A Scientometric Analysis of Worldwide Intercropping Research Based on Web of Science Database between 1992 and 2020

: Intercropping has been practiced worldwide in both traditional and sustainable agriculture to feed the growing population. This study aims to analyze the research status and evolution of intercropping, to identify the inﬂuential authors, research centers, and articles, and to reveal the main research topics between 1992 and 2020 based on the Web of Science Core Collection database. The results show that the volume of publications in this ﬁeld has increased rapidly over the past three decades. The analysis identiﬁes the top three authors (i.e., Meine Van Noordwijk, Wenyu Yang, and Teja Tscharntke), top three contributing organizations (i.e., the World Agroforestry Center (ICRAF), the Chinese Academy of Science, and the INRA), and three most productive countries (i.e., the USA, India, and China). Co-occurrence analysis demonstrates that studies on intercropping can be divided into four clusters as centered by keywords of intercropping/maize, biodiversity/conservation, agroforestry, and carbon, respectively. Lal 2004 is the most inﬂuential study with the greatest number of citations and Agroforestry Systems is the most utilized journal. Perspectives on future studies were also given. This study helps researchers to clarify the current research status in the ﬁeld of intercropping and put forward its future research.


Introduction
Intercropping is the agronomic practice of simultaneously growing two or more crop species in the same field in close proximity for a considerable proportion of their growing season [1][2][3]. The main types of intercropping include agroforestry, hedgerow intercropping, relay intercropping, mixed intercropping, row intercropping, and strip intercropping [4]. Intercropping has been widely applied around the world in both tropical and temperate regions for both traditional, intensive farming systems, and sustainable agriculture systems [4][5][6][7]. The intercropping systems vary from place to place as a result of variations in local climate, soil, and socio-economic structure and status. For instance, there are multistrata agroforestry and jungle rubber-based agroforestry in tropical areas of Indonesia, India, Niger, and Mali. There are silvopastoral systems, coffee agroforests in Central America, dehesa agroforestry in Spain/Portugal, and cocoa agroforestry worldwide [4,8,9]. In both Africa and Latin America, beans and peas climb tall cornstalks, whereas pumpkins and squash cover the ground below [10]. In temperate regions, peas are grown with barley or oat, wheat is grown with canola or pea, and broccoli is grown

Materials and Methods
The Web of Science Core Collection (WoSCC) is among the most comprehensive and widely utilized databases for scientometric analyses [54][55][56], it contains quality controlled full literature data (e.g., title, author, abstract, keywords, references, and citations) since 1985 up to present. The data between 1985 and 2020 were downloaded from the WoSCC on Jan 2, 2021 for analysis based on the preset query sets. The query sets used for the literature search are TS = ("intercrop*" OR "interplant*" OR "crop mixture" OR "undercrop sow*" OR "agroforest*" OR "hedgerow"), where TS is a field tag indicating topic in the Web of Science. The search results were further restricted by languages and document types and only articles, letters, notes, books, book chapters, data papers, database reviews, and reviews written in English were retrieved. This search process returned a total of Sustainability 2021, 13, 2430 3 of 14 14,001 publications and they were saved as tab-limited text files (UTF-8) containing the "full record with citation data". VOSviewer 1.6.15 (The Centre for Science and Technology Studies, Netherlands) and CiteSpace 5.7.R1 (Drexel University, USA) were used to perform scientometric analysis. VOSviewer 1.6.15, a Java-based software developed in 2009 by van Eck and Waltman [57], is a tool for building and visualizing a scientometric network, which can quickly observe knowledge and research in a specific field. The annual trend of publication volume, the main authors, countries and institutions, the most utilized journals, the highly impacted studies, and occurrence network of keywords were analyzed. CiteSpace is a Java-based software tool to present the rules and structure of scientific knowledge [58], which is used to analyze the burst time of keywords that reflect the development and revolution of research hotspots.

