3.1. Evolution of the Main Characteristics in the Field of CE and Agriculture
This work analyzes all the publications made in the field of CE applied to agriculture. The time horizon considered ranges from the first year in which investigations are registered (2005) to the present (2021). Likewise, the seventeen years studied were divided into three periods, one of seven years, namely 2005–2011, and two of five years each, namely 2012–2016 and 2017–2020. The main reason for this classification is that in the first period (2005–2011), no publications were recorded in 2006 and 2008, thus by including two more years, it was possible to obtain three periods with the same number of years.
Table 1 shows the main characteristics of this line of research, which has a sample of 1060 total investigations. The first period, between the years 2005 and 2011, recorded a total of 14 publications, while the last period (2017–2021) had 1005 documents. Regarding the authors, from 2005 to 2011, 40 participated compared to the 4208 between 2017 and 2021, which represents a percentage of representation of 0.92% and 96.78% of the total of the authors that make up the sample, respectively. In addition, if the two mentioned variables are related (active authors in the line of research and published documents), it is possible to calculate the average number of authors, which rises from 2.9 authors per article between 2005 and 2011 to a total of 4.2 between 2017 and 2021.
The countries also increase their participation in this line of research, which goes from three countries in the first period (China, Hong Kong and the USA) to registering 95 countries in the last period. In fact, the total sample is made up of 95 countries, thus the last period analyzed represents 100% of the countries active in this line of research. The latent interest in CE and agriculture is also reflected in the number of total citations since if all those made so far are counted, a total of 8886 citations are obtained. In this case, the first period (2005–2011) has 27 citations and the last period (2017–2021) has a total of 8761 citations. These values indicate that the first period has 1.93 citations per article, while the last one reaches an average of 8.72 citations per research document. On the other hand, the number of journals that publish on this subject rose from 13 in 2005–2011 to 401 journals between 2017 and 2021. Finally, special mention should be made of the last period analyzed, namely 2017–2021, as it has the largest percentages of variation in all the variables considered in the Table.
Figure 2 graphically represents the number of publications that have been made annually to make up the total sample of 1060 documents. The first year analyzed (2005) has three publications [
44,
45,
46], the first of the entire sample being identified under the name “Practical and theoretical issues on the sustainable development of Chinese ecological agriculture”. Additionally, the year 2021 recorded 440 documents. It must be clarified that the study was being carried out without having finished 2021, thus the number of investigations is likely to increase when considering the full year. From 2018, more than 100 annual investigations began to be published and the highest number of publications occurred in the last year analyzed (2021), with a total of 400 investigations. The year with the highest percentage of variation (231%) is 2017, with 53 publications, due to the increase produced when compared to the previous year (2016), which had 16 publications. Next, 2018 went on to register 113 publications with an increase of 113% compared to the previous year. On the contrary, the years with the lowest percentage of variation are 2006 and 2008 since they did not have publications. Finally, if reference is made to the established time periods, the variation that occurs in published research is 192% for the second period (2012–2016) and 2.351% for the third period (2017–2021).
3.2. Analysis by Subject Area
The investigations carried out may be classified in several thematic areas simultaneously depending on the interests of the authors and the publisher themselves. For this reason, it is common to find publications that are grouped into several categories due to the various topics of interest investigated. In this study, we highlight those that registered the highest number of publications regardless of whether they are classified only in this discipline or in others at the same time. In this case, the sample of 1060 documents was classified in the Scopus database into a total of 26 subject areas.
Figure 3 represents the evolution of the main categories, that is, those that exceed 5% representation of the total sample. In total, 1718 documents are grouped among the five main disciplines and represent 74.05% of the total sample.
The category that Lead
Figure 3 is Environmental Science with 604 investigations and a representation percentage of 26.03%. It is considered a reference topic in the research line since publications are framed from the first year analyzed (2005). In addition, in regard to the last period (2017–2021), a percentage increase of 2.643% was experienced since it went from 21 investigations in the previous period to a total of 576 publications. In second position, with 312 documents and a 13.45% representation, is Energy. This discipline stands out for its late incorporation into the topics of interest; its first investigation was framed in 2015. However, due to the growing interest in this category, the number of investigations went from five in the period (2012–2016) to 307 between 2017 and 2021. The discipline that follows, in third position in the ranking, is Engineering. This subject area has a total of 241 publications and represents 10.39% of the total sample analyzed.
