1 Introduction

The production of plastics has increased significantly since the nineteenth century, rising from 1.5 million tons in the 1950s to 368 million tons in 2019 [1] which has been further projected to reach 1.2 billion tons by 2050 [2]. The rapid increase in plastic production has led to the generation of considerable amount of waste. Reports indicate that in 2015, 55% of plastic products were discarded, 25% recycled, and 20% destroyed. It has been predicted that the waste volume will increase by nearly 13% by 2060 [3]. The discarded plastics degrade into microplastics (MPs), ranging from 1 to 100 mm. The European Union alone is producing 75,000–300,000 metric tons of plastic debris annually, impacting various ecosystems and environmental components [4,5]. Information is available on how MP pollution has emerged as potent threat to the oceans, freshwater bodies, and soils at a global scale [6]. The impact of MPs on soil organisms varies based on factors such as particle size, shape, and polymer composition [7,8]. However, the ecological interactions play a crucial role in determining the fate and effects of MPs on terrestrial ecosystems [9]. The research on MPs has been directed towards several key aspects, such as examining the exposure of MPs to soil microbial communities, understanding bioaccumulation processes, assessing soil pollution levels, investigating oxidative stress responses, and evaluating heavy metal contamination [10,11,12]. None the less, understanding the dynamics of MP in soil requires knowledge of the soil's biodiversity and ecology. Research over the past decade has extensively explored the ecotoxicological effects of MPs on earthworms, vital contributors to soil ecosystem processes [13,14]. Earthworms play a crucial role in agricultural vermiremediation, improving soil ecosystem health [15]. They also act as biofilters, bioindicators, bioaccumulators, and transformers of MPs and other pollutant hydrocarbons [16,17]. Recent reports have indicated how the MPs alone and in combination with other soil contaminants impact the growth, behavior, avoidance and reproduction in diverse earthworm species [13,18]. Microorganisms in earthworms' gut help in the fixation, accumulation, and transformation of these toxicants, mitigating their impact [19]. MPs can influence the bioaccumulation of other contaminants in earthworms, such as heavy metals and organic chemicals [20,21]. Polyethylene and polystyrene, the predominant forms of MPs induce histopathological damage in earthworms and serve as carriers of complex pollutants via adsorption–desorption mechanisms [22,23,24]. It has been reported that MP toxicity induced oxidative stress, resulting in neural and DNA damage in earthworms [25]. Previous studies indicate that MPs can impair earthworm spermatogenesis, alter gut bacterial communities, and exhibit minimal short to long-term toxicity on earthworms [10,26]. It is noteworthy that surpassing MP content of 0.1% in soil detrimentally affects earthworm growth and triggers oxidative stress [27,28]. In contrast, Mondal et al. found that the epigeic earthworm Eisenia fetida exposed to low-density polyethylene (LDPE) MPs (≤ 125 μm) at concentrations ranging from 0 to 3% w/w showed no mortality or notable weight changes [29]. Nevertheless, elevated MP concentrations have been correlated with increased ingestion rates and skin damage, indicating potential long-term growth implications [29,30]. Both MP fibers and nanoplastics can be ingested by earthworms, affecting egestion rates and potentially accumulating in their tissues over time [18]. Earthworms show a preference for ingesting smaller MP particles, particularly those under 50 μm [13]. Their burrowing activity can facilitate the leaching of MPs into deeper soil layers and potentially into groundwater systems [9]. Higher concentrations of MPs tend to have more severe effects on earthworms [10,11,12,13]. These findings highlight the significant role of earthworms in MP distribution and the potential environmental risks associated with MP pollution in soil ecosystems. While numerous studies have explored the ecological risk posed by MP toxicity to earthworms, a systematic review of the literature is still absent. To explore the biological effects of the MPs notably their toxicity on soil organisms including earthworms, it is necessary to summarize the results from the previous studies. This review employs bibliometric analysis to delve into extensive scientific data, helping in understanding the evolutionary trends within a specific field and highlighting emerging areas. Despite numerous publications in this domain [10,22,24,27], there has not been an attempt to conduct bibliometric analysis on the relationship between microplastics, soil, and earthworms. By elucidating the interplay of keywords and evaluating the research impact, this review seeks to provide a more comprehensive understanding of the environmental risks associated with MP pollution in soil ecosystems.

1.1 Search strategy and data sources

Even though several workers across the globe have been working on the impact of MPs on soil biotas including earthworms, there is a lack of systematic description on the research trends in the field. Therefore, we tried to find out the hierarchical development of the field and current research hotspots using the Scopus core dataset for visualization and analysis of data from the existing literature. The key words selected were “microplastic”, “soil”, “biota”, and “earthworms” to retrieve relevant papers from the Scopus database. The type of documents was open and there was no restriction on the year of publication, author name, subject area, document type, source title, source, and language. About 38 records were obtained and the data set was exported to files in the “Comma Separated Values (.csv)” format. The extracted record was analyzed by Scopus database (https://www.scopus.com) for the period from 2017 to 2023.