Overview of Publication Trend
It is noted that query sets in Section 2 returned no data between 1985 and 1991, which was also reported in other studies as the main SCI-expanded database of WoSCC began archiving since 1992 [54,55], where SCI is short for Science Citation Index. There is an increasing trend in the number of publications, from 241 (1.72%) in 1992 to 1258 (8.99%) in 2020 ( Figure 1). This indicates that research on intercropping has received increasingly more attention. For the 14,001 publications, most of them fall into the Web of Science category of agronomy (4638 or 33.13%), forestry (2555 or 18.25%), environmental science (2158 or 15.41%), agriculture multidiscipline (1907 or 13.62%), ecology (1847 or 13.19%), plant science (1461 or 10.44%), and soil science (1460 or 10.43%). The dominant document type is an article, which accounts for 13,287 or 94.90% of the total publications, followed by a review that accounts for 620 papers or 4.43% of the total publications. guages and document types and only articles, letters, notes, books, book chapters, data papers, database reviews, and reviews written in English were retrieved. This search process returned a total of 14,001 publications and they were saved as tab-limited text files (UTF-8) containing the "full record with citation data". VOSviewer 1.6.15 (The Centre for Science and Technology Studies, Netherlands) and CiteSpace 5.7.R1 (Drexel University, USA) were used to perform scientometric analysis. VOSviewer 1.6.15, a Java-based software developed in 2009 by van Eck and Waltman [57], is a tool for building and visualizing a scientometric network, which can quickly observe knowledge and research in a specific field. The annual trend of publication volume, the main authors, countries and institutions, the most utilized journals, the highly impacted studies, and occurrence network of keywords were analyzed. CiteSpace is a Java-based software tool to present the rules and structure of scientific knowledge [58], which is used to analyze the burst time of keywords that reflect the development and revolution of research hotspots.

Overview of Publication Trend
It is noted that query sets in Section 2 returned no data between 1985 and 1991, which was also reported in other studies as the main SCI-expanded database of WoSCC began archiving since 1992 [54,55], where SCI is short for Science Citation Index. There is an increasing trend in the number of publications, from 241 (1.72%) in 1992 to 1258 (8.99%) in 2020 ( Figure 1). This indicates that research on intercropping has received increasingly more attention. For the 14,001 publications, most of them fall into the Web of Science category of agronomy (4638 or 33.13%), forestry (2555 or 18.25%), environmental science (2158 or 15.41%), agriculture multidiscipline (1907 or 13.62%), ecology (1847 or 13.19%), plant science (1461 or 10.44%), and soil science (1460 or 10.43%). The dominant document type is an article, which accounts for 13,287 or 94.90% of the total publications, followed by a review that accounts for 620 papers or 4.43% of the total publications.

The Co-Authorship of Authors
There are 291 out of 38,704 authors meeting the threshold of a minimum of 10 publications ( Figure 2). They are grouped in 77 clusters, where a cluster indicates a group of closely collaborating authors. The largest set of connected authors consists of 184 researchers centered in Figure 2a. The colors in Figure 2b indicate the active periods of authors, with the "yellow" indicating that researchers published intercropping studies in recent years, "green" indicates that most papers of the authors were published around 2010, while "blue" around 2000. For instance, Drs. Wenyu Yang and Taiwen Yong from Sichuan Agriculture There are 291 out of 38,704 authors meeting the threshold of a minimum of 10 publications ( Figure 2). They are grouped in 77 clusters, where a cluster indicates a group of closely collaborating authors. The largest set of connected authors consists of 184 researchers centered in Figure 2a. The colors in Figure 2b indicate the active periods of authors, with the "yellow" indicating that researchers published intercropping studies in recent years, "green" indicates that most papers of the authors were published around 2010, while "blue" around 2000. For instance, Drs. Wenyu Yang and Taiwen Yong from Sichuan Agriculture University (China) have been publishing on intercropping [59][60][61], they are also the top productive authors on these topics (Table 1). This is similar to Drs. Qiang Chai and Zhilong Fan from Gansu Agriculture University (China, Table 1) and Dr. Lijin Lin from Sichuan Agriculture University (China) [27]. Other productive researchers such as Dr. Fusuo Zhang (China Agriculture University, China, Table 1) and Meine van noordwijk (Wageningen University and Research, Netherlands, Table 1) were active around 2010, while Dr. Chin K. Ong (World Agroforestry Center, Kenya, Table 1) was active around 2000 [49].

