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Systematic Review

A Systematic Review and Meta-Analysis of the Sustainable Impact of Sewage Sludge Application on Soil Organic Matter and Nutrient Content

by
Enzo Antonio Lecciolle Paganini
1,
Rafael Barroca Silva
1,2,
Ludmila Ribeiro Roder
1,
Iraê Amaral Guerrini
2,
Gian Franco Capra
1,
Eleonora Grilli
3 and
Antonio Ganga
1,*
1
Department of Architecture, Design and Urban Planning, University of Sassari, Viale Piandanna 4, 07100 Sassari, Italy
2
Department of Forest, Soil and Environmental Sciences, College of Agricultural Sciences, São Paulo State University-UNESP, Botucatu 18610-034, SP, Brazil
3
Department of Environmental, Biological and Pharmaceutical Sciences and Technologies, University of Campania “Luigi Vanvitelli”, Via Vivaldi 43, 81100 Caserta, Italy
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(22), 9865; https://doi.org/10.3390/su16229865
Submission received: 1 October 2024 / Revised: 3 November 2024 / Accepted: 8 November 2024 / Published: 12 November 2024
(This article belongs to the Special Issue Recycling Materials for the Circular Economy—2nd Edition)
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Abstract

The increasing scarcity of natural resources makes the linear production model unsustainable, highlighting the need for more sustainable practices under the umbrella of circular economic principles. Sewage sludge emerges as a promising solution to provide soil organic matter (SOM) and nutrients. This meta-analysis evaluated the impacts of three levels of sludge application (low, medium, and high) on organic matter (OM), nitrogen (N), phosphorus (P), copper (Cu), and zinc (Zn) contents, considering different areas and experimental conditions worldwide. The analysis included 37 studies and 355 comparisons, after screening 7625 records, following the 2020 PRISMA protocol. The effects of sludge concentrations, continents, and types of experiment (field or greenhouse) were evaluated. Sewage sludge application significantly increased OM, N, Cu, and Zn levels, mainly at medium and high rates. The largest effects were observed in greenhouses, suggesting variation by location and environment. Moderators explained a part of the variation in the results, but the residual heterogeneity test revealed that there is still unexplained variability.

