect. A single application of a high dose (180 Mg·ha−1) of composts, especially with the addition of sewage sludge and fly ash, can be recommended as an effective technology for the reclamation of degraded areas. An additional ecological advantage of this reclamation model is waste management (SS and FA), which is part of the circular economy strategy.

Author Contributions

Conceptualization, M.M.-D. and G.Ż.; methodology, G.Ż.; validation, M.M.-D.; formal analysis, M.M.-D.; investigation, G.Ż. and M.M.-D.; data curation, G.Ż.; writing—original draft preparation, M.M.-D.; writing—review and editing, G.Ż.; visualization, M.M.-D.; supervision, G.Ż.; All authors have read and agreed to the published version of the manuscript.

Funding

The research was financially supported by the Polish Ministry of Science and Higher Education. As part of the subsidy for the maintenance and development of research potential, task number RGL/S/34/2022—Natural and anthropogenic transformations of the main elements of the environment and their protection.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Degraded area of the Jeziórko sulfur mine. (b) The area after technical reclamation and the use of post-flotation lime. (c) Area after biological reclamation.
Figure 1. (a) Degraded area of the Jeziórko sulfur mine. (b) The area after technical reclamation and the use of post-flotation lime. (c) Area after biological reclamation.
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Figure 2. Ward’s tree dendrogram for TOC, TN, and C/N.
Figure 2. Ward’s tree dendrogram for TOC, TN, and C/N.
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Figure 3. Change in the average amounts of Adh on individual dates for individual variants.
Figure 3. Change in the average amounts of Adh on individual dates for individual variants.
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Figure 4. Change in the average amounts of Aph on individual dates for individual variants.
Figure 4. Change in the average amounts of Aph on individual dates for individual variants.
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Figure 5. Change in the average AU quantities on individual dates for individual variants.
Figure 5. Change in the average AU quantities on individual dates for individual variants.
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Figure 6. Tree diagram—Ward’s dendrogram for Adh, Aph, and AU.
Figure 6. Tree diagram—Ward’s dendrogram for Adh, Aph, and AU.
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Figure 7. Biplot (combination of a 2D factorial plot for cases (CS, S + SSC, S + SSFAC) with a 2D factorial plot for variables (TOC, TN, C/N, Adh, Aph, AU).
Figure 7. Biplot (combination of a 2D factorial plot for cases (CS, S + SSC, S + SSFAC) with a 2D factorial plot for variables (TOC, TN, C/N, Adh, Aph, AU).
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Table
Table 1. Selected soil properties and materials used. Average values [47].
Table 1. Selected soil properties and materials used. Average values [47].
PropertyUnitControl Soil (CS)Sewage Sludge Compost (SSC)Sewage Sludge + Fly Ash Compost (SSFAC)Post-Flotation Lime (PfL)
pH1 mol KCl4.76.26.67.1
Hcmol(+)·kg−14.28.95.04.5
S2.0229.7287.648.8
T6.22306.6292.654.3
TOCg·kg−15.6408372-
TN0.5931.626.2-
C/N ratio 9.512.914.2-
Explanation: pHKCl—pH in 1 mol·dm−3 KCl, H—hydrolytic acidity, S—sum of basic cations, T—total sorption capacity, TOC—Total Organic Carbon, TN—Total Nitrogen Content, and C/N ratio.
Table
Table 2. Total Organic Carbon (TOC), Total Nitrogen (TN) Content, and C/N ratio.
Table 2. Total Organic Carbon (TOC), Total Nitrogen (TN) Content, and C/N ratio.
VariantTermTOCTNC/N
g·kg−1
CSI2.64 ± 0.01 c0.20 ± 0.01 a13.2
II2.03 ± 0.05 a0.19 ± 0.01 a10.68
III2.52 ± 0.02 b0.26 ± 0.01 b9.69
IV3.18 ± 0.02 d0.33 ± 0.01 c9.63
S + SSCI13.69 ± 0.04 j1.01 ± 0.01 f13.55
II13.20 ± 0.02 i1.00 ± 0.01 f13.2
III10.86 ± 0.02 g0.99 ± 0.01 f10.97
IV9.12 ± 0.02 e0.84 ± 0.00 d10.86
S + SSFACI13.21 ± 0.03 i1.57 ± 0.00 g8.41
II12.82 ± 0.01 h0.89 ± 0.00 e14.40
III10.18 ± 0.02 f0.89 ± 0.02 e11.43
IV10.21 ± 0.01 f0.99 ± 0.01 f10.31
Explanation: different lowercase letters in the upper index indicate significant differences. Mean ± standard deviation.
Table
Table 3. ANOVA for TOC factorial systems.
Table 3. ANOVA for TOC factorial systems.
EffectSsDfMSFp
Expression free2686.5212686.522,819,6730.000
Variant658.262329.13345,4400.000
Term34.83311.6112,1850.000
