Overview of Research on Sustainable Agriculture in Developing Countries. The Case of Mexico

: One of the principal challenges faced by Mexican agriculture is the development of management models that are able to increase production while respecting the environment and generating wealth for society as a whole. In recent years, the number of studies analysing the sustainability of Mexican agriculture has grown signiﬁcantly. The purpose of this study is to analyse the evolution of the research on the sustainability of agriculture in Mexico. For this purpose, a review and bibliometric analysis of a sample of 867 articles was carried out. The results reveal that the research has focused on the development of sustainable agricultural models in vulnerable rural areas, the sustainable exploitation of agroforestry systems, the development of energy crops for different uses, water resource management and land uses and their changes, conservation farming and climate change. Although research focused on sustainability is still in its early stages, it has become a priority ﬁeld. A need to promote research from the economic and social disciplines may be observed, together with holistic projects that include the three pillars of sustainability (social, economic and environmental). This study could be useful to researchers in this ﬁeld as it identiﬁes the recent trends and principal agents that drive knowledge.


Introduction
Humankind has modified 75% of the earth's surface in an effort to meet the demand for food, fibre and bioenergy [1][2][3]. Currently, about one third of the Earth's land surface is used for agriculture [4]. The expansion and intensification of agriculture in recent decades has resulted in increased production, reducing the number of malnourished people [5][6][7][8]. On the other hand, the human population is experiencing the largest growth of its history and is expected to peak at 9.7 billion people in 2050 [9]. In addition, new consumer trends and the increased availability of income are factors driving higher demand [10]. In the worstcase scenario, this growth in the population will require an additional food production increase of up 110% [11]. In turn, this increase in production will necessitate a greater expansion of the cultivated area or the development of new intensive production systems. Moreover, there is the technological change as a factor that influences production growth, so required changes include not only intensification but sustainable innovation [10].
However, fulfilling these objectives may have strong impacts on the environment, which could be irreversible. Agriculture is the world's largest consumer of water resources [12,13], using from 60% to 90% of total available water, according to the climate and the economic development of the region [14,15]. An increase in irrigation to meet growing food demand could severely affect the availability of water for the natural ecosystems, and even for human supply [16,17]. On the other hand, due to deforestation practices, agricultural expansion is the world's second biggest threat to biodiversity conservation [18,19]. ture is an essential component for the country's development for several reasons [41]. The agricultural activity i) is a useful tool to help ensure food security, ii) is key to strengthening the development and growth of production, iii) contributes to improving standards of living, and iv) is a vital sector for rural areas, home to a quarter of the country's population, and accounts for 50% of the rural population's income [40,42,43]. In recent years, the share of Mexican agricultural goods in foreign markets has increased, thanks to their quality and diversity and to the tariff advantages resulting from the North American Free Trade Agreement (NAFTA). As a result, it has developed a strong specialisation in fruit and vegetable exports, principally to the United States. The eradication of poverty is another argument to strengthen agriculture. This is mainly due to the fact that about 20% of the population is considered to be in food poverty, and 5% is categorised as undernourished [40]. It is estimated that children in rural areas are 43.4% stunted, more than double the national average in Mexico, with negative effects on motor and cognitive development; this demonstrates that short stature is not an adaptation, but a condition of vulnerability that reduces the body size and capabilities of the subjects who present it, since mental functions (intelligence, memory and learning) affect learning abilities [44,45].
However, Mexico's agricultural development has certain limitations that put the sustainability of the sector at risk. First, there exist problems concerning the availability of water for irrigation in different parts of the country [38]. On the other side, the surrounding ecosystems are being severely degraded due to the over-exploitation of the water resources, particularly in the river basins and aquifers. Of the 653 aquifers in Mexico, a total of 105 are over-exploited, whereas about 69 of the country's 757 river basins have a water deficit, as the allocated flow is higher than the recharge capacity of renewable water resources [46][47][48]. Furthermore, the unequal development of water infrastructures generates inefficiencies and inequality between territories [48,49]. Therefore, in Mexico, there is an urgent necessity to develop management models in agriculture that ensure the stability of a sensitive sector for the economy, by increasing production in an environmentally friendly way and generating wealth for society as a whole [28,29,50].
A growing number of contributions studying the adoption of sustainable practices in Mexican agriculture have been published. To date, however, no study has analysed these contributions collectively. The purpose of this paper, therefore, is to study the evolution of the research on the sustainability of agriculture in Mexico. Given the nature and objectives of this work, bibliometric analysis has been considered the most appropriate methodology, due to its capacity to extract information by synthesising data from a large number of literature sources [51,52]. The findings will allow us to identify the main drivers of knowledge in these fields and the most significant lines of research. Finally, given that the extensive area of Mexico covers a diverse range of climate zones, the results of the study on Mexican agriculture could be extrapolated to other regions, including mountainous areas, regions with arid or semi-arid climates or those with mild or tropical climates [53,54]. In contrast to previous studies, this paper is the first to apply the bibliometric method to analyse research on sustainable agriculture using Mexico as a case study, given that previous works focus on different specific aspects of agricultural sustainability and their results mainly represent developed countries [19,[55][56][57]. This is its main contribution.