1. Introduction

Population growth has been significant in recent years, with the world population surpassing 8 billion people in 2022 and projected to reach approximately 10 billion by 2060 [1]. This increase puts enormous pressure on food production, which needs to meet this growing population demand [2]. However, increasing agricultural productivity faces major challenges, including the growing scarcity of finite natural pivotal soil macronutrients, such as phosphorus and potassium [3]. As natural resources become increasingly scarce, the traditional linear production model is losing strength, and the need to adopt more sustainable practices at all stages of the production process becomes evident. The exploitation of resources, followed by the disposal of waste, is gradually being replaced by approaches that prioritize efficiency and a circular economy. Practices such as reuse, recycling, and recovery are gaining prominence, enabling us to extract the maximum value from resources and extend their lifespans, in accordance with the principles of sustainability [4].
Historically, soils have followed a linear production model characterized by the intensive exploitation of natural resources for agriculture and other activities, without considering the long-term environmental consequences. This model has resulted in degradation, nutrient depletion, and loss of biodiversity, compromising productivity and increasing susceptibility to erosion. However, soils play crucial roles not only in maintaining agricultural and forestry productivity but also as physical supports for vegetation, nutrient cycling, water regulation and purification, carbon storage, and habitats for organisms and microfauna, in addition to serving as carbon stocks [5].
Soil loss is a natural process. However, frequent anthropic activities, particularly within linear production systems, and the accelerating pace of climate change have significantly intensified this process [6]. Therefore, it is crucial to find solutions that restore physical and chemical soil properties and to explore alternatives that reduce production costs. With these more sustainable practices and the growing global awareness of the scarcity of finite natural resources [7], there is increasing interest in using sewage sludge, which is no longer considered as a waste product but as a valuable resource.
The use of sewage sludge is directly linked to the development and improvement of sanitary collection systems, contributing to the efficiency and sustainability of processes [8].
Global sewage sludge production reaches hundreds of millions of tons per year. The United States is the largest producer, generating approximately 65 Mt/year, followed by China, with approximately 30 Mt/year, and Japan and Germany, each with approximately 20 Mt/year [9]. In the USA, approximately 47% of the collected sludge is applied to the soil, while in China, 98% is still sent to landfills or disposed of inappropriately [9]. In the European Union, 27% is reused in agriculture, followed by incineration (25%), composting (21%), landfills (9%), and other purposes (18%) [10]. The low percentage of sludge use highlights the great potential for expansion of this resource in soils.
Research on sewage sludge applications is constantly advancing, as sewage sludge comprises numerous organic compounds and macro and micronutrients that are beneficial for soil and plants [11].
Anaerobic digestion, incineration, composting, and landfilling are conventional methods for sludge treatment, each with its own advantages and challenges. Anaerobic digestion, a widely used method, reduces sludge volume and generates biogas as an energy source but requires significant operating and infrastructure costs. Sludge incineration effectively eliminates pathogens but consumes high amounts of energy and generates ash with high concentrations of heavy metals, posing technical and economic challenges for its safe disposal. However, this ash has gained importance because of new processes that can degrade emerging contaminants and recover valuable components [12]. Composting and landfilling, although simpler methods, have environmental and health limitations because of the presence of organic and inorganic contaminants in the sludge, which can infiltrate the soil and groundwater [13].
New methods for valorizing sludge, such as pyrolysis, have received attention because they convert sludge to reusable products, such as biochar, bio-oil, and pyrolysis gas, which can be applied to soil and used as energy sources. Biochar, in particular, enriches soil fertility, improves water and nutrient retention capacities, and sequesters carbon, contributing to the reduction of greenhouse gas emissions [13] in addition to having the potential to purify wastewater [14]. However, processing these materials still faces economic and technical challenges at a large scale, especially when compared to the direct application of sludge, which offers immediate fertilization and less technological demand [14].
To reduce risks from the direct application of sewage sludge to the soil, pretreatment techniques, such as composting and chemical stabilization, can be used to reduce contaminants and make the use of sewage sludge safer. Sewage sludge has different origins, and understanding these variations is extremely important, as the concentrations of substances, including heavy metals, vary depending on the source, requiring adaptations to local regulations [15].
To determine the appropriate application rate and consider this material as a resource, studies are needed to validate its safety for use, as many of these sludges also contain substantial amounts of potentially toxic elements (PTEs) and pathogens, which can result in bioaccumulation in various plant tissues and the soil [16].
Several studies have focused on identifying more sustainable destinations for sewage sludge, recognizing its potential in various applications. These studies highlight its suitability for enhancing forestry and ornamental species [17,18,19,20,21], increasing food productivity [22,23,24,25], and restoring degraded soils [26,27,28,29,30].
To comprehensively understand the impacts of sewage sludge application on soil organic matter (OM), N, P, Cu, and Zn contents, systematic reviews and meta-analyses are valuable tools. These approaches enable the integration and interpretation of data from multiple studies, providing a more comprehensive and transparent understanding of this topic [31]. Systematic reviews guide the search for relevant studies, while meta-analyses employ statistical techniques to compare and analyze their findings, offering more precise estimates [32]. It is crucial to conduct these analyses carefully, considering potential issues, such as publication bias and heterogeneity among studies, which may compromise the reliability of the results [32].
Most studies on sewage sludge application have focused on local results, typically employing sludge from a single treatment plant and applied in a specific context. To date, there is no awareness of any global-scale research that examines the effects of sewage sludge application in different regions, climates, and experimental conditions.
The main objectives of this study were to use the PRISMA protocol to conduct a systematic review, with a predefined methodology and clear inclusion and exclusion criteria, and to perform a meta-analysis (quantitative analysis) based on the selected studies. This meta-analysis also aims to understand the responses of three different classes of sewage sludge application intensity (low, medium, and high) regarding the increases in OM, N, P, Cu, and Zn contents present in soils in different regions of the world and in studies carried out in greenhouses or in the field, when compared to a control.

