Global research on soil contamination from 1999 to 2012: A bibliometric analysis

Abstract

We evaluated soil contamination research based on a bibliometric analysis of 14,090 articles published in journals in the Science Citation Index and Social Sciences Citation Index bibliographic databases from 1999 to 2012, which revealed scientific outputs, subject categories and major journals, international collaboration and geographic distribution of authors and countries, keywords, and hot issues. The results suggested that research on soil contamination developed well with increasing scientific production and research collaboration. Environmental science, engineering environment, soil science, and applied microbiology were the most frequently used subject categories in soil contamination studies. Chemosphere was the most active journal in this field. The clusters of authors were more in the USA, Western European countries, China, Japan, and India. Q. X. Zhou of Nankai University was the most productive author, and S. P. Mcgrath of Rothamsted Research England published the most influential articles. The USA exceeded all other countries with the most independent and collaborative papers in research on global soil contamination. Heavy metal pollution was the hottest issue, and bioremediation is the most promising research topic in combating against heavy metal pollution of soils. The status of publication on soil contamination research described here is significant for researchers on soil contamination in their future work.

Keywords:

• soil contamination
• bibliometrics analysis
• geographic distribution
• hot issues

Introduction

Soil contamination is becoming a global problem coupled with rapid industrialization and urbanization, unbridled mining and emissions, uncontrolled discharge of waste water, application of sewage irrigation, and long-term use of pesticides over the last few decades, both in developed and developing countries (Kirchmann & Thorvaldsson 2000; Li & Thornton 2001; Liu & Diamond 2005). Soil contamination is defined as the build-up in soils of persisted toxic compounds, chemicals, salts, radioactive materials, or disease causing agents, which have adverse effects on plant growth and animal health (Patra & Sharma 2000; Semple et al. 2001; Bastida et al. 2008; Khan et al. 2008). Loss of soil functions is detrimental to man and the environment, including damaged control of substance and energy cycles as compartment of ecosystems (Myers et al. 2001; Muller et al. 2002; Frey et al. 2005); threat to the life of plants, animals, and man (Duruibe et al. 2007; Rodrigues et al. 2012); unsafe agriculture production and food (McLaughlin et al. 1999; Fischer et al. 2009); movement of contaminants or other agents into groundwater (Jia et al. 2006; Shi et al. 2011; Aleksander-Kwaterczak & Ciszewski 2012); and disturbance of soil microbial community and microbial processes (Kandeler et al. 2000; Girvan et al. 2005). Therefore, research on soil contamination has been highlighted in various environmental research fields.

The existing studies on soil contamination involve the potential mechanism of soil contamination (O'Reilly et al. 2001; Li et al. 2006; Khan et al. 2010), radioactive contamination of soil (Papp et al. 2002), treatment of soils contaminated with organic pollutants (Alkorta & Garbisu 2001; Semple et al. 2001), spatial distribution for sources of contaminated soils (Einax & Soldt 1999; Facchinelli et al. 2001; Shivaramaiah et al. 2002), techniques for monitoring contaminated areas (Stenberg 1999; Kooistra et al. 2003; Rathod et al. 2013), human health risks associated with soil contamination (McLaughlina et al. 1999; Türkdogan et al. 2003; Khan et al. 2008), lost arable land and food security (Jansson & Oborn 2000; Zhao et al. 2011; Liu et al. 2013), conservation of contaminated soils (Soares & Siqueira 2008; Scanferla et al. 2009), advanced methods of soil remediation (Chen et al. 2000; Margesin et al. 2000), and heavy metal pollution (Peters 1999; Smolders et al. 1999; Sun et al. 2001; Evangelou et al. 2007), with the last topic being the mainstream of current research. Investment in soil contamination research is considered a major means for facilitating the control of environmental quality and food security, and the protection of human health, and for limiting the loss of arable land.