During the first period (2005–2011), six publications were framed in this category; in the second period (2012–2016), nine investigations were reached; and in the last period (2017–2021), the total number of registered documents amounted to 226. These values represent the exponential increase that is taking place in the investigation of subjects focused on engineering. Fourth, indicated in yellow in
Figure 3, is Agricultural and Biological Sciences. This thematic area registered 223 publications and a representation percentage of 9.61% with respect to the total sample analyzed. The year 2005 recorded the first three publications; it is from the year 2011 when it began to be considered annually in the documents. The succeeding discipline is Social Sciences with 187 investigations and an 8.06% representation. This category, which in the first period registered two publications, reached 147 in the last (2017–2021). Finally, Business, Management and Accounting occupies the sixth position. This discipline has 6.51% and 151 investigations. As can be seen in the Figure, it is at the bottom of the rest of the main categories, although it also registered an exponential increase for the last years analyzed.
The rest of the disciplines are not represented graphically since their representation percentage is under 5%. Even so, variety is mentioned in the rest of the disciplines framed since there are publications in the field of Nursing, Veterinary, or Neuroscience [
47,
48]. Finally, to obtain an overview of the number of categories in which the investigations are classified, it must be said that a total of 2320 documents are registered for the 26 categories. These data imply that each investigation is classified into an average of 2.19 disciplines.
3.3. Identification of the Most Prolific Journals
This section presents the 20 most active journals in CE research in the agricultural field out of the 429 that make up the sample.
Table 2 refers to the publications made by each journal, the citations received, the average number of citations, the H-index [
38,
49], the nationality, or the impact factor according to the Scimago Journal Rank. Moreover, in relation to the publications in the sample, the H-index of the documents was analyzed as well as the publication date of the first and last article. According to the data provided, the countries with the highest percentage of representation are Switzerland and the Netherlands, registering seven and four journals, respectively. Regarding the year of publication, all the journals are still publishing to this day, that is, in 2021. If reference is made to the quartile to which they belong, 13 journals are classified in the first quartile, representing 65% of the journals in the table; six journals in the second quartile (30%), and only one in the third quartile (5%).
The journals are ordered according to the number of published investigations; hence, Sustainability is the journal that leads the ranking, with 83 investigations. This journal is Swiss and ranks in the first quartile with an impact factor of 0.61. The Journal of Cleaner Production, in second place, is the journal with the highest values for the total citations received (1282) and H-index for articles (21). Some publications made by this journal with a high number of citations are “Remanufacturing challenges and possible lean improvements” and “Redesigning a bioenergy sector in EU in the transition to circular waste-based Bioeconomy-A multidisciplinary review” with 84 and 57 citations, respectively [
50,
51]. Regarding the average number of citations per article, the journal with the highest value (23.58) is Resources Conservation and Recycling, ranked fourth. By way of addition, the journal with the lowest values for the three variables mentioned, namely total citations (11), average citations (1.38), and H-indices for articles (2), is Frontiers in Sustainable Food Systems. This discipline, which is in position 18, registered these values due to its short history since its first two publications were made in 2020, entitled “Co-benefits and Trade-Offs From Agro-Food System Redesign for Circularity: A Case Study With the FAN Agent-Based Model ”and“ Sustainability Assessment of Biorefinery Systems Based on Two Food Residues in Africa” [
52,
53]. On the contrary, Nongye Gongcheng Xuebao Transactions of the Chinese Society of Agricultural Engineering is characterized by its long research activity since it is the only journal in
Table 2 that publishes in the first period analyzed, specifically in 2005. Finally, in terms of the characteristics of the journals themselves, Renewable and Sustainable Energy Reviews registered the highest value in the journal’s H-index and in the impact factor, with 295 and 3.52, respectively.
3.4. Most Prolific Authors
The total sample was made up of 4348 authors.