The Top Contributed Organizations and Countries
There are 8037 organizations that published research on intercropping, the World Agroforestry Center (ICRAF) ranked first on the publication volume (n = 326), followed by Chinese Academy of Science (n = 307), INRA (n = 260), China Agriculture University (n = 217), and University of Gottingen (n = 213). The top 25 organizations are tabulated in Table 2. In terms of average per publication citation times, the University of Gottingen ranked first (C/N = 42), followed by the University of California (C/N = 35), and the University of Florida (C/N = 34). The average per-publication citations of the University of Missouri, the China Agriculture University, and INRA are equal to or greater than 30, which indicates the high influence of these institutions. In addition, the World Agroforestry Center (ICRAF), the China Agriculture University, and INRA collaborated more with other organizations as indicated by their greater TLS (over >10,000), CSIC and the University of California Davis had the least inter-organization collaborations among the top 25 organizations.
In addition, there are 169 countries that published research on intercropping, with the USA (n = 2468), India (n = 1470), China (n = 1437), and Brazil (n = 1164) are the top four countries ( Table 2, Table 3 and Figure 3). They consist of the research centers for study on intercropping. It is interesting to notice that the average citations of Indonesia ranked first (C/N = 29), followed by the USA, Australia, Denmark, and Costa Rica that had C/N = 28.
The USA collaborated most with other countries with the greatest TLS = 35,512, followed by France and Germany with TLS = 21,916 and 21,538, respectively.

The Co-Occurrence and Burst Time of Keywords
There are 37,398 keywords in the title, abstract, and author-provided keyword list, and 338 keywords meeting the threshold of 60 occurrences. These keywords are grouped into four clusters: red, green, blue, and yellow (Figure 4), and the top 25 keywords were tabulated in Table 4. The red-colored cluster in Figure 4 is pertaining to crop intercropping as represented with high-frequency keywords of "maize", "yield", "wheat", and "soybean" etc. [59,62,63]. Among them, the "maize" (corn) based intercropping system is the dominant type around the world [31,53,62,63]. The green-colored cluster is mainly related to "biodiversity" or "conservation" and management of intercropping systems [34,36,64]. One of the popular topics is to increase "biodiversity" without compromising yield in the intercropping system [65]. The blue-colored cluster is themed with "agroforestry" or "sustainability" [66][67][68][69], while the yellow-colored cluster is featured with "nitrogen", "carbon sequestration", and "microbe biomass" etc. [70][71][72][73].

The Co-Occurrence and Burst Time of Keywords
There are 37,398 keywords in the title, abstract, and author-provided keyword list, and 338 keywords meeting the threshold of 60 occurrences. These keywords are grouped into four clusters: red, green, blue, and yellow (Figure 4), and the top 25 keywords were tabulated in Table 4. The red-colored cluster in Figure 4 is pertaining to crop intercropping as represented with high-frequency keywords of "maize", "yield", "wheat", and "soybean" etc. [59,62,63]. Among them, the "maize" (corn) based intercropping system is the dominant type around the world [31,53,62,63]. The green-colored cluster is mainly related to "biodiversity" or "conservation" and management of intercropping systems [34,36,64]. One of the popular topics is to increase "biodiversity" without compromising yield in the intercropping system [65]. The blue-colored cluster is themed with "agroforestry" or "sustainability" [66][67][68][69], while the yellow-colored cluster is featured with "nitrogen", "carbon sequestration", and "microbe biomass" etc. [70][71][72][73].   Table 4. Table 4. High-frequency keywords for research on intercropping and word frequency analysis from 1992 to 2020 with VOSviewer 1.6.15. The total link strength (TLS) indicates the total strength of the links of an item with other items.

The Highly Impacted Publications
There are 361 out of 14,001 publications meeting the threshold of 100 citations, they are divided into 98 clusters with the largest set of connected documents consisting of 270 papers ( Figure 6). Lal [74], Smith et al. [75], and Reich et al. [76] are the top three studies that were cited over 1000 times based on the record of WoSCC, with the first two being mainly about carbon sequestration or greenhouse gas emission mitigation in agriculture.

