Microplastics in Terrestrial Ecosystems: A Scientometric Analysis

: Microplastics, as an emerging contaminant, have been shown to threaten the sustainability of ecosystems, and there is also concern about human exposure, as microplastic particles tend to bioaccumulate and biomagnify through the food chain. While microplastics in marine environments have been extensively studied, research on microplastics in terrestrial ecosystems is just starting to gain momentum. In this paper, we used scientometric analysis to understand the current status of microplastic research in terrestrial systems. The global scientiﬁc literature on microplastics in terrestrial ecosystems, based on data from the Web of Science between 1986 and 2020, was explored with the VOSviewer scientometric software. Co-occurrence visualization maps and citation analysis were used to identify the relationship among keywords, authors, organizations, countries, and journals focusing on the issues of terrestrial microplastics. The results show that research on microplastics in terrestrial systems just started in the past few years but is increasing rapidly. Science of the Total Environment ranks ﬁrst among the journals publishing papers on terrestrial microplastics. In addition, we also highlighted the desire to establish standards / protocols for extracting and quantifying microplastics in soils. Future studies are recommended to ﬁll the knowledge gaps on the abundance, distribution, ecological and economic e ﬀ ects, and toxicity of microplastics.


Introduction
Over 426 million metric tons (Mt) of plastic products were produced globally in 2018, including 359 Mt of resins, according to PlasticsEurope, and 67 Mt of synthetic fibers, according to The Fiber Year. Plastic production is expected to continue growing in future to meet the improving living standards of the world's population [1,2]. However,~85% of these plastics are not recycled and enter the environment (i.e., ocean and terrestrial ecosystems) [3,4]. Plastics smaller than 5 mm are defined as microplastics [5], and their existence in the marine ecosystem was first reported in the early 1970s [6,7]. Small microplastics have become a big issue drawing global concern [8]. Because they adsorb pollutants or other chemical substances on their surface, microplastics can also be ingested by biota and accumulate in the food chain [8][9][10][11][12]. This is in addition to direct human exposure to microplastic-contaminated air, table salt, and drinking water [13][14][15]. Microplastics are also toxic to other organisms, including animals and plants, and threaten global biodiversity [16,17]. Great efforts have been devoted to studying their occurrence, adverse ecological effects, and toxicity in the marine ecosystem and coastal environment or on shorelines [18][19][20][21][22][23][24][25].
The search returned a total of 877 publications pertaining to microplastics in the terrestrial ecosystem. The publications can be mainly divided into environmental science (number of publications (N = 679), environmental engineering (N = 162), marine freshwater biology (N = 106), water resources (N = 80), and multidisciplinary science (N = 36), according to the Web of Science categories. It was noted that one publication or journal may belong to two or more categories, and the sum of papers in different categories was greater than the actual number of papers. The number of publications on terrestrial microplastics was small compared with the 2882 publications that were focused on microplastics in the marine ecosystem, which was searched with query sets of "TS = ((microplastic* OR nanoplastic*) AND (marine OR ocean OR sea)) AND PY = (1985-2020)" in the WOSCC following an approach similar to that of Pauna et al. [18].
Publications on microplastics in both marine and terrestrial ecosystems have increased rapidly since 2009, but studies on the terrestrial ecosystem took longer to come out than those on the marine ecosystem ( Figure 1). It is noteworthy that the 2012 publication on microplastics in the terrestrial ecosystem written by Rillig [9] started a wave of study on microplastics in terrestrial ecosystems. The number increased exponentially thereafter to 155 papers in 2019, and there were already 366 papers as of 3 October 2020 ( Figure 1). This indicates that terrestrial microplastics have become a hot topic, attracting growing attention. It is expected that publications on this topic will increase remarkably in the near future.

Figure 1.
Annual publication trend of research on microplastics in the marine ecosystem (blue) and terrestrial ecosystem (red). In total, 2882 and 877 publications were distributed from 1998 to 2020 from the Web of Science Core Collection (WOSCC) database for marine and terrestrial ecosystems, respectively.