Table 3 shows the 10 most prolific authors for having the largest number of investigations carried out on CE applied to the agricultural context. Together, these groups have a total of 68 articles, which represents a percentage of 6.42% compared to the total sample analyzed.
Table 3 provides the total citations received by each author [
54,
55], the average number of citations, the H-index [
56], the institution to which they belong, the nationality, and the first and last year of publication on this topic. As can be seen, the European nationality predominates among the most active authors with a percentage of 90% representation. Regarding the period of publication, all the authors registered research from the second period analyzed (2012–2016), Sergio Ulgiati being the one with the longest career. In addition, all authors continue to publish today in the year 2021. This section was analyzed with the aim of identifying outstanding authors since they can serve as a reference for political decision-makers or individuals who want to expand their knowledge on this line of research. In this case, its commitment to updating the state of the art and contributing significantly to scientific advancement is detailed in the table.
The ranking of the most active authors is headed by Xavier Gabarrell i Durany with a total of 10 investigations. This Spanish author works for the Autonomous University of Barcelona (UAB) Institute of Environmental Science and Technology and has an H-index of six. In second place, Sergio Ulgiati, recorded eight investigations and is the only prolific author from China. In third position, with a total of seven publications and 60 total citations, is Luis Jesús Belmonte-Ureña. This Spanish author, who works for the University of Almeria, registered an H-index of four. Among the research he has, “The management of agricultural waste biomass in the framework of circular economy and bioeconomy: An opportunity for greenhouse agriculture in Southeast Spain” and “Effects of circular economy policies on the environment and sustainable growth: Worldwide research” are the two papers with the highest number of citations [
3,
28]. Furthermore, Anastasia A. Zabaniotou stands out for being the author with the highest number of total citations (206) and average number of citations (34.33). This Greek author works for the Aristotle University of Thessaloniki and registered the highest H-index (six) together with Xavier Gabarrell i Durany.
Finally, special mention must be made to the research developed by the authors Roberta Bernini and Annalisa Romani since both recorded the same data in the published publications (6), total citations (137), average of citations (22.83), nationality (Italy), and H-index (3). The only difference between Roberta Bernini and Annalisa Romani is the institution, as they work for the University of Tuscia and University of Florence, respectively.
The VOSviewer tool allows for organizing groups with different topics. In this case, it was carried out according to the activity of the main authors. Therefore,
Figure 4 represents a collaboration map based on co-authorship. The colors indicate the different collaboration groups and the size of each author’s contribution, as well as the number of investigations carried out in the line of research on CE applied to agriculture.
Figure 4 distinguishes seven clusters, although there are connections between authors from different clusters.
The first collaboration group appears in purple. This cluster is directed by Anna Petit-Boix and Gara Villalba Méndez, both with seven published articles. The other three authors that make up the group are Tarik Serrano, Sina Leipold, and Martí Rufí-Salís. These five authors publish numerous investigations focused on CE and agriculture, such as “Recirculating water and nutrients in urban agriculture: An opportunity towards environmental sustainability and water use efficiency?”, among many others [
57,
58,
59]. The second group of authors is represented by Joan Rieradevall I. and Felipe Parada, and has other collaborators such as Verónica Arcas-Pilz. In fact, these three authors share their institution of origin since they all belong to the UAB Institute of Environmental Science and Technology. The publication “Comparison of organic substrates in urban rooftop agriculture, towards improving crop production resilience to temporary drought in Mediterranean cities” [
60] is one of the investigations carried out by said authors; some of those that make up the purple cluster can also be found in this publication. The light blue and orange clusters are the smallest, with four and three authors, respectively. The light blue one is represented by Xavier Gabarrell i Durany and has the participation of Pere Muñoz, Ana Manríquez-Altamirano, and Jorge Sierra-Pérez [
61,
62]. The orange one, made up of Wenhao Chen, Nicholas M. Holden, and Thomas L. Oldfield, recorded three joint investigations, namely one from 2019 and two from 2020 [
63,
64,
65]. In addition, the red cluster is the largest, registering the collaboration of 10 authors. Among the members who collaborate in the investigation are Sergio Ulgiati, from China, and Gabriella Fiorentino, Amalia Zucaro, and Cristina Cavinato, from Italy [
66].
