Assessing the effectiveness of sewage sludge in the reclamation of disturbed areas in the Kola subarctic zone (a case study of a sand quarry)

Funding The study was carried out within the framework of research topics FMEZ-2022-0022, FMEZ-2022-0010.

Introduction

Mining operations in Russia’s Murmansk region involving open-pit mining of sand, crushed stone, block stone, and others mineral resources widely used in the construction and production of construction materials industry has led to the emergence of numerous small and large quarries. This approach optimizes the economics of the region’s construction industry but involves a number of environmental problems associated with the emergence of new anthropogenic landscape forms with low biological productivity and distinct geomorphological, hydrophysical, and geochemical properties. The mineral reserves of Murmansk region include 68 deposits of sand and gravel mixture with estimated reserves of 92,690 thousand m³ in categories A + B + C1, while the reserves under active development amount to 48,721 thousand m³.  

Quarry mining is in greatest demand close to urban areas, but lead to air pollution, soil disturbance, and upset surface and groundwater and biota balance, which also negatively affects human health [1-3].

In Russia, in accordance with the Law on Subsoil, upon completion of the development of a deposit in areas disturbed by mining operations, mandatory reclamation measures are required to prevent negative environmental impacts and bring the area to a condition suitable for further use. However, unproductive anthropogenic soils typical of former quarries are unsuitable for reclamation both in terms of physical properties and chemical composition (GOST 17.5.1.03-86) [4-6], especially in the harsh climatic conditions of the Far North. Reclamation is problematic without additional investment in expensive materials and fertilizers, therefore it is necessary to search and develop non-conventional, science-based methods adapted to the region’s environment and economics involving the use of alternative ameliorants that improve the biogenicity of human-modified soils [7, 8].

In this context, the products of regional water and sewerage treatment facilities – sewage sludge (SS) – are of great relevance [9, 10]. Many Russian and international researchers are studying the possibilities of using SS as ameliorants to restore vegetation cover at tailings storage facilities of bauxite [11], copper [12, 13] and rare metal mines [14, 15]. The introduction of SS, characterized by a high content of organic matter and nutrients in bioavailable forms, helps to improve the edaphic factors of reclaimed soils [16-18]. However, the material needs to be carefully studied on a case-by-case basis due to the potential presence of heavy metals and pathogenic microorganisms [19].

The use of organic ameliorants for the establishment of erosion-proof phytocenoses on the sandy soils of quarries in the Arctic context without conventional land cultivation can help solve the dusting problem. A number of indicators should be studied – available forms of nutrients (nitrogen, phosphorus, potassium), organic carbon, dissolved organic matter, soil respiration (CO₂ emissions from the soil) – widely used to assess the state and productivity of the resulting ecosystems [20].

The purpose of this study is to assess the effectiveness of soil reclamation using SS for the accelerated establishment of stable erosion-proof vegetation cover in sand quarries. The practical goal of the research is to develop a cost-effective ecotechnology for reclaiming disturbed landscapes using an unconventional organomineral ameliorant that otherwise requires disposal.

Methods

Description of experimental sites and experimental design

In 2017, at the model site of a sand quarry operated by the regional water utility AO Apatityvodokanal, staff of the Federal Research Center KSC RAS laid out a small-plot field experiment on the establishment of erosion-proof grass cover by applying SS. The ameliorant was provided by the indicated utility. According to earlier studies, SS of AO Apatityvodokanal was considered waste of hazard class 5 fully compliant with the requirements of GOST R 54534-2011 for SS when used as engineered soils for biological or engineering reclamation (Table 1) [8, 21, 22]. The content of organic matter was 62 %, of potassium 0.38 %. The approximate age of the applied SS was 3 to 5 years of storage in aeration tanks.