Co-occurrence Analysis of Keywords
The analysis of co-occurrence of all keywords (in title, abstract, or keyword list) generated 3509 results, and 77 were selected based on the threshold of 20 co-occurrences (Figure 2a), while 2122 results were generated for author-provided keywords, and 99 met the threshold of five occurrences (Figure 2b), according to outputs generated by the VOSviewer based on the WOSCC data. The two scenarios were used to show that researchers usually highlighted the differences in the research status of microplastics in marine and terrestrial ecosystems as research rationale/background. Therefore, they demonstrated different occurrences and total link strength of keywords, such that terms often associated with marine microplastics like "fish", "marine-environment", "ingestion", "sea", "marine", "ocean", and "accumulation" were shown in Figure 2a, but did not appear in Figure 2b.
It was not surprising to note that "microplastics" was the keyword with the strongest total link strength (TLS) for both scenarios, as indicated by its yellow color and larger font size. Only the top five terms in titles, abstracts, and keyword lists and author-provided keyword lists were tabulated ( Table 1). The proximity of keywords indicated their relatedness; the further they were from "microplastics", the more distant the relationship or the less they were studied. For instance, microplastics in "food chain" and "sediments" and "microbial community" were less studied ( Figure 2b) [74,75]. Another example was "sludge" and "sewage sludge", which were closely related to microplastics in the "agroecosystem" (Figure 2b) [76]. Figure 2b also reveals that microplastics may associate or interact with other pollutants such as heavy metals and antibiotics [77][78][79]. Figure  2b also shows that current studies on microplastics in the terrestrial ecosystem focused on their sources (e.g., polyethylene, polyester, microbeads, plastic waste, sewage sludge, sludge), distribution and impact (freshwater, rivers, sediments, microbial community, degradation), transport and fate (e.g., fate, food chain, biota, ingestion, sorption, antibiotics, heavy metals), and analysis (e.g., Raman spectroscopy, Fourier transform infrared-FTIR, quantification, and identification).

Co-Occurrence Analysis of Keywords
The analysis of co-occurrence of all keywords (in title, abstract, or keyword list) generated 3509 results, and 77 were selected based on the threshold of 20 co-occurrences (Figure 2a), while 2122 results were generated for author-provided keywords, and 99 met the threshold of five occurrences (Figure 2b), according to outputs generated by the VOSviewer based on the WOSCC data. The two scenarios were used to show that researchers usually highlighted the differences in the research status of microplastics in marine and terrestrial ecosystems as research rationale/background. Therefore, they demonstrated different occurrences and total link strength of keywords, such that terms often associated with marine microplastics like "fish", "marine-environment", "ingestion", "sea", "marine", "ocean", and "accumulation" were shown in Figure 2a, but did not appear in Figure 2b.
It was not surprising to note that "microplastics" was the keyword with the strongest total link strength (TLS) for both scenarios, as indicated by its yellow color and larger font size. Only the top five terms in titles, abstracts, and keyword lists and author-provided keyword lists were tabulated ( Table 1). The proximity of keywords indicated their relatedness; the further they were from "microplastics", the more distant the relationship or the less they were studied. For instance, microplastics in "food chain" and "sediments" and "microbial community" were less studied ( Figure 2b) [74,75]. Another example was "sludge" and "sewage sludge", which were closely related to microplastics in the "agroecosystem" (Figure 2b) [76]. Figure 2b also reveals that microplastics may associate or interact with other pollutants such as heavy metals and antibiotics [77][78][79]. Figure 2b also shows that current studies on microplastics in the terrestrial ecosystem focused on their sources (e.g., polyethylene, polyester, microbeads, plastic waste, sewage sludge, sludge), distribution and impact (freshwater, rivers, sediments, microbial community, degradation), transport and fate (e.g., fate, food chain, biota, ingestion, sorption, antibiotics, heavy metals), and analysis (e.g., Raman spectroscopy, Fourier transform infrared-FTIR, quantification, and identification).
Initially, the study on microplastics in the terrestrial ecosystem concentrated on its source and occurrence [48,[80][81][82][83]; the transport and fate in soils [84,85] and the soil-plant system [86,87]; and the test, verification, and development of analytical methods [88][89][90]. However, there is still a lack of analytical protocol and monitoring data on the occurrence, abundance, and distribution of microplastics in the terrestrial ecosystem under various climatic environments [26,29]. In addition, more studies should be conducted to investigate the occurrence, risk and toxicity, interactions, transport, and fate of microplastics in the terrestrial ecosystem [91,92]. Studies pertaining to the effects of microplastics are emerging for soil physical properties [92], soil macrofauna (e.g., snail and earthworms) and microbiota [93][94][95][96], plant growth [1,97], and toxicity to animal and human beings [98,99]. Table 1. Occurrences and total link strength (TLS) of top ten keywords in title/abstract/keyword list with 20-occurrence threshold and in author-provided keywords with 5-occurrence threshold.