The sixth cluster, identified in yellow, is made up of a total of six authors. Two of the investigations carried out by this collaborative group are “Gasification biochar from horticultural waste: An exemplar of the circular economy in Singapore” and “Closing the food waste loop: Food waste anaerobic digestate as fertilizer for the cultivation of the leafy vegetable, xiao bai cai (Brassica rapa)” [
67,
68], which have the collaboration of Yen Wah Tong, Hugh TWTan, and Ee Yang Lim. Finally, the sixth cluster is identified in dark blue and records numerous investigations on CE applied to agriculture [
69,
70]. Of the five authors that make up the cluster, the authors Maria Cristina Righetti, P. Cinelli, and Andrea Lazzeri are the ones with the highest number of publications.
3.5. Characteristics of the Main Organizations from 2005 to 2021
Table 4 lists the 10 most prolific organizations in this line of research. The information provided is the nationality, articles produced, total citations, average citations, H-index, percentage of international collaboration, and total citations for international and national research. These organizations produced 178 publications and this translates into a representation of 16.79% of the total sample analyzed. In addition, they are all of European origin and, with a representation percentage of 60%, Italy stands out over the rest of the countries.
The first in the ranking is the National Research Council of Italy, which has 25 articles and 231 total citations. This Italian organization registered 9.24 citations per article and has the highest H-index (9) together with the Autonomous University of Barcelona. In second place, the University of Catania, which has the lowest values in the total citations (58), average citations (2.9), and H-index (4), is the organization to which the prolific author Francesca Valenti belongs. The said Italian institution has the lowest values in the total citations (58), average of citations (2.9) and H-index (4). On the contrary, the one with the highest values in total citations (293) and average citations (17.24) is the University of Tuscia, to which the author Roberta Bernini belongs.
Regarding the international cooperation, it is possible to ascertain which ones carry out numerous investigations with the collaboration of other countries and which ones opt for national publications. The second in
Table 5, University of Catania, once again positioned itself with the lowest values in percentage of collaboration (5%) and in total citations in national articles (2.74). The highest percentage of collaboration, with a value of 88.89%, was registered by Wageningen University and Research, carrying out 16 investigations internationally out of the 18 published. Finally, the University of Naples Federico II registered the highest number of total citations for international articles, with 19.83, and the University of Tuscia has the highest value for citations in national articles, with 18.31. This last organization, of Italian origin, also stands out for not registering citations in international articles since, out of the 17 publications, 16 were prepared without other countries.
3.6. Main Countries in the Field of CE and Agriculture Research
In total, 95 countries made up the sample studied. However,
Table 5 only considers the 10 most active in research on CE applied to agriculture. In total, these countries group 942 publications of the total sample, which represents 88.87%. Of the 10 countries considered, all are currently publishing in 2021 and that with the longest research history is China, specifically with its first publication in 2005.
The first country, Italy, has the highest value in the number of total citations received (2552) and the H-index (26). This European country possesses the largest number of collaborating countries, with a total of 51, of which Spain, China, Germany, France, and Greece stand out due to the close research relationship established. In addition, it is among the main collaborators of the rest of the countries in the Table, except for Brazil and Poland. Spain is in second place with a total of 165 publications and 1163 total citations received. The prolific authors Xavier Gabarrell i Durany, Luis Jesús Belmonte-Ureña, and Gara Villalba Méndez belong to this country, which explains their second position in the ranking of the most active countries. In this case, the number of collaborators amounts to 45 and the countries with which it conducts more research on CE and agriculture are Italy, Portugal, Germany, the UK, and the USA. On the other hand, Brazil, in sixth position, registered the lowest values for total citations and average citations, with 275 and 4.66, respectively.
In relation to cooperation in these countries, the percentage of international collaboration, total citations in international research, and total citations in national publications are also mentioned. The highest percentage of international collaboration is held by the UK and Germany, both with 69%, and the lowest by Poland with 26%. In addition, the latter also has the smallest number of collaborators, specifically with a total of 16 countries. The average of the total citations is balanced in national and international research, as five countries receive more citations in international articles and five other countries receive more citations in articles of a national nature. Finally, the highest average of citations in international research is received by Poland with 12.83 and France with 15.54 in national publications.