1.2 Global research trends

A bibliometric analysis of scientific publications on MPs and their impact on earthworms was conducted from 2017 to 2023. The analysis revealed a steady increase in the number of publications on MPs and earthworms up to 2022 (Table 1). The data further indicated that China was the first country to conduct research based on the selected keywords followed by India and Australia (Fig. 1).

Table 1 Year wise distribution of Documents
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Fig. 1
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Number of articles published by different countries of the world by 2023, considering microplastics, soil and earthworms as keywords in Scopus database

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The number of publications increased towards 2019, the trend continued in 2020 and 2021 and reached the maximum in 2023 (Fig. 2). Duarte and Pereira have published the highest number of articles in Scopus followed by Mishra and the group [24,31,32]. The VOS viewer software, version 1.6.20, was employed for the visualization of data and the execution of cluster analysis procedures. The keywords and co-author references of the published literature were analyzed by mapping social networks [33].

Fig. 2
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Number of articles published per year considering microplastics, soil and earthworm as keywords in Scopus database

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In bibliometric analysis, the research hotspots are reflected in two-dimensional graphics [34,35]. Specifically, in the VOS viewer, the created social network map shows nodes with varying thickness. The frequency of occurrence is shown by the node sizes, and the strength of the association between nodes is presented by the thickness of the lines between the nodes [24]. In prior bibliographic analyses concerning the ecological interaction of MPs, the extensively cited literature delves into the intricate relationship between MPs and earthworms, exploring the adaptation and survival of earthworms in response to pollutants and polyester fibers [36,37,38]. Nevertheless, the accumulation and biological impacts of organic pollutants and heavy metals within earthworms may undergo alteration due to MPs via the adsorption–desorption process [24,39]. However, our review of the highly cited literature revealed only five studies addressing this phenomenon.

1.3 Keyword co-occurrence and co-authorship analysis

Co-occurrence and keyword analyses were conducted to visualize the correlation and evolution of various research themes using bibliometric data (Fig. 3). Full counting methodology was implemented, with a minimum threshold of 5 occurrences for a keyword to be considered. Among 866 keywords, 65 met this criterion. Centrality measures the extent to which a specific node lies on the shortest path between other nodes. A higher centrality value signifies a stronger influence of that particular node.

Fig. 3
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Keyword co-occurrencemap of studies published until December 2023 focusing on the interactions between microplastics and earthworms. The size ofthe nodes indicates the frequency of occurrence, and the thickness of the lines between the nodes indicates the strength of the association

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The analysis identified the top 10 keywords with high centrality, namely "microplastics," "soil," "pollution," "earthworm," "exposure," "accumulation," "toxicity," "oxidative stress," and “ecosystem". Among them, three clusters were identified, comprising 1855 links, which underscore keyword groups emphasizing the connection among " microplastics," "soil," and "earthworms." Notably, " microplastics " and "earthworm" emerged as research hotspots, exhibiting the highest frequency of occurrence. This observation suggests that research in this domain draws upon a multitude of disciplines, spanning engineering, metabolomics, and microbiology. Moreover, the analysis unveiled the interconnectedness of research trajectories, underscoring the significance and present developmental status of diverse research domains within this field.

Co-author analysis was conducted to visually depict the correlation and evolution of various research themes using bibliometric data (Fig. 4).

Fig. 4
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Co-author map of studies focusing on the interactions between microplastics and earthworms, published until December 2023. The frequency of occurrence is represented by the size of the node, and the strength of the association is shown by the thickness of the lines between the nodes

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Each document involved a maximum of 25 authors, with at least one document attributed to each author. A total of 209 authors met these criteria, resulting in the identification of the largest connected set comprising 14 items, with two clusters and 55 links observed. Additionally, the interrelationship among countries concerning the same research domain is illustrated in Fig. 5, with 21 countries meeting the threshold criteria, each contributing at least one document.

Fig. 5
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Number of documents produced in different regions or countries of the world considering microplastics in soil and earthworms as keywords in Scopus database

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Four clusters with 13 links were identified. Furthermore, citation analysis of documents revealed 38 documents meeting the threshold, organized into 11 clusters with 27 links (Fig. 6 and Table 2).

Fig. 6
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Citation analysis represented by overlay visualization map

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Table 2 Cited literature on microplastics and their impact on soil biota
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Rodriguez-Seijo et al., placed within cluster 3, exhibited a higher citation count relative to other documents [40]. The analysis revealed that the most representative types of MPs studied in relation to earthworm toxicity were polyethylene and polystyrene. In a previous section of the bibliography, a group of researchers disclosed their findings from a keyword co-word analysis, which categorized keywords into two principal themes: the implications of adsorption–desorption properties of MPs on the accumulation and bio-effectiveness of other contaminants in earthworms, and the adverse effects of ingested MPs on earthworms [24]. Furthermore, authorship distributions over the years represent a critical aspect of scholarly output and collaboration dynamics within scientific communities. The analysis of authorship trends provides insights into the evolution of research networks, collaboration patterns, and the dissemination of scientific knowledge over time. Through an analysis of author distribution across various publication years, the data unveiled a higher prevalence of publications involving multiple authors as opposed to joint or single-author contributions (Table 3).