It can be seen that research on soil contamination is abundant in quantity and rich in achievement. A systematic review of literature in studies on soil contamination would be helpful for us in understanding the achievements by scientists in this field, reveal research trends in the world, identify research directions in the future, raise public awareness of soil contamination, and facilitate research for pollution control (Mermut & Eswaran 2001; Zhao et al. 2011). Summaries have been published on the development of soil contamination studies, such as outlines of genotoxicity assays (Watanabe & Hirayama 2001; Proctor et al. 2002; White & Claxton 2004), accumulation of heavy metals (Rascio & Izzo 2011; Boyd 2009), bioremediation of soil contaminated with heavy metals (Diels et al. 1999; Kramer & Chardonnens 2001; Lasat 2002; Clemens et al. 2002; McGrath & Zhao 2003; Kramer 2005; Vamerali et al. 2010), mechanism of soil damage by organic pollutants (Semple et al. 2001; Meharg 2003; Ehlers & Loibner 2006), occurrence of semivolatile organic chemicals (Cai et al. 2008), attenuation mechanisms for soil contaminants (Yong & Mulligan 2004; Wang & Mulligan 2006), and applications of geochemistry (Rao et al. 2008; Wilson et al. 2008). However, no bibliometric review of the global research on soil contamination has not been tried, which may help further understanding research on soil contamination.

The bibliometric method has been widely applied in various fields to summarize the research trends by investigating the publication characteristics, including publication output; category of subjects and journals; distribution of authors, countries and research institutes, and keyword frequencies, etc. (Almeida-Filho et al. 2003; Chen et al. 2005; Chiu & Ho 2007; Liu et al. 2012). The current bibliometric research tends to focus on the international collaboration between authors, institutions and countries (Glanzel 2000; Zhuang et al. 2013), on academic exchange between different disciplines (Liu et al. 2012), and on geographic distribution of authors (Liu 2011; Wang et al. 2012). Bibliometric analysis provides a series of visual and quantitative parameters that help to generalize the patterns and dynamics in the selected scientific research fields (Pritchard 1969).

In this research, we employed a bibliometric analysis of published studies on soil contamination from 1999 to 2012 in the hope of providing a better understanding of research on soil contamination in the world. The topic was dealt in the following aspects: document types and languages, publication outputs, subject categories and major journals, author productivity, geographic distribution of publications, publications and GDP and population, and keywords.

Data and methods

We built our bibliometric database of “soil contamination” based on the Science Citation Index (SCI) and Social Sciences Citation Index (SSCI) bibliographic databases. A keyword search was performed to identify “soil contamination”-related studies published between 1999 and 2012, using the following flexible retrieval conditions: “soil contaminat*” or “soil pollut*”or “pollut* soil” or “contaminat* soil” or “soils contaminat*” or “soils pollut*” or “pollut* soils” or “contaminat* soils,” to locate publications that contained these words in their titles, abstracts, or keyword lists. These searching terms are designed based on previous reviews of “soil contamination” and related published researches (Wilcke 2000; Kramer & Chardonnens 2001; Semple et al. 2001; Horrigan et al. 2002; McGrath & Zhao 2003; Kabra et al. 2004; Kramer 2005; Kavamura & Esposito 2010). We did not use the search term of “soil*,” as it would contain some derived meaning such as “soiled,” “soilage,” “soiling,” and “soilless”; but we adopted the keyword “soils” since it represents different types of soil. Using the above-mentioned searching strategy, a total of 16,075 papers were obtained in the SCI and SSCI databases which were soil contamination-related. Following the conventions used in other bibliometric studies, we restricted our further analysis to articles (14,090), which are peer-reviewed and represent original scientific development.

These articles published in 1,237 SCI and SSCI-indexed journals and covered 151 ISI identified subject categories in the SCI and SSCI databases in the period of 1999–2012. Articles originating from England, Scotland, Northern Ireland, and Wales were reclassified as being from the UK. Because of some inconvenience on data collecting, articles from Hong Kong, Macau, and Taiwan were counted separately (Ho 2007; Wang et al. 2012). Based on the author addresses, there were 131 countries/regions with soil contamination research. Among the 30,990 authors who (co)authored these articles, in which 13,711 or 97.3% of the total journal articles were in English. Collaboration type was determined by the addresses of the authors, where the term “single country” was assigned if the authors' addresses were from the same country; “international collaboration” was designated to those articles that were by authors from multiple countries (Zhang et al. 2010). In determining collaborated works among authors, institutions, or countries, each signatory on publications was treated equally. Applying NetDraw (Borgatti 2002; Liu et al. 2011) to the network centralities in the collaboration network of countries/regions, we visualized a core group of countries in the collaboration network. The impact factors (IF) were taken from the Journal Citation Report (JCR) published in 2012, which was the latest data available. The IF was used to evaluate a journal's relative importance, especially when compared with other journals in the same field. We then plotted the geographic distribution of authors using CiteSpace (Chen 2004) and ArcGIS software, and using the data of the number of articles for individual countries as background for a comparative analysis. We also used regression analysis for the relationship between the number of papers and GDP and population. Despite our efforts, some articles were missed because the full address was not registered or because of the numerical errors without identification. An analysis of 21,386 keywords were listed, 15,395 (71%) of which were used only once, and 2,778 (13%) were used twice.