2. Materials and Methods

2.1. Literature Collection

The most recent version of the PRISMA protocol (Preferred Reporting Items for Systematic Reviews and Meta-Analyses, 2020 version) was used to select relevant publications that would be included in this work. This protocol outlines a detailed 27-step process to ensure transparency, consistency, and quality in the systematic reviews and meta-analyses [33].
The bibliographic review sought to identify studies that compared sewage sludge treatments to controls in terms of changes in soil chemical parameters. These studies were focused on areas such as degraded land restoration, agricultural fertilization, and landscapes and soil recovery. The specific parameters of interest included OM, N, P, Cu, and Zn contents, which presented sufficient quantitative data to enable a meta-analysis. Although nutrients, such as potassium (K), and micropollutants, such as cadmium (Cd) and lead (Pb), are relevant, their inclusion was limited by the availability of consistent data in the selected studies. Thus, the chosen parameters represent the variables with the greatest quantitative and comparable presence among the studies analyzed.
The search was completed in August 2023 using the University of Sassari (UNISS) credentials on the Scopus and Web of Science platforms. These platforms were chosen after preliminary tests revealed their comprehensiveness compared to smaller platforms with numerous duplicate works. Thus, the final search terms used were (“sewage sludge” OR “byproduct”) AND (“urban soil” OR “landscape” OR “soil remediation” OR “forestry”).
Duplicate studies were initially excluded. The inclusion criteria for articles were (a) English language; (b) published articles only; (c) subject area within biological sciences related to sustainability and soil sciences—Thus, for the Scopus platform, the filters used were “environmental sciences”, “agricultural sciences”, and “biological sciences”, while on the Web of Science platform, they were “environmental sciences”, “environmental engineering”, “soil sciences”, and “ecology”. It was not possible to specify the number of studies for each thematic area, as many of them are classified in more than one area in the search platform filters; (d) publication dates between 2013 and August 2023; (e) studies where the abstract made it clear that domestic sewage sludge was used in soils; (f) studies that presented quantitative data. Qualitative studies, book chapters, case studies, studies using materials other than sewage sludge that could mask the result strictly from the addition of the sewage sludge, and studies with missing information were excluded.
After these steps, the selected articles were independently reviewed by at least two of the authors of this study. The papers were sequentially evaluated based on language, document type, subject area, year of publication, abstract, and papers that presented quantitative data to establish their eligibility. To strictly adhere to the PRISMA protocol guidelines, our approach was divided into four main phases: identification, screening, eligibility, and inclusion.

2.2. Data Extraction

The PRISMA diagram outlines the detailed process for screening the papers (Figure 1). After selecting the papers, we included 37 studies (Supplementary Material S1) in our analysis.
Data extraction was conducted manually through a comprehensive reading of the selected articles by two different reviewers. The data were extracted and organized into spreadsheets, where all the significant data were described individually for each work [34].
Because of the wide range of sewage sludge concentrations employed in the analyzed studies, three categories were adopted: low concentration (up to 25 Mg ha−1 or 30% mixed), medium concentration (25–50 Mg ha−1 or 30–70% mixed), and high concentration (above 50 Mg ha−1 or 70% mixed) [35].
Because different units of measurement were used across the studies, the values of the response variables were converted to a single unit of measurement for this meta-analysis: The values of the OM, N, and P contents were expressed as percentages, while the values of the Cu and Zn contents were presented in mg kg−1. In studies that did not provide a soil density, a density of 1 g cm−3 was assumed to convert the values to percentages. When the studies did not present the standard error, a value corresponding to 10% of the measured value was adopted [36]. For studies which data were not shared but that allowed for their extraction from graphics, measurements were performed using ImageJ software, version 1.54 [37].
Not all the studies included in this meta-analysis reported data on all five analyzed parameters, and some studies provided multiple comparisons, either using different controls at the same sewage sludge dosage or by comparing the same control with different types of sewage sludge. This resulted in a discrepancy between the number of analyzed articles and the total number of analyses performed, as shown in Table 1.
Of the 37 studies included in this analysis, 24 [7,11,17,21,26,27,29,30,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53] were used to discuss OM concentrations. Among these, 19 studies compared a control treatment with a low concentration of sewage sludge. For the comparisons between the control and medium concentrations, 13 studies were used, and 12 studies were used for comparisons between the control and high application concentrations. Overall, 60 comparisons were discussed. Individual studies may have presented multiple comparisons at the same application concentration or even across different concentrations. A detailed list of the selected articles and the number of comparisons made for all the parameters and concentrations can be found in Supplementary Material S2.
To discuss the comparisons between N concentrations in the treatments, 19 studies were analyzed [17,26,27,29,30,38,40,42,43,44,45,46,48,49,50,52,53,54,55]. Of these, 15 studies compared a control treatment to a low concentration of applied sewage sludge, resulting in 21 comparisons. For comparisons between a control and a medium concentration, 13 studies were used, while 11 studies were used for comparisons between a control and high applied concentrations. In total, 52 comparisons were discussed.
To compare P concentrations across the treatments, we analyzed 26 articles [7,11,17,21,26,27,29,38,39,40,41,42,43,44,45,46,47,48,49,50,52,53,54,55,56,57]. Of these, 18 were used to compare a control treatment with a low concentration of sewage sludge, resulting in 35 comparisons. For the comparison between a control and a medium concentration, 15 articles were used, while 16 articles were analyzed for the comparison between a control and a high application concentration. In total, 71 comparisons were discussed.
Copper concentrations had the most data in the selected studies, with 28 studies [7,11,17,20,21,24,25,26,27,29,30,38,41,42,44,45,47,48,49,53,54,57,58,59,60,61,62,63]. The highest number of comparisons occurred between the control treatment and the application of low concentrations of sewage sludge, resulting in 47 analyses. When the control was compared with the application of medium concentrations of sludge, data from 18 studies were used, and 16 studies were considered for comparisons with high concentrations.
For Zn concentrations, 25 records [7,11,17,21,24,25,26,27,29,38,41,44,45,47,48,49,53,54,57,58,59,60,61,62,63] were identified in the analyzed studies. Comparisons between the control treatment and the application of a low concentration of sewage sludge resulted in 45 analyses. When a control treatment was compared with the application of medium concentrations, data from 16 studies were used, and 14 studies were used for comparisons with high concentrations.