The experiment was carried out on 18 plots measuring 1 m² each, laid at a distance of 0.5 m from each other. The experimental design included three treatments (n = 6): control plot (no SS applied), experimental treatment 1 – fragmentary application of SS (50 % of the area – pointwise, in a checkerboard pattern on each of the plots), experimental treatment 2 – continuous application of SS (100 % of the area). The thickness of the applied SS layer was about 5 cm. The SS had a creamy texture, eliminating the possibility of sediment spreading beyond the treatment area. The humidity of the applied material was 95-97 %. A grass seed mixture was used as seed material, including zoned seeds of cereal and legume plant species. During the first three years, the quality of the established phytocenoses was assessed annually: height of plants, projected area of cover, density of the grass cover, above-ground phytomass, thickness of the grass turf, increase in plant biodiversity in the established phytocenoses, and later, photosynthetic efficiency [8, 21, 22]. Condition of vegetation in the experimental plots from 2019 to 2023 is presented in Fig.1.

Table 1

Comparative analysis of the chemical composition of the SS of AO Apatityvodokanal compared to GOST R 54534-2011

Sampling and analysis

Samples of soil and anthropogenic soils at the experimental site in the quarry were taken in the sixth growing season using a cutting ring with a diameter of 10 cm and a height of 5 cm. Samples were taken one at a time in each sampling round, i.e. for each treatment, six samples were collected, n = 6. Soil density was calculated as the ratio of the dry mass of the sample to the volume of the sampler.

Laboratory ion meter I-160MI fitted with glass laboratory electrode ES-10603 and reference electrode ESr-10103 was used to measure the pH of the aqueous and salt (KCl) extracts. The extract for measuring pH_H2O was prepared at a solid to liquid ratio (S : L) of 1 : 5, stirred for three minutes, settled for five minutes in accordance with GOST 26423-85.

The extract for measuring pH_KCl was prepared at an S : L ratio of 1 : 25, stirred in a laboratory shaker for one hour in accordance with GOST 26490-85. The determination of the hygroscopicity coefficient of the samples was carried out at the Shared Resource Center, INEP KSC RAS, in accordance with GOST 28268-89.

Mobile phosphorus and exchangeable potassium were determined using the Kirsanov method as modified by TsINAO (GOST P 54650-2011). 0.2 N HCl solution was added to the soil sample (S : L 1 : 5), the mixture was stirred in a laboratory shaker for 15 minutes, then filtered through a blue ribbon paper filter. The resulting solutions were analyzed at the Shared Resource Center, INEP KSC RAS, using atomic absorption spectrometry (atomic absorption spectrometer Quantum-2mt) and photometry (photoelectric photometer KFK 3-01).

Organic carbon in solid samples was determined using the Tyurin method (GOST 26213-91). The content of labile forms of carbon and nitrogen was analyzed after extraction with cold and hot water [23]. Extraction with cold water was carried out at room temperature (distilled water as extractant, S : L ratio 1 : 10, extraction time 30 min), the solutions were centrifuged (universal laboratory centrifuge Dlab DM0636, 3500 rpm, 30 min) and filtered through a Vladipor membrane filter with a pore size of 0.45 μm. For hot water extraction, a fresh portion of distilled water was added to the remaining solid phase and kept in a thermostat at 80 °C for 16 h. The resulting extracts were centrifuged and filtered as described. The concentrations of nitrogen and carbon in the resulting filtrates were determined using an elemental composition analyzer Topaz NC.

Measurements of CO₂ emissions from soils were carried out twice during the growing season using a portable gas analyzer EGM-5 with an SRC-2 camera (PP Systems). One hour before the measurement, the open chambers were deepened into the soil by 3-4 cm with the preliminary removal of living biomass. At the same time, soil temperature was measured at a depth of 1 and 10 cm with a thermometer Checktemp-1 (Hanna Instruments) and soil moisture was measured at a depth of 10 cm with a moisture meter SM-150 (Delta-T Devices) [24]. 

Statistical processing of the results was carried out in MS Excel 2016, StatPlus suite (v7, AnalystSoft Inc.). The statistical significance of differences in measured parameters among the treatments was assessed using one-way analysis of variance ANOVA (p < 0.05).