No. Keyword Occurrences TLS
In title/abstract/keyword list

Citation Network of Authors, Countries and Organizations
A total of 3529 authors contributed to the 877 publications (Figure 3), and 44 authors published a minimum of five documents. They were composed of four clusters (i.e., four colored groups or four groups of authors that worked closely) with a total of 3484 links. The resulting citation network map reveals a high contribution from environmental scientists based on their number of publications (N), links (L), total link strength (TLS), and citations (C) as shown in Table 2. The top ten contributing authors are listed in Table 2, and they generally published eight or more papers in this field. It is noteworthy that Geissen, Huerta Lwanga, and Yang worked or studied in Wageningen University and Research and used polystyrene microbeads, which are now called "microplastics", as model colloids. It should also be noted that eminent researchers from soil physics, such as Keith Bristow, Markus Flury, and Violette Geissen have focused on the transport and fate of microplastics in agroecosystems [51]. However, there is still a call for more input in this field, and the involvement of researchers from multiple disciplines is encouraged to solve the microplastic issues in the terrestrial system with interdisciplinary collaborations [18]. more studies should be conducted to investigate the occurrence, risk and toxicity, interactions, transport, and fate of microplastics in the terrestrial ecosystem [91,92]. Studies pertaining to the effects of microplastics are emerging for soil physical properties [92], soil macrofauna (e.g., snail and earthworms) and microbiota [93][94][95][96], plant growth [1,97], and toxicity to animal and human beings [98,99].

Citation Network of Authors, Countries and Organizations
A total of 3529 authors contributed to the 877 publications (Figure 3), and 44 authors published a minimum of five documents. They were composed of four clusters (i.e., four colored groups or four groups of authors that worked closely) with a total of 3484 links. The resulting citation network map reveals a high contribution from environmental scientists based on their number of publications (N), links (L), total link strength (TLS), and citations (C) as shown in Table 2. The top ten contributing authors are listed in Table 2, and they generally published eight or more papers in this field. It is noteworthy that Geissen, Huerta Lwanga, and Yang worked or studied in Wageningen University and Research and used polystyrene microbeads, which are now called "microplastics", as model colloids. It should also be noted that eminent researchers from soil physics, such as Keith Bristow, Markus Flury, and Violette Geissen have focused on the transport and fate of microplastics in agroecosystems [51]. However, there is still a call for more input in this field, and the involvement of researchers from multiple disciplines is encouraged to solve the microplastic issues in the terrestrial system with interdisciplinary collaborations [18].   (Table 2). It is interesting to note that 8 of the top 10 organizations are from China, which may indicate that China has invested more and more in sustainable environment [100]. In addition, the Chinese Academy of Science (CAS) and the University of Chinese Academy of Science (University of CAS) have close collaborations because the graduate students belongs to the University of CAS, while their supervisors are affiliated with the CAS and some of them may teach in the University of CAS as well ( Table 2).
There were 40 out of 77 countries that published a minimum of five publications on microplastics in terrestrial ecosystems. These countries were grouped into four clusters (Figure 4), where China, the USA, and Mexico had the strongest collaborative relationship based on their joint publications and the proximity of their nodes. Table 2. Top ten authors, countries, organizations, and journals focusing on publications on terrestrial microplastics with indices of number of publications (N), links (L, the number of collaborations or lines between investigated author/country/organization/journal and others), total link strength (TLS), and citations (C). A threshold of five documents was used.

No.
Authors N L TLS C CAS and some of them may teach in the University of CAS as well (Table 2). There were 40 out of 77 countries that published a minimum of five publications on microplastics in terrestrial ecosystems. These countries were grouped into four clusters (Figure 4), where China, the USA, and Mexico had the strongest collaborative relationship based on their joint publications and the proximity of their nodes.

Most Used Journals and Citation Network of Journals
There were 24 out of 178 journals with publications on terrestrial microplastics that met the threshold of a minimum of five publications ( Figure 5). Science of the Total Environment, Environmental Pollution, Environmental Science and Technology, and Water Research and Marine Pollution Bulletin have the strongest citation relationship, as they belong to the same cluster and as evidenced by the thick link between them. Publications with these journals are also highly cited, with over 2800 total citations (Table 2).

Most Used Journals and Citation Network of Journals
There were 24 out of 178 journals with publications on terrestrial microplastics that met the threshold of a minimum of five publications ( Figure 5). Science of the Total Environment, Environmental Pollution, Environmental Science and Technology, and Water Research and Marine Pollution Bulletin have the strongest citation relationship, as they belong to the same cluster and as evidenced by the thick link between them. Publications with these journals are also highly cited, with over 2800 total citations ( Table 2).