2.3. Statistical Analysis

The variables analyzed in this meta-analysis were the soil OM, total N, total P, Cu, and Zn contents. Because of the incomplete reporting of these variables in the selected publications, random effect models were applied for each variable and for each comparison between the control and the three analyzed doses of the sewage sludge application (low, medium, and high), as explained in the previous section, with their respective lists of studies (Supplementary Material S2).
To obtain the effect sizes of the treatments in relation to the control for each comparison, the response ratio, i.e., the log-transformed ratio of means, was calculated, as the variables are always quantitative and continuous [35,64], for the sampling variance. The random effect models applied were of the multilevel type for including more than one comparison within a study [65].
Random effect models were chosen because of the great degree of heterogeneity in the studies, resulting from different objectives, methodologies, and comparisons. The multilevel model allowed for the inclusion of moderators, adjusting for influences both within and between studies [65].
Three moderators were applied to the multilevel random effect models: (a) sewage sludge application concentration, (b) continent where the studies were developed, and (c) experimental conditions (field or controlled environment). The first moderator covers different sewage sludge application concentrations, which can be low, medium, or high; the second covers studies developed on different continents; the third includes experiments conducted under field conditions or in a controlled environment (greenhouse or laboratory). Heterogeneity was calculated using the QE test [66].
To calculate the effect sizes, random effect models, and forest plot, the “metafor” package [66] was used in the statistical software R Studio version 2024.04 [67]. The publication bias was accessed through applying the random effect model, with the standard error of the effect size as a moderator, as performed by Habeck et al. [68]; if the model’s intercept is different from zero, we can assume that there is bias in the relation between the precision and the sample size of the studies [69].