Figure 5 was made with the VOSviewer tool, in which the international cooperation of the countries is graphically represented through co-authorship. The 45 countries that make up
Figure 5 are represented in different colors depending on the cluster to which they belong and the size in relation to the investigations carried out by each.
The first red cluster is led by the UK, China, and India. The countries that make up this collaboration group are Australia, Belgium, Austria, and Japan. In total, they produced 427 articles and represent 40.28% of the total sample. Its publications are characterized by being framed in greater presence in the categories of Environmental Sciences, Energy, and Engineering.
The second cluster, which is green in color, is represented by Germany, the Netherlands, Finland, Sweden, and Poland. The rest of the countries that make up this collaboration group are South Africa, the Czech Republic, Denmark, Hungary, and Switzerland. In total, these 10 countries that constitute the cluster produced 263 investigations and represent 24.81% of the total sample. In addition, among the main categories of this line of research, the publications made by this group of countries are mainly framed in Environmental Sciences, Energy, and Social Sciences.
The third collaboration group, in blue, is made up of seven countries, specifically Canada, Chile, Mexico, Portugal, Russia, Brazil, and Spain. The last two countries mentioned are the most representative due to the number of investigations carried out and in total, with a representation percentage of 31.13%, they produced 330 articles, most of them belonging to the disciplines of Environmental Sciences, Agricultural and Biological Sciences, and Energy.
The fourth cluster in yellow and directed by Italy, France, and Greece has 67 investigations and represents 6.32% of the total sample. These last two collaboration groups have the same number of active countries, however, the number of publications made by both differs significantly.
The last cluster, in purple, is the smallest in terms of the countries that it comprises, as it only has five, specifically Bulgaria, France, Greece, Italy, and Morocco. Even so, their research activity is quite strong since they have carried out a total of 361 investigations together, which represents 34.06% of the total. This is largely thanks to Italy since it is the country that publishes the most in this line of research. These countries have Environmental Science, Energy, and Engineering as the most representative subject areas of the publications made. Finally, it should be clarified that the countries that make up the collaboration groups not only carry out research among themselves but also documents that include countries belonging to different clusters can be registered.
To conclude this section,
Figure 6 is presented. This world map indicates, based on color, the number of publications with which it has contributed to CE research in the agricultural field. In this sense, the three most prolific countries (Italy, Spain, and China) are identified in dark red because they registered the highest numbers. On the contrary, the countries indicated with light and dark green stripes are those that publish the least. This group, made up of a total of 43 countries, is consolidated as the most numerous, with each country registering between one and five investigations for the entire period analyzed. Finally, those countries indicated in gray do not have any publications to date for this line of research.
3.7. Keywords from 2005 to 2021
The terms used to investigate CE in agriculture have evolved. In this sense, it is common to find keywords in the first years of research, completely different from those used today. This is due to the evolution in the research line and to the interests according to the moment of study.
Table 6 shows the 20 keywords with the highest number of occurrences, that is, those that appear most frequently in the total sample of 1060 documents. The concepts are used in research to define the fields of study and interest of the article. Therefore, it is common to find more than one keyword in each of the analyzed publications. In this case, the total keyword sample is 8565, which would reflect the use of an average of eight keywords per published document.
Table 6 indicates some concepts with a high number of occurrences due to their interest throughout the entire time horizon, while others are among the most used due to their high current interest. This allowed us to know if the study topics have evolved over the years or if, on the contrary, an attempt is still being made to answer the same initial questions. In addition, the study eliminated words that did not add value to the analysis, such as those that were used in the search itself (circular economy and agriculture) or those that refer to characteristics of no great significance (priority journal, article, humans, etc.).