Citation Network of Highly Cited Papers
The number of citations of the 877 publications range from 0 to over 1250 times based on the WOSCC database as of 3 October 2020. Of the 877 publications, 68 were cited over 100 times, and the citation network is shown in Figure 6. The bigger the circle of a paper, the more times it was cited. The most cited paper was that of Geyer et al. [2], which described the production, use, and end-of-life fate of plastics produced on a global scale and had 1255 citations; it was followed by a paper by Eerkes-Medrano et al. [101] that reviewed microplastics in freshwater systems and had 588 citations. Many of the other highly cited papers (e.g., with between 300 and 500 citations)

Citation Network of Highly Cited Papers
The number of citations of the 877 publications range from 0 to over 1250 times based on the WOSCC database as of 3 October 2020. Of the 877 publications, 68 were cited over 100 times, and the Sustainability 2020, 12, 8739 9 of 15 citation network is shown in Figure 6. The bigger the circle of a paper, the more times it was cited. The most cited paper was that of Geyer et al. [2], which described the production, use, and end-of-life fate of plastics produced on a global scale and had 1255 citations; it was followed by a paper by Eerkes-Medrano et al. [101] that reviewed microplastics in freshwater systems and had 588 citations. Many of the other highly cited papers (e.g., with between 300 and 500 citations) generally pertain to microplastics in freshwater [29,[102][103][104][105]. Rillig's seminal paper [9] that initiated the study of microplastics in terrestrial ecosystems is a perspective paper that is not included in the database but is also highly cited (>400 citations). Rillig and Lehmann [5] highlighted the shifts in microplastic studies from ecotoxicology to ecosystem effects and feedbacks, including effects on soil properties [92,106] and soil biota [107,108].

Citation Network of Highly Cited Papers
The number of citations of the 877 publications range from 0 to over 1250 times based on the WOSCC database as of 3 October 2020. Of the 877 publications, 68 were cited over 100 times, and the citation network is shown in Figure 6. The bigger the circle of a paper, the more times it was cited. The most cited paper was that of Geyer et al. [2], which described the production, use, and end-of-life fate of plastics produced on a global scale and had 1255 citations; it was followed by a paper by Eerkes-Medrano et al. [101] that reviewed microplastics in freshwater systems and had 588 citations. Many of the other highly cited papers (e.g., with between 300 and 500 citations) generally pertain to microplastics in freshwater [29,[102][103][104][105]. Rillig's seminal paper [9] that initiated the study of microplastics in terrestrial ecosystems is a perspective paper that is not included in the database but is also highly cited (>400 citations). Rillig and Lehmann [5] highlighted the shifts in microplastic studies from ecotoxicology to ecosystem effects and feedbacks, including effects on soil properties [92,106] and soil biota [107,108].

Conclusions and Perspectives for Future Studies
The global scientific literature on microplastics in the terrestrial ecosystem was explored with scientometric software (i.e., VOSviewer), based on data from the Web of Science Core Collection. The small number of publications (N = 877) and considerable increase in the number of annual publications indicate that this is an emerging research field. It is drawing growing attention, and more publications on this topic are expected in the coming years. This study identified the top authors, organizations, countries, and journals focusing on terrestrial microplastics. The most influential publications on this topic were also analyzed through the citation network of papers. The scientometric method provided a useful tool for conducting comprehensive reviews.
Compared to the marine ecosystem, issues of microplastics in terrestrial ecosystems and soils are usually ignored, given the fact that they might be the main source for plastics emissions to rivers and oceans [38,102,109]. Considering the low recycling rates (i.e.,~15%) for plastic products, disseminating recycling technology and improving the demand for recycled plastics are the keys to reducing the source of microplastics entering the environment. There is a desire to develop reliable equipment and to establish standards/protocols for extracting and quantifying microplastics in soils [38]. Researchers with an interdisciplinary background are encouraged to work on terrestrial microplastics. For instance, accurate, sensitive, cost effective, and harmonized detecting methods and high-throughput sample processing are required for a better understanding of the transport, fate, and transformation of microplastics in soils. Future research should address the knowledge gap on the abundance, distribution, magnitude of ecological and economic effects, and toxicity of microplastics in drylands, deserts, grasslands, forests, and tundra, in addition to the agricultural system and freshwater, which receive the most attention. Furthermore, stricter measures should be adopted to control the use of plastic products. Although biodegradable polymers are assumed to be an alternative in agriculture, their risk should also be assessed, considering the difficulties in removing the plastic waste from soils. Moreover, studies are currently mostly laboratory-based. Studies that investigate microplastics in a natural environment, with and without controlled conditions, are needed. International cooperation in microplastic research is needed, as microplastic pollution is an international problem of mounting concern.

Conflicts of Interest:
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.