Variant × Term31.9765.3355930.000
Error0.02240.00
Explanation: Ss—sum of squared deviations, Df—degrees of freedom, MS—mean squared deviations, F—test statistic value, and p—significance level.
Table
Table 4. ANOVA for Nt factorial systems.
Table 4. ANOVA for Nt factorial systems.
EffectSsDfMSFp
Expression free20.99120.99151,1400.000
Variant4.9322.4717,7530.000
Term0.3230.117630.000
Variant × Term0.7360.128730.000
Error0.00240.00
Explanation: Ss—sum of squared deviations, Df—degrees of freedom, MS—mean squared deviations, F—test statistic value, and p—significance level.
Table
Table 5. Analysis of variance (k-means method) for TOC, TN, and C/N.
Table 5. Analysis of variance (k-means method) for TOC, TN, and C/N.
VariableAnalysis of Variance
Between SsDfWithin SsDfFSignificant p
TOC2.00010.00018664.9170.007 *
TN1.96310.037153.7690.086
C/N0.92111.07910.8540.525
Explanation: TOC—Total Organic Carbon; TN—total nitrogen; C/N ratio; Ss—sum of squares; Df—degrees of freedom, F—test statistic value, and p—significance level. * Statistically significant differences (p < 0.05).
Table
Table 6. Soil enzymatic activity.
Table 6. Soil enzymatic activity.
VariantTermAdhAphAU
mg TPF kg−1 24 h−1mmol PNP kg−1 h−1mg N-NH4+ kg−1 h−1
CSI0.25 ± 0.00 a19.34 ± 0.01 a3.33 ± 0.01 e d
II0.33 ± 0.01 b22.79 ± 0.01 b3.20 ± 0.01 e c
III0.23 ± 0.01 a23.09 ± 0.01 b3.80 ± 0.01 e g
IV0.24 ± 0.01 a25.37 ± 0.00 c2.75 ± 0.00 a
S + SSCI0.26 ± 0.00 a36.27 ± 0.48 f3.73 ± 0.01 e f
II0.52 ± 0.02 d34.51 ± 0.01 e4.31 ± 0.01 e j
III1.90 ± 0.01 h32.35 ± 0.01 e d4.21 ± 0.01 e i
IV1.83 ± 0.01 g38.41 ± 0.01 e g h2.95 ± 0.03 b
S + SSFACI0.24 ± 0.01 a38.90 ± 0.01 e h3.90 ± 0.01 e h
II0.44 ± 0.02 c41.41 ± 0.01 e i4.54 ± 0.02 k
III1.22 ± 0.01 e37.91 ± 0.01 e g4.60 ± 0.02 l
IV1.42 ± 0.01 f41.96 ± 0.01 e j3.50 ± 0.01 e
Explanation: Adh—dehydrogenase activity, Aph—phosphatase activity, and AU—urease activity. Different lowercase letters in the upper index indicate significant differences. Mean ± standard deviation.
Table
Table 7. ANOVA for Adh factorial systems.
Table 7. ANOVA for Adh factorial systems.
EffectSsDfMSFp
Expression free19.70119.7084,423.440.000
Variant4.6222.319891.580.000
Term5.9231.978450.520.000
Variant × Term3.7360.622664.330.000
Error0.01240.00
Explanation: Ss—sum of squared deviations, Df—degrees of freedom, MS—mean squared deviations, F—test statistic value, and p—significance level.
Table
Table 8. ANOVA for Aph factorial systems.
Table 8. ANOVA for Aph factorial systems.
EffectSsDfMSFp
Expression free38,476.78138,476.781,326,9130.000
Variant1946.182973.0933,5580.000
Term94.24331.4110830.000
Variant × Term55.4269.243190.000
Error0.70240.03
Explanation: Ss—sum of squared deviations, Df—degrees of freedom, MS—mean squared deviations, F—test statistic value, and p—significance level.
Table
Table 9. ANOVA for AU factorial systems.
Table 9. ANOVA for AU factorial systems.
EffectSsDfMSFp
Expression free502.131502.131,807,6800.000
Variant4.6022.3082780.000
Term6.7732.2681250.000
Variant × Term0.9160.155440.000
Error0.01240.00
Explanation: Ss—sum of squared deviations, Df—degrees of freedom, MS—mean squared deviations, F—test statistic value, and p—significance level.
Table
Table 10. Analysis of variance (k-means method) for Adh, Aph, and AU.
Table 10. Analysis of variance (k-means method) for Adh, Aph, and AU.
VariableAnalysis of Variance
Between SsDfWithin SsDfFSignificant p
Adh1.77010.23017.6930.220
Aph1.86610.134113.9420.167
AU1.70310.29715.7290.252
Explanation: Adh—dehydrogenase activity, Aph—phosphatase activity, AU—urease activity, Ss—sum of squares, Df—degrees of freedom, F—test statistic value, and p—significance level.
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Myszura-Dymek, M.; Żukowska, G. The Influence of Sewage Sludge Composts on the Enzymatic Activity of Reclaimed Post-Mining Soil. Sustainability 2023, 15, 4749. https://doi.org/10.3390/su15064749

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Myszura-Dymek M, Żukowska G. The Influence of Sewage Sludge Composts on the Enzymatic Activity of Reclaimed Post-Mining Soil. Sustainability. 2023; 15(6):4749. https://doi.org/10.3390/su15064749

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Myszura-Dymek, Magdalena, and Grażyna Żukowska. 2023. "The Influence of Sewage Sludge Composts on the Enzymatic Activity of Reclaimed Post-Mining Soil" Sustainability 15, no. 6: 4749. https://doi.org/10.3390/su15064749

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