Results and discussion

Document types and languages

Among the total 16,075 publications, 10 document types were identified: peer-reviewed journal articles, proceeding papers, reviews, abstracts of conference papers, editorial materials, corrections, book chapters, news items, letters and reprints, with the peer-reviewed journal articles being the most published (14,090), accounting for 87.7% of the total publications. Following the conventions used in other bibliometric studies, we adopted the data of the peer-reviewed journal articles for analysis. The data of peer-reviewed journal articles here indicate research on soil contamination has been a hot topic in the world, as was agreed to by other researchers (Cheraghi et al. 2012; Hu et al. 2012).

A total of 13,711 or 97.3% of the total journal articles were in English. This observation was consistent with the fact that English is the international academic language and that most SCI and SSCI-indexed journals are published in English (Liu et al. 2011). Authors have to write in English to have their research published. Because of a large number of articles published in English, the data here also seem to confirm the above interpretation that research on soil contamination has been a popular research topic in the world.

Publications outputs

The trend of publications on soil contamination in the period of 1999–2012 is shown in Figure 1. The number of papers increased yearly from 1999 to 2012. The increasing interest in research on soil contamination was derived the increasing global awareness of environmental degradation resulting from various soil contaminations. Soil contamination is the consequence of fast global economic development embodied in unlimited industrialization and urbanization (Cai et al. 2008; Zeng et al. 2014). Therefore, soil contamination became the focus of attention in the academia (Alexander 2000; Sun et al. 2012). Great importance has been attached to the issues of soil contamination in China (China SEPA 2003; Zhao et al. 2011), e.g., China's Ministry of Environment and Ministry of Land and Resources conducted a national soil pollution survey from 2006 to 2010 (2006), the Chinese State Council (2013) has planned to comprehensively investigate soil environment in the country by 2015, monitor 60% of its arable lands on a regular basis, and endeavor to establish a national soil environmental protection system by 2020. The similar situation can be found in Europe (Schwitzguebel et al. 2009; Desaules et al. 2010) and other countries in the world.

Figure 1. Scientific outputs descriptors during 1999–2012.

Table 1 confirms Figure 1 by providing more information besides the annual increase in the number of papers published. The number of authors per paper increased from 3.02 to 3.74. We understand that this increase is probably caused by higher level of research involving more scholars from different disciplines. The number of articles cited per paper increased from 28.09 to 41.24 in the period of 1999–2012, which, as we observed, was brought about by more and more abundant results of research on soil contamination; citing from other related disciplines is an additional increase in the number of articles cited. The last interesting fact is the earlier the articles were published, the higher their times of being cited, and with the articles published in 1999 enjoying the highest rate of citation of 29.37. This result suggests that soil contamination remained a hot topic of research in the period of 1999–2012. The articles on soil contamination have a long half-life.

Table 1. Scientific outputs descriptors during 1999–2012.

Year	TP	AU	AU/TP	PG	PG/TP	NR	NR/TP	TC	TC/TP
1999	514	1,551	3.02	5,155	10.03	14,438	28.09	15,096	29.37
2000	522	1,616	3.10	5,031	9.64	15,214	29.15	16,310	31.25
2001	561	1,739	3.10	4,627	8.25	14,909	26.58	14,438	25.74
2002	594	1,935	3.26	6,449	10.86	18,156	30.57	15,223	25.63
2003	733	2,342	3.20	7,351	10.03	22,904	31.25	19,001	25.92
2004	760	2,477	3.26	7,279	9.58	24,608	32.38	18,024	23.72
2005	883	2,866	3.25	8,265	9.36	28,764	32.58	18,409	20.85
2006	1,022	3,463	3.39	9,757	9.55	35,015	34.26	17,485	17.11
2007	1,105	3,666	3.32	9,896	8.96	37,014	33.50	15,248	13.80
2008	1,303	4,573	3.51	11,410	8.76	44,933	34.48	13,626	10.46
2009	1,401	4,909	3.50	12,072	8.62	50,523	36.06	11,503	8.21
2010	1,422	5,118	3.60	12,707	8.94	54,994	38.67	7,967	5.60
2011	1,599	5,866	3.67	14,709	9.20	64,712	40.47	5,423	3.39
2012	1,671	6,243	3.74	15,268	9.14	68,919	41.24	1,538	0.92
Total	14,090	48,364		281,887		495,103		189,291
Average			3.35		9.35		33.52		17.28