3. Results and Discussion

3.1. Characteristics of the Included Studies

Table 1 summarizes the studies included in this meta-analysis. Of the 37 studies analyzed, 28 were used for Cu comparisons, 25 for P, 24 for OM and Zn, and 19 for N. In total, 355 comparisons were performed: 98 for Cu, 88 for Zn, 61 for P, 57 for OM, and 51 for N.
Most comparisons involved low concentrations of sewage sludge compared to the control, with 144 comparisons (40.6%). Medium concentrations were used in 111 comparisons (31.3%), while high concentrations were used in 100 comparisons (28.2%).
The study environments differed significantly. Of the 355 comparisons, 245 (approximately 69%) were from open-field studies, while only 110 (approximately 31%) occurred in greenhouses. Notably, 80 of these 110 greenhouses comparisons focused on Cu and Zn, representing approximately 72.3% of the analyses conducted in this controlled environment.
After the systemic analysis, the Americas had the highest number of studies (11), followed by Asia (9), Africa (8), Europe (6), and Oceania (3).
Figure 2 shows the global distribution of the studies by continent. America and Asia account for 29.7% and 24.3% of the studies, respectively, followed by Africa (21.6%), Europe (16.2%), and Oceania (8.1%). Brazil had the highest number of studies, totaling 11, followed by Tunisia, with 6, and China, with 5 (Figure 3).
These countries’ interest in sewage sludge is evident, especially considering Brazil and China’s leadership in global agricultural production. Brazil, the world’s largest soybean, coffee, and sugarcane producer, seeks sustainable alternatives to maintain large-scale productivity while reducing dependence on chemical fertilizers. China, the world’s largest agricultural producer, with an estimated production of US$1.14 trillion, faces soil depletion due to intensive agriculture and sees sludge as a solution to replenish and improve soil nutrients and health [70]. Tunisia, with predominantly arid and semi-arid infertile soils, finds sewage sludge application to be an effective strategy to increase soil fertility [27].
In total, this meta-analysis includes studies form 14 countries, with the mentioned countries representing approximately 60% of the total. There is a significant concentration of studies in a few territories. Of the 355 analyses performed, 117 were for the Asian continent (33%), 104 for Africa (29.3%), 92 for the Americas (25.9%), 25 for Europe (7%), and 13 for Oceania (3.7%).
The African and American continents had well-balanced comparisons among the five parameters. Conversely, the Asian continent focused more on analyses on PTEs, such as Cu and Zn, with 80 of the 117 analyses performed on these parameters, representing approximately 68.4% of the total. China’s interest is justified by the challenges facing agriculture because of population growth and accelerated urbanization. Indeed, there is concern about applying high sewage sludge concentrations, which can lead to soil and groundwater pollution through leachate processes [71,72].