Results and discussion

The results of measuring the basic physicochemical and agrochemical properties of soil samples taken from the experimental plots and their statistical processing are presented in Table 2.

Table 2

Physicochemical and agrochemical properties of samples

Note. Means ± standard errors are shown; letters in superscripts mean the reliable presence (letters are different) or absence (letters are the same) of differences between the experimental options at p < 0.05.

The application of SS and its long-term deposition led to a significant decrease in soil density and an increase in its hygroscopicity on the experimental plots. The density of anthropogenic soil samples in comparison with the control area decreased by 1.3 and 1.5 times on the experimental plots with fragmentary and continuous application of SS, respectively. Significant differences were found (p < 0.05) in soil density between the experimental and control treatments without a significant difference between the treatment methods. The hygroscopicity coefficient was statistically significantly different for samples from control plots and continuous application plots. Previous research [19, 25, 26] noted the positive effect of SS on the physicochemical properties of soils, in particular density and water-air exchange.

According to the pH of the aqueous extract, all samples are classified as slightly acidic, close to neutral. The application of SS significantly reduced (p < 0.05) the actual acidity in the continuous application samples. The pH values of the salt extract changed significantly: the quarry soil was close to neutral, while the soil from the experimental plots was close to being slightly acidic. Not only the fact of treatment, but also the area exposed to the SS treatment did affect the potential acidity level.

Chemical analysis showed a natural increase in the content of mobile potassium along the gradient control treatment < fragmentary application of SS < continuous application of SS. Similar results were obtained during an experiment on apatite-nepheline tailings [21]. However, the content of available phosphorus was significantly higher than in the control only in the continuous application treatment.

When comparing the content of mobile phosphorus and potassium in the soil with the scale of soil nutrient supply presented in the guidelines, it was found that anthropogenic soils belonged invariably to the very high phosphorus supply class. In terms of potassium content, the soil of the continuous application plots showed a very high level, while that of the fragmentary application plots demonstrated an elevated level, and the control plots – an average level. Thus, the content of mobile nutrients in the soil six years after a single application of SS allows one to judge the prolonged action of the treatment.

Levels of carbon and nitrogen in various fractionsand the statistical processing results are presented in Table 3.The amount of organic carbon naturally increased along the gradient control treatment < fragmentary application of SS < continuous application of SS. Significant differences were found between the control and experimental treatments involving continuous application of SS (p < 0.05).

Table 3

Carbon and nitrogen content in various fractions

The fraction of carbon extracted by distilled water at room temperature characterizes the content of water-soluble organic matter, which is a substrate for soil microflora and vegetation [27-29]. As expected, the content of carbon and nitrogen increased with increasing amount of added SS. Significant (p < 0.05) differences in the content of water-soluble carbon and nitrogen were found between the control and experimental samples with continuous application of SS.

The carbon and nitrogen content in the extract prepared using hot water as an extractant characterizes the carbon and nitrogen content of microbial biomass [30-32]. Similarly to the cold-water extracts, the content of carbon and nitrogen in the hot-water extracts increased along the gradient control treatment < fragmentary application of SS < continuous application of SS. Significant differences were also found in the carbon and nitrogen content in the experimental treatment involving continuous application of SS when compared to the control.

An additional indicator of the humus status of soils is the C/N ratio. As found in [33], soils whose C/N ratio exceeds 14 correspond to a very low level of nitrogen in the humus. The C/N ratio of water-soluble varieties in the cold-water extract of the studied samples was significantly higher than the indicated value. At the same time, in both experimental treatments using SS, this indicator is two times lower than in the control one, which may be due to an increase in the nitrogen content together with carbon and correlated with the treatment method. The value of the C/N ratio in the hot-water extract decreased along the gradient control treatment – fragmentary application of SS – conti-nuous application of SS.