Note: TP, number of publications; AU, number of authors; PG, page count; NR, cited references; TC, total citation counts; AU/TP, PG/TP, NR/TP, and TC/TP, average of authors, pages, references, and citations in a paper, respectively.

Subject categories and major journals

Figure 2 shows that published papers on soil contamination covered 151 ISI identified subject categories in the SCI and SSCI databases in the period of 1999–2012. The four most common categories were environmental sciences (7,448 articles; accounting for 52.9% of the total), environmental engineering (2,017; 14.3%), soil science (1,518; 10.8%), and applied microbiology (1,447; 10.3%). These top categories also suggest a high priority of ecological and environmental issues in soil contamination research. The current global environmental pollution is very severe, and soil pollution is one of the most serious threats to the human beings. Therefore, issues on soil contamination have been the most concerned, and scientists and researchers naturally respond to this social demand by doing the greatest amount of research on these issues. The governments in the world have attached great importance to controlling soil contamination by sponsoring more and more research programs on soil contamination, with the Chinese government being the good example (China SEPA 2006; Khan et al. 2008; Song et al. 2012; Liu et al. 2013).

Figure 2. Annual growth rates of articles in the most active subject categories.

We summarized the annual growth rates of articles in the top four subject categories from 1999 to 2012 in Figure 2. The annual growth rates of articles in Applied Microbiology were the highest for seven times during the period, because bioremediation is now the most popular technology in dealing with soil contamination, with the microbial technology being the most applied. For example, bacteria, proteins, and enzymes are the most used means of soil remediation indicating that biotechnology is most widely applied in research on soil contamination. Technique of genetic engineering in microbes is easier and more mature than in plant cells, and, using transgenic technology to create an optimum plant + soil + microbes combination would be a promising way in the future development (Karen et al. 2009; Wu et al. 2010).

Table 2 shows that, among the 1,237 SCI and SSCI-indexed journals that publish articles on soil contamination, 20 journals published the largest number of studies on soil contamination in the period of 1999–2012, with their total publications, total citations, and IF all ranking the top 20. This means that the editors of these journals are the most interested in soil contamination-related studies and research articles submitted to these journals would be most likely accepted. Chemosphere and Journal of Hazardous Materials are the two journals that published the largest numbers (736 and 687) of articles on soil contamination, implying that these journals welcome the submission on soil contamination the most. Incidentally, we found that, 265 of the 736 articles and 282 of the 687 articles (data not shown) were on heavy metal pollution, which is a significant hint for people doing research on heavy metal pollution. Also, 152 (20.65 of the total) articles in Chemosphere and 201 (29.29 of the total) articles Journal of Hazardous Materials were written by Chinese authors. The articles by Chinese authors were the most accepted obviously because environmental pollution in China is the most serious in the world and therefore the most studied, and their research results are the most instructive. Applied and Environmental Microbiology enjoyed the highest total citation (6,126). This confirms the observation in the last paragraph that microbial technology is the most applied in research on soil contamination. The journal of Environmental Science & Technology received the highest IF (5.257), which means that articles published in this journal have the highest academic value and were the most referred to in the academia. For instance, some articles contained the most advanced techniques of research, such as transgenetic technology, infrared spectroscopy analysis, and remote sensing monitoring, to be applied by other researchers in the same field (Ruiz et al. 2003; Lee et al. 2006; Chakraborty et al. 2010; Lourenco et al. 2010; Stenberg et al. 2010).

Table 2. Journals publishing soil contamination articles.