3.2. Meta-Analysis

Table 2 presents the results of the moderator test (QM) and the residual heterogeneity test (QE) for the five analyzed parameters. These results can be verified in Supplementary Material S3.
Phosphorus had the lowest QM value, indicating that moderators have a less-pronounced influence on the results’ variability compared to the other analyzed parameters. In terms of the increasing impact on the results’ variability, the concentrations increased in the sequence N, OM, Zn, and, finally, Cu, with Cu’s value being approximately 59 times greater than that of P.
All the parameters analyzed show a high significant p-value of 0.0001, suggesting a 99.9% reliability for the influence of the moderators, confirming a strong and consistent moderating impact on the results.
Although the moderator test revealed that the assessed factors explain some variation between the comparisons, the residual heterogeneity test indicates significant unexplained heterogeneity. This is evident in the high QE values for all the parameters, regardless of the analyzed degrees of freedom. The heterogeneity in these results is notably higher than that observed in clinical meta-analyses, which often compare control groups with patients undergoing specific treatments [73]. Because the research subjects are human beings with more standardized characteristics, these analyses do not usually include high-valued variables, resulting in less variability in the results.
Phosphorus, with the highest QE value (14,676.6805), demonstrated the largest unexplained residual variation. This suggests that the included moderators were not sufficient to fully explain the heterogeneity between the comparisons. In all cases, the p-values were below 0.0001, confirming significant residual heterogeneity for all the parameters. As a matter of fact, this analysis included studies from five continents, thus assuming highly variable environmental conditions for the experiments (soil attributes, lithologies, climate descriptions, and plant and crop species). These implicit variables are not consistently shown in all the selected papers, so their effects are likely associated with the high degree of heterogeneity.
Figure 4 clearly illustrates that sewage sludge application favored increased concentrations of OM, N, P, Zn, and Cu in the soil compared to the control, especially at medium and high dosage levels [26,27,42,58]. Even at the low application rate, OM showed a significant effect (confidence interval not exceeding 1), reaching 1.51 (approximately 50% higher than the control), while the high dosage resulted in an average effect ratio of 2.2. The nitrogen effect was significantly higher than the control only at medium (1.45) and high (1.84) residue application rates [45,46,54]. In relation to phosphorus, it was possible to observe that the medium and high concentrations presented significant results in comparison with the low concentration; however, there was no significant difference between the medium (1.86) and high (1.92) concentrations [42,47].
The effects of sewage sludge application rates on Zn and Cu contents followed very similar trends, with increasing and significant effect ratios in high application classes [61], demonstrating mean effects of 2.2 for Zn and 2.9 for Cu. Thus, it is evident that even under different environmental conditions, the increase in the doses of sludge applied resulted in higher concentrations of both micropollutants [26,29,54].
When continents are used as moderators, each studied variable exhibits a distinct pattern (Figure 5). This is attributed to specific regional characteristics, such as climatic variations, water regimes, and soil types. Studies comparing sandy and sandy loam soils revealed different results for the analyzed parameters [26,48].
The variables OM and P did not show any significant mean differences between continents. Although the P variable was not significantly affected by the application of the sewage sludge, studies conducted in the Americas and Europe showed mean effects of 4.07 and 4.13, respectively, with confidence intervals not intercepting 1. This can be explained by the naturally high acidity and low retention capacity of Brazilian soils, which facilitate greater release and availability of macronutrients compared to soils with lower acidity and higher levels of fixing minerals, such as iron and aluminum [74], while the European studies can be explained by the temperate climate and fertile and overfertilized soils, favoring the retention of phosphorus [75].
The average effects of Zn and Cu levels were also higher in the Americas, with estimated values approximately nine times higher than the average for the control, without sewage sludge application. Notably, these effects are linked to increased application rates. The studies representing the Americas exclusively originate from Brazil, where the soils are highly weathered and tend to be more acidic, fostering greater Cu and Zn availabilities following sludge application [49]. In other regions, even with high doses of applied sewage sludge, the presence of clays, iron oxides, and aluminum can immobilize portions of these metals, reducing their soil availability [45,53].
When the experimental site was considered as a moderator, either in the field or in controlled environments, positive effects were observed on the levels of the variables OM, N, P, Zn, and Cu (Figure 6). The main reasons for these positive effects include the controlled conditions of these environments, which have more stable temperatures and humidity levels, less leaching potential, and, thus, longer soil nutrient availability and reduced N volatility, making the soil less susceptible to wind-driven losses [76].
The only significant difference between controlled environments and field experiments was observed in N levels. In the controlled environments, sewage sludge application resulted in an average N effect ratio of 2.7, whereas in field experiments, this ratio was only 1.84. This difference can be attributed to slower leaching processes and, consequently, reduced N loss in controlled environments compared to field experiments [77].
Although controlled environments generally produced higher average effects for OM, P, Zn, and Cu compared to the field studies, their confidence intervals were very wide and overlapped significantly. This indicates a high degree of uncertainty in the results, making it difficult to identify statistically significant differences between the effects observed in controlled environments and in the field. The incomplete description of environmental conditions (soil properties, temperature ranges, and water nutrient contents) across the selected studies makes it difficult to identify and apply them as moderators that explain the variability in the response ratios.
There were indications of publication bias in all the variables (p < 0.001) except for copper (p = 0.53) (Supplementary Material S3). This is a significant indication that for these variables, there is a preference for the publication of positive results in relation to the application of sewage sludge, especially when the study-filtering criteria are applied. This draws attention to the need for investigating the criteria for study selection and filtering in the scope of the use of residuals in soils. In agronomic research, the presence of publication bias is not uncommon [78].

3.3. Sustainability and Applicative Aspects

This meta-analysis indicates that applying sewage sludge to agricultural soils can significantly increase soil organic matter (SOM) and soil macronutrient (N and P) contents, along with important micronutrient (such as Zn and Cu) contents. However, the environmental sustainability of this practice is complex and influenced by various factors, including application rates, soil properties, climate conditions, and the specific characteristics of the sewage sludge.
The most positive impacts are (i) enhanced soil fertility and productivity, reducing the need for synthetic fertilizers, and (ii) reduced reliance on conventional chemical fertilizers; by providing essential nutrients, sewage sludge can help to decrease the use of synthetic fertilizers, which can contribute to water pollution and greenhouse gas emissions.
However, its application in the soil environment must be carefully considered, taking into account issues such as (i) the potentially toxic element (PTE) concentration—high concentrations of PTEs, like Cu and Zn, in sewage sludge can accumulate in the soil and potentially contaminate groundwater and crops if not managed properly; (ii) the soil salinity—the excessive application of sewage sludge can increase the soil salinity, which can adversely affect plant growth and soil quality; (iii) pathogen and parasite transmission—sewage sludge may contain pathogens and parasites that can pose health risks to humans and animals. Proper treatment and composting can help to mitigate or eliminate these risks.
Further considerations for sustainable sewage sludge application include (i) strict quality control—rigorous testing and treatment of sewage sludge are essential to ensure it meets quality standards and minimizes the risk of contamination; (ii) appropriate application rates—applying sewage sludge at recommended rates can optimize nutrient delivery and minimize potential negative impacts; (iii) regular soil monitoring—a regular soil monitoring program should be recommended to assess the long-term effects of sewage sludge application.
By carefully considering these factors and adopting the best management practices, the environmental sustainability of sewage sludge application can be maximized.