Table 3 Authorship distribution of published documents
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This indicates emerging interdisciplinary collaborations, and the impact of individual researchers or research groups within their respective fields. The frequency of documents published across various journals and books utilizing specified keywords are mentioned in Table 4.

Table 4 Published documents in different journals and books
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The vertical column denotes the sources of the published literature, while the horizontal row signifies the number of manuscripts published in these sources across different years, spanning from 2017 to 2023. Following data collection, a time series analysis was conducted based on the annual publication counts, as demonstrated in Table 5. Notably, there is a consistent upward trend in the number of publications each year. The peak volume of articles was observed in 2023. Particularly noteworthy are the journals "Environmental Pollution" and "Science of The Total Environment" which have contributed significantly to this body of literature. "Environmental Pollution" has contributed to this field annually, except in 2022, while "Science of The Total Environment" has maintained continuous publication from 2021 to 2023. The cumulative count of published articles across 20 different journals and books until 2023 amounts to 38, denoted as "y" in Table 5.

Table 5 Time series analysis
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1.4 Time series analysis and forecasting for microplastic literature growth

The time series is a sequence taken at consecutive equally spread-out points in time. Time series analysis comprises methods for analyzing the data to extract meaningful statistics and other characteristics. Time series forecasting is the use of a mathematical model to predict future values based on the previously observed values.

In this study, time series analysis has been applied to forecast the growth of microplastic literature. The table number 5 shows the time series analysis of 7 years, from 2017–2023. The number of papers published is represented as “y” and “x” is the deviation from the median value which is 5. To arrive at the assessment for future growth, straight line equation is applied.

The straight-line equation is

$$Y=a+bX$$

where, Y= number of literatures published in future,

$$a=\frac{\sum y}{N},b=\frac{\sum xy}{\sum {x}^{2}}$$

Here, y is number of papers published each year, N is maximum number of papers published and x is the deviation.

since Σx = 0,

$$a\, = \,\frac{\sum y }{N}\, = \,\frac{38}{7}\, = \,5.429,$$
$$b=\frac{\sum xy}{\sum {x}^{2}}=\frac{45}{28}=1.607$$

Now applying the values in the straight-line equation, we can find the estimated literatures in 2030.

Here, X = The time duration between 2030–2019 which is 11 years.

Putting the values of a, b and X in the straight-line equation we find,

$${\text{Y }} = { 5}.{429 } + \, \left( {{1}.{6}0{7 }*{ 11}} \right) \, = { 23}.{1}0{7}$$

From the results, it is evident that there is an increasing trend of research literature in the future years. Here, the time series predicts the future trend in upcoming 11 years. This increasing trend signifies the future scope and availability of more literature resources in the mentioned area and has been depicted in Fig. 7.

Fig. 7
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Time series analysis and forecasting for microplastic literature growth

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1.5 Future research perspectives

The results of majority of research on MP-earthworm interactions are from laboratory short duration experiments in controlled environmental conditions. In the natural environment, the soil properties and earthworm types are widely variable so also the environmental factors which could impact the interaction of MPs with other potent contaminants and their intake by the earthworms. The epigeic, endogeic and anecic earthworms too could respond differently to these contaminants since they inhabit different soil strata. It will also be interesting to observe the fate of the adsorption–desorption of MPs with pesticides, heavy metals and other contaminants in the field conditions over a long period of time. Whether long term exposure of earthworms to the MPs and the co-contaminants in the soil environment could cause irreparable DNA damages need to be explored further. Therefore, future studies should focus more on field experiments with emphasis on the long-term impact of MPs in soil with or without co-contaminants on earthworms inhabiting diverse terrestrial habitats with a wide range of environmental variables.

2 Conclusions

The current review delves into the temporal dynamics of MP research, spanning 2017–2023, employing bibliometric and time series analyses. A meticulous scrutiny of Scopus data underscores escalating global contributions, with a preeminence from China. VOS viewer elucidates thematic clusters, spotlighting interrelations among "microplastics," "soil," and "earthworms." The nuanced examination of cited literature unveils divergent research focus. Concurrently, time series analysis prognosticates sustained growth in MP literature, yielding an estimated 23.107 publications by 2030. This integrative methodology not only discerns current MP research paradigms but also prognosticates future trajectories, serving as a roadmap for environmental science stakeholders and policymakers in this burgeoning domain.