Journal title	TP	TP (%)	TC	TC/TP (ranking)	IF (ranking)	ISI category
Chemosphere	736	5.23	15,096	20.51 (7)	3.137 (8)	Environmental Sciences
Journal of Hazardous Materials	687	4.86	8,553	12.45 (11)	3.925 (3)	Environmental Sciences
Environmental Pollution	555	3.94	14,146	24.49 (5)	3.73 (4)	Environmental Sciences
Environmental Science & Technology	497	3.53	15,569	31.33 (2)	5.257 (1)	Environmental Engineering
Water Air and Soil Pollution	420	2.98	4,125	9.82 (14)	1.748 (15)	Meteorology & Atmospheric Sciences; Water Resources
Science of the Total Environment	400	2.84	7,940	19.85 (8)	3.258 (7)	Environmental Sciences
Environmental Toxicology and Chemistry	217	1.54	3,289	16.54 (10)	2.618 (10)	Environmental Sciences; Toxicology
Journal of Environmental Quality	208	1.48	5,306	25.51 (3)	2.353 (11)	Environmental Sciences
International Journal of Phytoremediation	207	1.47	1,527	7.38 (15)	1.179 (18)	Environmental Sciences
Soil & Sediment Contamination	195	1.38	1,127	5.78 (17)	0.509 (20)	Environmental Sciences
Plant and Soil	173	1.23	4,251	24.57 (4)	2.638 (9)	Agronomy; Plant Sciences; Soil Science
Environmental Monitoring and Assessment	162	1.15	1,067	6.52 (16)	1.592 (16)	Environmental Sciences
Journal of Environmental Science – China	160	1.14	805	5.03 (18)	1.773 (14)	Environmental Sciences
Bio-resource Technology	152	1.08	2,547	16.77 (9)	4.75 (2)	Applied Microbiology
Soil Biology & Biochemistry	149	1.06	3,328	22.34 (6)	3.654 (6)	Soil Science
Fresenius Environmental Bulletin	137	0.97	272	1.99 (20)	0.641 (19)	Environmental Sciences
International Bio-deterioration & Biodegradation	137	0.97	1,504	10.99 (13)	2.059 (13)	Applied Microbiology
Journal of Environmental Science and Health Part A-Toxic/Hazardous Substances & Environmental Engineering	137	0.97	638	4.66 (19)	1.252 (17)	Environmental Engineering
Ecotoxicology and Environmental Safety	133	0.94	1,529	11.5 (12)	2.203 (12)	Environmental Sciences; Toxicology
Applied and Environmental Microbiology	131	0.93	6,126	46.76 (1)	3.678 (5)	Applied Microbiology

Note: TP, number of publications; TP (%), percentage of the journal in the study field; TC, total citation counts; TC/TP, the average number of citations; IF, 2012 ISI Impact factor; ISI category, ISI identified subject categories in the SCI and SSCI databases.

Author productivity and geographic distribution of authors

Table 3 is the result of the author productivity analysis. Twenty authors were the most productive in the period of 1999–2012. Evidently, they had the greatest amount of achievement in their research on soil contamination. Of these authors, Q. X. Zhou produced the largest number (81) of papers, but L. Q. Ma and S. P. Mcgrath A receive the highest rate of citations (43.45 and 56.19). This seems that for individual authors, their productivity is negatively related with their academic value. The pursuit for high level of research might be at the cost of quantity of research. Szakova, J published 47 articles with 39 being co-authored with P. Tlustos. This suggests that some studies require cooperation between authors to be successful and that both authors may also enjoy some convenience in publication.

Table 3. The 20 most productive authors and major collaborators.

Author	TP	TC	TC/TP	AU	CO	AU/TP	DC	DC/TP	MC
Zhou, QX	81	1,021	12.60	363	282	4.48	127	1.57	Wei, SH(20)
Naidu, R	67	1,053	15.72	322	255	4.81	86	1.28	Megharaj, M(26)
Ma, LQ	64	2,781	43.45	244	180	3.81	89	1.39	Cao, XD(10)
Luo, YM	54	1,121	20.76	328	274	6.07	116	2.15	Christie, P(26)
Zhu, YG	54	1,187	21.98	244	190	4.52	107	1.98	Chen, BD(13)
Mcgrath, SP	53	2,978	56.19	288	235	5.43	114	2.15	Zhao, FJ(31)
Schulin, R	50	1,477	29.54	238	188	4.76	105	2.1	Nowack, B(14)
Christie, P	49	973	19.86	269	220	5.49	83	1.69	Luo, YM(30)
Wong, MH	49	1,283	26.18	232	183	4.73	80	1.63	Ye, ZH(19)
Szakova, J	47	496	10.55	246	199	5.23	89	1.89	Tlustos, P(39)
Smolders, E	46	1,001	21.76	214	168	4.65	83	1.80	Degryse, F(13)
Lestan, D	45	810	18.00	123	78	2.73	35	0.78	Finzgar, N(9)
Tlustos, P	42	395	9.40	221	179	5.26	87	2.07	Szakova, J(39)
Yang, XE	41	578	14.10	233	192	5.68	84	2.05	He, ZL(22)
Mclaughlin, MJ	39	1,013	25.97	187	148	4.79	78	2.00	Lombi, E(10)
Chen, YX	38	700	18.42	237	199	6.24	72	1.89	Lin, Q(18)
Zhao, FJ	35	2,341	66.89	185	150	5.29	78	2.23	Mcgrath, SP(31)
Li, PJ	34	376	11.06	192	158	5.65	76	2.24	Gong, ZQ(14)
Li, SP	34	259	7.62	200	166	5.88	88	2.59	Jiang, JD(14)
Vangronsveld, J	34	1,245	36.62	215	181	6.32	105	3.09	Ruttensa, A(11)