4. Conclusions

The obtained outcomes indicate that sewage sludge application significantly increases OM, N, Cu, and Zn levels in soil, especially at medium and high application rates. The effects are more pronounced in greenhouse experiments compared to field studies, suggesting that environmental factors influence the impacts of sludge application. This study also identifies significant heterogeneity in the results, even after considering factors like the sludge concentration, continents, and experimental conditions. This suggests that other factors not accounted for in the analysis may contribute to the variability in the effects of sewage sludge application. Overall, this research highlights the potential of sewage sludge as a valuable resource for improving soil fertility and nutrient contents. However, it also emphasizes the need for further research to better understand the factors influencing the impacts of sludge application and to develop guidelines for its safe and sustainable use. Indeed, future steps will be focused on (i) identifying the moderators explaining the variations in the parameters and the observed heterogeneity and (ii) including additional parameters (such as K, Cd, and Pb contents) to provide a more comprehensive understanding of the effects of the application of sludge on soils.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/su16229865/s1: Table S1: Selected articles; Table S2: Comparison of parameters; Document S3: Meta-analysis script and publication bias. S4: PRISMA_2020_checklist.

Author Contributions

Conceptualization, A.G. and E.A.L.P.; methodology, A.G., E.A.L.P. and R.B.S.; validation, A.G.; formal analysis, A.G., E.A.L.P. and R.B.S.; investigation, E.A.L.P. and R.B.S.; data curation, E.A.L.P. and R.B.S.; writing—original draft preparation, E.A.L.P. and R.B.S.; writing—review and editing, E.A.L.P., A.G., R.B.S., G.F.C., L.R.R., I.A.G. and E.G.; supervision, A.G. All authors have read and agreed to the published version of the manuscript.

Funding

This work has received funding from the Coordination of Higher Education Personnel Development (CAPES–Print Program), Brazil (Grant Number: 88887.194785/2018-00, 2018).