Of the total sample of keywords (8565), that with the highest number of occurrences (148) in all research is sustainable development. This term represents 13.96% of the total sample analyzed and was used in the three periods. During the years 2005 and 2011, a total of two investigations mentioned this term compared to 139 publications between 2017 and 2021. The data indicate that it is from the last period that this concept began to gain relevance in the field of study. The term sustainability ranks second with a total of 132 occurrences. This concept, which represents 12.45%, has registered its greatest presence in the last period (2017–2021), with a total of 139 investigations. In fact, all the terms recorded in
Table 6 reach the highest percentage of variation when moving from the second period (2012–2016) to the third period (2017–2021). The terms with the latest incorporation in the research line are anaerobic Digestion and nitrogen. The two terms registered their first publications as of 2017 and, therefore, are framed in the last period. On the other hand, agricultural robots is the only concept that registered an occurrence between 2005 and 2011, and did not register any in the second period (2012–2016). In this sense, the next investigation that registered this concept dates to the year 2017, reaching 109 investigations for the last period analyzed (2017–2021).
It is common to find publications that combine keywords from different clusters, such as, for example, those that contain the concepts of “Wastewater Treatment” and “Sustainability”. Both terms belong to different topics of study and therefore to clusters of different color, although joint investigations can be identified, such as those entitled “Hybrid life cycle assessment of agro-industrial wastewater valorization” or “Reuse of Treated Effluents in a Food- Processing Industry” [
64,
71]. However, this study aims to identify future research trends, thus it analyzed the 330 keywords with the highest number of occurrences. In
Figure 7, five clusters are identified in different colors depending on the group to which they belong and the size based on the investigations registered for each term. Each cluster represents a topic of interest within the line of research analyzed. In this sense, the relationships established between the terms refer to the times that they were included together in the articles, reflecting the affinity between these concepts. Therefore, to easily identify the relationships established between the most used keywords, the analysis was limited to those that registered the most occurrences and not to all that were considered in the entire time horizon.
The first group of keywords is represented in red. This cluster registered a total of 98 keywords and is represented by the terms “Sustainable Development”, “Sustainability”, “Food Waste”, “Food Production”, and “Recycling”. These terms are directly related to the subject of study since they record research on practices that ensure a future framed in circularity. Among the proposed approaches are studies focused on presenting sustainable food value chains as well as alternatives to reduce waste in the agricultural sector [
13,
72,
73].
The second cluster, which is green in color, is defined by the terms “Chemistry”, “Biomass”, “Agricultural Robots”, “Fermentation”, and “Biodegradation”, and is made up of a total of 98 concepts. Research in this group of words focuses on proposing alternatives to managing the biomass created throughout the agricultural production process and, thus, that contribute favorably to EC. An example of this are the investigations “Circular economy in olive oil production-Olive mill solid waste to ethanol and heavy metal sorbent using microwave pre-treatment” and “From Agricultural Waste to Biofuel: Enzymatic Potential of a Bacterial Isolate Streptomyces fulvissimus CKS7 for Bioethanol Production” [
74,
75].
The third group of terms that registered the most occurrences is the one indicated in yellow. This cluster contains 44 concepts and publications that have the object of study of water treatment. In this case, the concepts “Wastewater Treatment”, “Water Conservation”, “Chemical Removal (water treatment)”, “Effluent Treatment”, or “Nutrient Recovery” are some of those framed in this object of study [
76,
77,
78,
79]. This issue is of vital importance in the line of research studied since agricultural activity is the one that consumes the most water worldwide. In this sense, the study of alternatives to be able to use wastewater in agriculture are far-reaching measures since they will reduce the harmful consequences derived from dumping wastewater while increasing the availability of water.
The fourth cluster appears in purple. This group of words, represented by the keywords “Fertilizers”, “Anaerobic Digestion”, “Biogas”, and “Biofuels”, focuses on finding answers to toxicity problems. In fact, the keywords used in this cluster suggest this subject of study [
80,
81]. This issue is considered of vital importance in agriculture since the uncontrolled use of insecticides and chemical products negatively affects the environment. For this reason, it is common to find publications that focus on presenting solutions to these problems based on more sustainable actions.
Finally, the fifth cluster is made up of a total of 49 terms and is represented in blue. The terms with the most occurrences are “Phosphorus”, “Life Cycle Assessment”, “Global Warming”, and “Environmental Technology”. The concepts that make up this cluster refer to air pollution and the negative consequences that bad agricultural practices imply, as well as to the lack of actions to implement CE. Examples of the investigations studying this issue are the documents under the titles “How to reduce the carbon footprint of an irrigation community in the southeast of Spain by use of solar energy” and “Mitigating ammonia and greenhouse gas emissions from stored cattle slurry using agricultural waste, commercially available products and a chemical acidifier” [
82,
83].