Note: TP, total publications; TC, total citation counts; AU, signatories on publications or number of authors; CO, co-authorships; DC, distinct collaborators; MC, major collaborator and number of collaborated publications; TC/TP, AU/TP, and DC/TP, average of citations, authors, and distinct collaborators in a paper.

Figure 3 displays the geographic distribution of authors on soil contamination research. The clusters of authors (black dots on the map) show that authors are mostly from the USA, Western European countries, China, Japan, and India, each country producing over 500 articles on soil contamination in the period of 1999–2012. We understand that the authors in the USA, Western European countries, and Japan have a longer tradition in research on environment and are more aware of environmental issues in their areas and in the world. Therefore, authors in those countries produce more articles. In China and India, the rapid economic development has brought with it serious environmental problems which have been attracting more and more attention of researchers. Soil contamination is the most concerned topic of research. Social need is a reasonable motivation for extensive research on environmental issues like soil contamination. In individual countries, the productive authors are clustered in the region where universities and research institutes are located, such as the eastern part of the USA, London in the UK, Paris in France, Beijing in China, and Tokyo in Japan. These institutions have long established tradition of research providing platforms for authors to study hot issues like soil contamination.

Figure 3. Geographic distribution of authors and the article outputs of different countries.

Geographic distribution of productive countries

Table 4 shows the 25 countries and regions in the world that produced the most articles on soil contamination in the period from 1999 to 2012. The USA and China are the two most productive countries, with 2,503 articles and 2,021 articles, respectively. These countries are the most productive in soil contamination research with different backgrounds. No literature is available for this phenomenon, but we speculate that the USA is with a longer and more multiple-faceted tradition of research and with more advanced techniques of research, while China is with a more urgent need for environmental research and more researchers involved in studies on soil contamination. The total publications are divided into those of research done in a single country and those on studies carried out by international collaboration. In single-country research, the USA and China also enjoy the highest total citations of publications, 32,152 and 13,441, respectively, which is reasonably supported by the total number of articles published. But, articles by the USA authors are obviously more cited than articles by Chinese authors. Understandably, the academic level of Chinese authors in research on soil contamination is not as high as their American counterparts. Switzerland and the UK enjoy the highest citations per paper, which suggests that authors of these two countries produce articles of the highest academic value. In international research, the USA and China still produce the most articles, but the USA and UK have the highest total citations with China falling far backward in this regard which again confirms that Chinese authors need higher level of international research. In terms of the rate of international research to total research, Demark, the Netherlands, and Switzerland (59.60%, 57.85%, and 56.56%, respectively) are the most active. It seems that authors in these countries are more interested in international research probably because they have a good tradition of international collaboration in scientific research.

Table 4. The 25 most productive countries/regions in soil contamination research.