Data Availability Statement

The data are available on request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Flowchart of the PRISMA approach, illustrating the steps involved in bibliographic searches and the inclusion and exclusion criteria.
Figure 1. Flowchart of the PRISMA approach, illustrating the steps involved in bibliographic searches and the inclusion and exclusion criteria.
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Figure 2. Distribution of articles found by continent at a global scale.
Figure 2. Distribution of articles found by continent at a global scale.
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Figure 3. Distribution of selected articles among countries at a global scale.
Figure 3. Distribution of selected articles among countries at a global scale.
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Figure 4. Applications of different concentrations of sewage sludge (low, medium, and high) as moderators of OM, N, P, Zn, and Cu.
Figure 4. Applications of different concentrations of sewage sludge (low, medium, and high) as moderators of OM, N, P, Zn, and Cu.
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Figure 5. Different continents used as moderators to analyze OM, N, P, Zn, and Cu.
Figure 5. Different continents used as moderators to analyze OM, N, P, Zn, and Cu.
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Figure 6. Different conditions under which studies were conducted as moderators to analyze OM, N, P, Zn, and Cu contents.
Figure 6. Different conditions under which studies were conducted as moderators to analyze OM, N, P, Zn, and Cu contents.
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Table 1. Characteristics of the articles included in this meta-analysis. Variable outcome—parameter analyzed; number of studies—number of studies analyzed for each parameter; number of comparisons—total number of comparisons within the selected studies; SS rate—sewage sludge application rate; environment—field or controlled (greenhouse); continent—continent where the study was conducted.
Table 1. Characteristics of the articles included in this meta-analysis. Variable outcome—parameter analyzed; number of studies—number of studies analyzed for each parameter; number of comparisons—total number of comparisons within the selected studies; SS rate—sewage sludge application rate; environment—field or controlled (greenhouse); continent—continent where the study was conducted.
Variable OutcomeNumber of StudiesNumber of ComparisonsOutcome
SS RateEnvironmentContinent
OM24 [7,11,17,21,26,27,29,30,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53]57Low (25)Field (45)Africa (17)
Medium (16)Controlled (12)America (18)
High (16) Asia (19)
Europe (1)
Oceania (2)
N19 [17,26,27,29,30,38,40,42,43,44,45,46,48,49,50,52,53,54,55]51Low (20)Field (42)Africa (22)
Medium (16)Controlled (9)America (13)
High (15) Asia (11)
Europe (3)
Oceania (2)
P25 [7,11,17,21,26,27,29,38,39,40,41,42,43,44,45,46,47,48,49,50,52,53,54,55,56,57]61Low (25)Field (52)Africa (25)
Medium (20)Controlled (9)America (20)
High (16) Asia (7)
Europe (4)
Oceania (5)
Cu28 [7,11,17,20,21,24,25,26,27,29,30,38,41,42,44,45,47,48,49,53,54,57,58,59,60,61,62,63] 98Low (39)Field (56)Africa (19)
Medium (31)Controlled (42)America (21)
High (28) Asia (44)
Europe (10)
Oceania (4)
Zn24 [7,11,17,21,24,25,26,27,29,38,41,44,45,47,48,49,53,54,57,58,59,60,61,62,63]88Low (35)Field (50)Africa (21)
Medium (28)Controlled (38)America (20)
High (25) Asia (36)
Europe (7)
Oceania (4)
Table 2. Moderator test (MQ) and residual heterogeneity test (EQ) for the five parameters analyzed. OM—organic matter; N—nitrogen; P—phosphorus; Cu—copper; Zn—zinc. The p-values for all the tests were <0.0001.
Table 2. Moderator test (MQ) and residual heterogeneity test (EQ) for the five parameters analyzed. OM—organic matter; N—nitrogen; P—phosphorus; Cu—copper; Zn—zinc. The p-values for all the tests were <0.0001.
Test of Moderators (QM)Test for Residual Heterogeneity (QE)
OMQM (df = 7) = 427.6831QE (df = 48) = 2518.4932
NQM (df = 7) = 339.2645QE (df = 43) = 854.8001
PQM (df = 7) = 238.2578QE (df = 53) = 14,676.6805
ZnQM (df = 7) = 3791.4413QE (df = 80) = 7287.3183
CuQM (df = 7) = 14,121.1364QE (df = 90) = 9102.8543
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Paganini, E.A.L.; Silva, R.B.; Ribeiro Roder, L.; Guerrini, I.A.; Capra, G.F.; Grilli, E.; Ganga, A. A Systematic Review and Meta-Analysis of the Sustainable Impact of Sewage Sludge Application on Soil Organic Matter and Nutrient Content. Sustainability 2024, 16, 9865. https://doi.org/10.3390/su16229865

AMA Style

Paganini EAL, Silva RB, Ribeiro Roder L, Guerrini IA, Capra GF, Grilli E, Ganga A. A Systematic Review and Meta-Analysis of the Sustainable Impact of Sewage Sludge Application on Soil Organic Matter and Nutrient Content. Sustainability. 2024; 16(22):9865. https://doi.org/10.3390/su16229865

Chicago/Turabian Style

Paganini, Enzo Antonio Lecciolle, Rafael Barroca Silva, Ludmila Ribeiro Roder, Iraê Amaral Guerrini, Gian Franco Capra, Eleonora Grilli, and Antonio Ganga. 2024. "A Systematic Review and Meta-Analysis of the Sustainable Impact of Sewage Sludge Application on Soil Organic Matter and Nutrient Content" Sustainability 16, no. 22: 9865. https://doi.org/10.3390/su16229865

APA Style

Paganini, E. A. L., Silva, R. B., Ribeiro Roder, L., Guerrini, I. A., Capra, G. F., Grilli, E., & Ganga, A. (2024). A Systematic Review and Meta-Analysis of the Sustainable Impact of Sewage Sludge Application on Soil Organic Matter and Nutrient Content. Sustainability, 16(22), 9865. https://doi.org/10.3390/su16229865

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