The lowest values of CO₂ emission in June were observed on the control plots at 0.15 ± ± 0.05 mg CO₂/m²·h at an average humidity of 2,2 % (Fig.2). On the fragmentary treatment plots, this figure was significantly higher at 0.69 ± 0.08 mg CO₂/m²·h at an average humidity of 10.7 %, and on the continuous treatment plots, 0.49 ± 0.04 mg CO₂/m²·h at an average humidity of 3.8 %.

At the end of the vegetation season, CO₂ emission was 0.22 ± 0.06 mg CO₂/m²·h (at a humidity of 14.7 %) from the control plot; 0.47 ± 0.02 mg CO₂/m²·h (at a humidity of 25.3 %) from the fragmentary treatment plots; 0.49 ± 0.05 mg CO₂/m²·h (at a humidity of 25.2 %) from the continuous treatment plots. In the control plots, no significant difference was found between the results obtained in different measurement periods. At the same time, emissions from the control plots were significantly (p < 0.05) lower compared to the experimental treatments both in June and September. On the fragmentary treatment plots, CO₂ emissions in June were statistically significantly higher than at the end of the season, whereas on the continuous treatment plots, no such trend was found. High values of CO₂ emissions in June from the fragmentary treatment plots were due to increased soil moisture at the beginning of the growing season [34, 35].

Our analysis of the correlations between CO₂ emissions and other labile soil parameters showed a high level of correlation between soil respiration, humidity, and the content of carbon and nitrogen in the hot-water extract (Table 4). A similar pattern was reported in [36].

Table 4

Pearson correlation coefficients for some parameters (n = 18)

Our analysis revealed two main factors that explain over 74 % of the variability in the parameters being examined – the application of SS and the treatment method (Fig.3, Table 5). The contribution of the first factor – the application of SS – was 60 %. The greatest effect on this factor was exerted by the content of bioavailable phosphorus, potassium, carbon, nitrogen, CO₂ emission from the soil, potential acidity, density, humidity, and hygroscopicity coefficient. Nearly all parameters had a direct relationship, and only soil density and pH of the salt extract had an inverse relationship [35].

The second factor – the treatment method, whose contribution is 14.2 % – was driven by the pH of the water extract, volumetric humi-dity, and the content of mobile phosphorus. An inverse relationship was found only for the content of the mobile factor.

The most important condition is the presence of a layer of ameliorant, which leads to loosening of the soil and an increase in its hygroscopicity, a decrease in the pH of the salt extract, an increase in the content of bioavailable phosphorus, potassium, carbon, nitrogen, and an increase in CO₂ emissions from the soil. The second factor is the area of application of SS.

Table 5

Contributions of individual components to the factor analysis

Analysis of the study results indicates that SS can be recommended as a promising ameliorant promoting the accelerated establishment of a stable erosion-proof vegetation cover in sand quarries. Taking into account the economic effect, we find it reasonable to recommend fragmentary application of SS to increase the biogenicity of the soil and create a stable erosion-proof phytocenosis in sand quarries.

Conclusion

Adding SS had a positive effect on the physicochemical and agrochemical properties of the sand quarry soil, expressed in a decrease in its in situ density, a slight increase in hygroscopicity, a decrease in the pH of aqueous and salt extracts, and an increase in the content of available phosphorus and potassium even after six years after the treatment.

The average carbon content in the soil samples of the control treatment was 0.29 ± 0.05 %, which is three times lower than in the experimental fragmentary treatment with SS and nine times lower than in the continuous treatment. An increase in the content of carbon and nitrogen in cold- and hot-water extracts was observed along the gradient control treatment – fragmentary treatment – conti-nuous treatment. At the same time, the estimated values of the C/N ratio correspond to a very low level of nitrogen in the humus.

Significant differences were found (p < 0.05) between CO₂ emissions in the control and experimental treatments in both measurement periods. A high level of correlation was revealed between soil respiration, moisture, and the content of carbon and nitrogen in the hot-water extract.

Our factor analysis identified two main factors controlling the change in the estimated parameters – the application of SS and the method of application. Moreover, 60 % of the variability in physicochemical, agrochemical, and labile soil parameters is explained by the first factor.

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