		Single-country				Internationally collaborated
Country	TP	SP	TC	Av. TC	SP (%)	CP	TC	Av. TC	CP (%)
USA	2,503	1,648	32,152	19.51	65.84	855	14,917	17.45	34.16
Peoples Republic of China	2,021	1,441	13,441	9.33	71.30	580	7,615	13.13	28.70
Spain	880	584	8,281	14.18	66.36	296	4,259	14.39	33.64
UK	796	360	9,903	27.51	45.23	436	9,608	22.04	54.77
Canada	740	481	6,553	13.62	65.00	259	4,910	18.96	35.00
India	687	577	5,045	8.74	83.99	110	1,417	12.88	16.01
Germany	682	310	5,056	16.31	45.45	372	6,088	16.37	54.55
Italy	612	445	6,665	14.98	72.71	167	2,408	14.42	27.29
Japan	495	334	3,094	9.26	67.47	161	2,117	13.15	32.53
Poland	434	345	1,886	5.47	79.49	89	1,255	14.10	20.51
Brazil	433	343	1,569	4.57	79.21	90	1,595	17.72	20.79
Australia	426	192	2,829	14.73	45.07	234	3,226	13.79	54.93
South Korea	422	266	2,167	8.15	63.03	156	1,823	11.69	36.97
Russia	356	271	801	2.96	76.12	85	1,318	15.51	23.88
Belgium	305	144	2,531	17.58	47.21	161	3,967	24.64	52.79
Czech Republic	287	209	2,159	10.33	72.82	78	1,127	14.45	27.18
Taiwan	277	203	1,941	9.56	73.29	74	662	8.95	26.71
Netherlands	261	110	2,420	22.00	42.15	151	2,195	14.54	57.85
Mexico	247	182	1,175	6.46	73.68	65	823	12.66	26.32
Switzerland	221	96	2,830	29.48	43.44	125	2,526	20.21	56.56
Iran	216	173	626	3.62	80.09	43	215	5.00	19.91
Sweden	213	116	1,845	15.91	54.46	97	1,586	16.35	45.54
Denmark	198	80	1,609	20.11	40.40	118	2,208	18.71	59.60
Turkey	198	173	1,386	8.01	87.37	25	416	16.64	12.63

Note: TP, total publications; SP, single-country publication; TC, total citations; Av. TC, the average number of citations; SP (%), percentage of single-country publication; CP, internationally collaborated publication; CP (%), percentage of internationally collaborated publication.

Figure 4 shows the countries involved in collaboration in research on soil contamination in the period of 1999–2012. Such collaboration mostly takes place between the USA and Canada, the USA and China, the USA and South Korea. This core group of countries in the collaboration network is formed, in our opinion, by the tradition that the USA takes the lead in research on soil contamination with authors in the other countries, especially China and South Korea, seeking cooperation with the USA in order to advance their research.

Figure 4. Core international collaboration networks.

Figure 5 shows that academic outputs were associated with GDP and population. Countries that have a high GDP and population are more likely to publish high qualitative research articles in this field. The correlation between the number of papers published and GDP is as high as 0.916, and that between population and the number of papers published is 0.82 (p < 0.001). In terms of GDP, the USA, China, Japan, India, and Germany' published the most papers. In terms of population, the USA, China, India, Germany, and Japan produce the most papers, although this trend does not completely follow order of the population size.

Figure 5. Relationships between papers published and GDP and population (data from the World Bank database 2012).

Analysis of keywords and hot issues

Figure 6 shows the 20 most frequently used keywords in the 14,090 articles published in the period of 1999–2012. “Heavy metals” is the most frequently (1,245 times) used keyword. This indicates that heavy metal pollution is the hottest issue in soil contamination research currently. Industrialization and urbanization, mining and emissions, discharge of waste water, application of sewage irrigation, and long-term use of fertilizer and pesticides, all these are responsible for heavy metal pollution in soils (Wei and Yang 2009; Park et al. 2011; Xia et al. 2011). Heavy metals are toxic to humans even at low concentrations (Albering et al. 1999; Cui et al. 2005). They are easily absorbed by and stay in body tissues, and are not easily eliminated from the body, which implies a serious risk of cancer (Zakharova et al. 2002; Pruvot et al. 2006). Even worse, heavy metals released into the soil remain in the pedosphere for many years even after removing of the pollution sources (Wu et al. 2010).

Figure 6. The top 20 most frequently used keywords.

Eight heavy metals are currently most studied in the order of lead, cadmium, zinc, copper, and so on (Figure 7) Pollution of these heavy metals contaminates foods and grains (AL et al. 2005; Liu et al. 2010), damages feed crops (Christelle et al. 2006), and destroys the cycle of the microbial system in soils (Groenenberg et al. 2010; Maderova et al. 2011), all of which pose health risks of human beings.

Figure 7. Eight polluting heavy metals dealt with in articles.