The Widely Utilized Journals
There are 1291 journals that published research on intercropping, among which 15 journals each published over 100 papers (Figure 7). Agroforestry Systems is the most utilized journal that published over 1500 papers, followed by Agriculture Ecosystems and Environment that published around 500 papers between 1992 and 2020 based on the Web of Science Core Collection (WoSCC).

The Highly Impacted Publications
There are 361 out of 14,001 publications meeting the threshold of 100 citations, they are divided into 98 clusters with the largest set of connected documents consisting of 270 papers ( Figure 6). Lal [74], Smith et al. [75], and Reich et al. [76] are the top three studies that were cited over 1000 times based on the record of WoSCC, with the first two being mainly about carbon sequestration or greenhouse gas emission mitigation in agriculture.  Figure 6. The network visualization map of citations for 361 publications with a minimum of 100 citations on intercropping using VosViewer based on data retrieved from the Web of Science Core Collection (WoSCC). Each node represents a publication, the size of the node represents the number of citations, the lines represent the co-citation relationship between documents, and the thicker the lines the stronger the links.

The Widely Utilized Journals
There are 1291 journals that published research on intercropping, among which 15 journals each published over 100 papers (Figure 7). Agroforestry Systems is the most utilized journal that published over 1500 papers, followed by Agriculture Ecosystems and Environment that published around 500 papers between 1992 and 2020 based on the Web of Science Core Collection (WoSCC).   citations on intercropping using VosViewer based on data retrieved from the Web of Science Core Collection (WoSCC). Each node represents a publication, the size of the node represents the number of citations, the lines represent the co-citation relationship between documents, and the thicker the lines the stronger the links.

The Widely Utilized Journals
There are 1291 journals that published research on intercropping, among which 15 journals each published over 100 papers (Figure 7). Agroforestry Systems is the most utilized journal that published over 1500 papers, followed by Agriculture Ecosystems and Environment that published around 500 papers between 1992 and 2020 based on the Web of Science Core Collection (WoSCC).

Conclusions and Perspectives
The results show that the volume of publications in this field has increased rapidly over a period of 29 years with 241 papers (1.72%) in 1992 to 1258 (8.99%) in 2020. The analysis shows that Meine Van Noordwijk, Wenyu Yang, and Teja Tscharntke are the top three authors that published over 70 papers on this topic. The Chinese Academy of Science, INRA, and the China Agriculture University are the top three contributing organizations, while the USA, India, and China are the most productive countries for research centers on intercropping. The most influential studies are Lal [74] and Smith et al. [75] that investigated the carbon sequestration or mitigation of greenhouse gas emission. Agroforestry Systems is the most utilized journal that published over 1500 papers on intercropping in total.
The co-occurrence analysis demonstrates that intercropping studies generally focus on four aspects as indicated by keywords of intercropping/maize, biodiversity/conservation, agroforestry, and carbon, respectively. Given that applications of synthetic fertilizers significantly increased grain yield over the past decades, but overuse of chemical fertilizer leads to soil and environmental pollution and threatens agricultural sustainability [77], more studies are required to develop efficient approaches so as to improve the crop quality while maintaining or increasing yield to meet food security and higher demands of food quality [78][79][80]. As intercropping has been well studied in well-developed countries including the USA and the mostly populated countries including India and China, more intercropping studies in undeveloped countries in Africa should be conducted to fight starvation and malnutrition and to adapt to climate change [63,81]. In addition, the paradox between intercropping and automation should be properly addressed with the design and development of new compatible intercropping machineries [63,82]. Moreover, future studies are recommended to investigate the below-ground competition for water and nutrients between intercropped species, root-shoot equilibrium of intercropping, inter-zone water migration in water-saving irrigation agriculture systems, and regions with a low input level [27,83,84]. Increased attention should also be paid to innovative perennial systems with intercropping to facilitate root interactions and microbial abundance and diversity [85]. Furthermore, previous studies are generally based on experimental studies, it is desired to develop functional structural plant modeling to better understand and optimize species mixture [86]. This study helps researchers to clarify the current research status in the field of intercropping and put forward its future research.