Figure 8 was prepared from the keywords considered in
Figure 7. In this case, instead of identifying the groups of words according to the study topic, the maturity of each of the main terms used in the research of CE and agriculture is indicated. The timeline selected for
Figure 8 starts from the end of 2017 and extends to the present, that is, 2021. The terms indicated in dark blue refer to the keywords with greater maturity and the terms indicated in light green and yellow to the more recent maturity since they appear in the last months. The format of
Figure 8 is different because it wants to highlight, in addition to the maturity of the concepts, those that stand out from the rest of the sample due to their high number of occurrences. In addition, to complete this analysis and to know in which specific period they were referenced, the most prolific terms of each period are specified.
In the first period analyzed, namely 2005–2011, the investigations include the terms of environmental protection, pollution, air pollution, and cleaner production. Neither of these concepts receives numerous occurrences in the other two periods, hence, in
Table 6, they are not among the most prolific keywords. In the second period, namely 2012–2016, the research object is extended to other topics associated with the terms of recycling, agricultural wastes, and sustainable development. However, it is not the only interest that arises in this period since economic terms begin to draw attention, such as economics and social effects, industrial economics, and economic development. In the third period (2017–2021), the aforementioned concepts are maintained and new ones are incorporated, such as fertilizers, phosphorus, biogas, or nitrogen. In addition, numerous publications begin to be registered with the concepts of agricultural robots, innovation, energy utilization, or energy efficiency; these terms are indicated in yellow and light green due to their low maturity in the research, which means that they are new terms that are emerging.
3.8. Discussion
The variables that have been analyzed in this results section show the current interest of the documents published in Scopus on CE applied to agriculture. Throughout the 17 years analyzed, scientific productionresearch has been increasing exponentially and the last year (2021) is the one with the highest number of annual publications. The line of research studied is characterized by having a multidisciplinary nature since it registers very diverse thematic areas. In addition, the research framed in the main categories is directly related to the main groups of keywords used. In the case of environmental science, the terms of sustainable development and agroecology are framed among other concepts since the purpose of this field of study is to ensure a sustainable future [
13,
72,
73]. On the other hand, the terms energy and engineering are related to the search for actions that optimize waste management processes [
84,
85,
86]. Therefore, both subject areas represent the terms of agricultural robots or nutrient recovery. Finally, if the disciplines of agricultural and biological sciences, Business, Management, and Accounting are addressed, reference should be made to the line of research analyzed, that is, to agricultural activities and food production, as well as to the possibility of implementing CE [
74,
75].
The journals that stand out for their contribution are Sustainability and the Journal of Cleaner Production. These two registered the highest values for their number of investigations. Even so, other journals are registered with high values in other variables, such as the average of citations (23.58) in Resources Conservation and Recycling, or the H-index (295) in Renewable and Sustainable Energy Reviews. As for the most active authors in CE research in the agricultural field, Xavier Gabarrell i Durany is worth mentioning, in first position, together with Sergio Ulgiati, in second position, and Luis Jesús Belmonte-Ureña, in third position. In addition, it is interesting to pay attention to the nationality of the most prolific authors since those of Italian and Spanish origin stand out. This is the reason, among others, why out of the 95 countries that publish, Italy and Spain are the most active. On the other hand, in regard to the country with the highest number of international publications and, therefore, with the highest percentage of the collaboration index, the UK and Germany are noteworthy, both with a value of 69%. Next, with a similar value, specifically 68%, is the USA.
These commented variables provide information about the interests that currently represent the line of research. Hence, the latest network map (
Figure 8) reveals the new issues that arise, which are related to energy efficiency and innovation [
87,
88]. These relevant issues can bring together several areas and obtain favorable results since the incorporation of efficient and intelligent processes can satisfactorily promote the introduction of CE in agricultural activity and thus improve not only the production of the food that is grown, but also the reduction of the biomass and waste generated [
89,
90].