Therefore, heavy metal pollution is attracting increasing attention worldwide. And, remediation is the focus of research to combat against heavy metal pollution. Chemical and physical remediation, animal remediation, phytoremediation, and microbial-remediation (Chen et al. 2000; Margesin et al. 2000) were investigated and recommended (Figure 8) Of these four methods of remediation, phytoremediation and microbial-remediation can be categorized into bioremediation and are currently more popular, as they are more economical and practically effective (Rascio & Izzo 2011; Sarma 2011). For instance, a number of plants, genetic engineering, and cell engineering techniques are widely used in removing hazardous heavy metals from soils. Many heavy metal resistant genes have been introduced into the cells of the chosen plants which extract heavy metals from soils by hyperaccumulation of heavy metals in themselves (Hanikenne & Nouet 2011; Rossatoa et al. 2011). However, an integrated approach of physical, chemical, and biological degradation has been adopted in removing heavy metals from soils, as this approach is the most effective and, more importantly, the most environment friendly (Udiwal & Patel 2010; Alessandra et al. 2012) and therefore the most promising topic of research.

Figure 8. Remediation and related publications.

Conclusions

In this article, we have reported a bibliometric analysis of articles of global research on soil contamination during the period from 1999 to 2012. We followed the tradition of research and included only research articles in our investigation. Parameters of document type and languages, scientific outputs, subject and journal categories, author productivity and geographic distribution, geographic distributions of productive countries and regions, and keywords frequencies, and hot issues were investigated. The articles were generally in English. Scientific productivity increases step by step. Environmental sciences, environmental engineering, soil science, and applied microbiology are the four most popular subject categories of research. The journals of Chemosphere and Hazardous Materials published the most articles on soil contamination, but Environmental Science & Technology had the highest IF, and Applied and Environmental Microbiology received the highest citations per publication. The most productive authors, Q. X. Zhou of Nankai University and S. P. Mcgrathand of Rothamsted Research England, enjoy the highest citations of their articles. J. Szakova and P. Tlustos from Czech University have coauthored the most articles. The authors are mostly distributed in the USA, Western European countries, China, Japan, and India. The USA and China are the two most productive countries in research on soil contamination; however, the UK and Switzerland produced articles of the highest citations and the Netherlands, Demark, and Switzerland are characterized with the most international collaboration in research. Cooperated research was conducted more between the USA and China, the USA and Canada, the USA and South Korea than between other countries. Scientific productivity was related with GDP and the size of population. “Heavy metals” was the most frequently used keyword, and eight heavy metals were the most studied, and remediation was recommended with bioremediation being the most popular means.

The present study indicates that, as English is in fact an international language, academic communication has been in this language. The increased scientific productivity is related with stronger public awareness of environmental problems and with higher level of research techniques. The popular subjects of research are environmental oriented. Articles were more likely to be published in a few journals that are with environmental subject. The most productive author is Chinese but the English author produces the highest quality of articles. The distribution of authors seems to be related with tradition of research and with current social demand. Productivity of individual country may be related with its tradition and technology of research or the number of researchers involved but is not positively related with level of research. The USA takes the lead in research on soil contamination and the countries would be more interested in academic exchange with the USA by more international research. Increasing GDP and large population of a country seems likely to invest more in research on soil contamination. Heavy metal pollution is currently the most concerned issue on soil contamination because it is the threatening for humans and the most difficult to deal with.

The present results are significant for authors in the world in doing research on environmental issues and in publishing their articles.

However, there are limitations to this study. We used only the JCR database to identify journals for inclusion in the study. Articles published in non-JCR-cited journals were not included, but they also reflect scientific production in the world. If they had been included, the results would be interpreted more reliably. However, we are not able to obtain all these non-JCR-cited journals in the world.

The time period designed in this study may not be long enough so that the data were not big enough to reflect the real trend in publication on issues of soil contamination. There were considerable studies on this issue in earlier years. If we had a longer period for data collection, the results would be more significant. However, the period for data collection was what we could do in our university.

Therefore, there is room for improvement in our future research in publication on soil contamination.

Acknowledgments

The authors are thankful to Prof. Yuh-Shan Ho (Peking University, China) for his pioneering work in the bibliometric methods that were employed in this study. The authors would also like to thank Mr. ZhongQiu Liu, Mr. JiWei Li, Mr. QingHu Jiang, and Ms. BeiBei Niu (Wuhan University, China) for their helpful advices on the methodology. Comments from the anonymous referees and the editor are much appreciated.