Methodology
In order to meet the purpose of this study, a bibliometric analysis of scientific literature is used, based principally on the traditional approach of co-occurrence analysis. In order to evaluate the information relating to the bibliometric data, indicators of productivity, quality and structure are included [51]. First, we identified the most productive agents (including authors, journals, institutions and countries) based on the amount of contributions and, second, we examined the impact of the research of these authors. This analysis was carried out in the same way for journals, institutions and countries. This information could be relevant to researchers when assessing the relevance of the media in which they publish their findings, the centres with which to collaborate or the countries in which to undertake research [58,59]. Finally, we use mapping techniques to analyse the trends in this line of research.
Scopus was used to collect the studies to be analysed. The main reason for using Scopus instead of Web of Sciences is that, according to Mongeon and Paul-Hus [52], the former offers a wider coverage of abstracts and journals. Therefore, Scopus allows to obtain a more representative sample of papers from the field of study. In addition, this database has a series of advantage over other repositories, such as (i) it is easy to access, (ii) it enables the visualisation of data analysis, and (iii) it makes it possible to download data in different formats for further processing by software applications [60]. It is very common for data available in statistical databases, or valuable grey literature, to be used in studies and published in the form of reports, conference papers, and/or book chapters. Therefore, to avoid duplication of information, only original articles have been included in the sample, which, in addition, have undergone a peer-review process, demonstrating the quality of the information [58,61].
The search parameters were used in the search engines based on the keywords, titles and abstracts. The intention of the authors in carrying out this work is to focus on the field of sustainability. For this purpose, general parameters have been used, which are already used in a similar way in other works in which only terms derived from the root sustain* are used [62][63][64][65]. In addition, to include the perspective of the 2030 Agenda, terms drawn from the definition of the target on sustainable agriculture, contained in SDG 8, have been included. The study period selected was 2000 to 2020, in order to assess the contribution made in this century so far, as the greatest development of this topic in this area of study has taken place in this period. To compare full annual periods, only documents up to 2020 have been included. Similarly, to avoid duplication, the sample only includes original articles [56,66]. The search was last updated in February 2021.
For the selection of the final sample of documents, a sequential process was followed, as shown in Figure 1. In this way, for a document to form part of the sample to analyse, it should address an aspect of agriculture from a perspective of sustainability and be related to the geographical area of Mexico (as part of the area of study of an empirical analysis or as an area of reference of a theory development study). Summing up, the introduced sample selection criteria are the following: (i) topic of study (sustainable agriculture), (ii) time period (from 2000 to 2020), (iii) geographical location (country of Mexico as case study), (iv) language (English and Spanish as dominant language in the field of study, finding no bias between them) and (v) type of document (only original articles). The sample of papers that will finally be analysed in this study consists of 867 articles. In parallel, a search for papers related to agriculture in Mexico with the same restrictions was also conducted to analyse the relative importance of sustainability in relation to the general topic (see Figure 1). We should remember that this study is conditioned by the selection process of the sample and that other search parameters or databases, or another subsequent updating of the search, could generate different results. Literature on the area is becoming more and more specific and beginning to consider other possible aspects, which may or may not mention the chain "sustain *", such as climate smart agriculture, resilience, agroecology, agroecosystems.
The number of articles, the name of the journals and the year in which they were published, the authors and institutions and countries of affiliation, the subject area in which Scopus classifies the documents, and the keywords were the variables analysed. The first task was to download the information in the appropriate formats. Next, duplications that could lead to errors in the counting of the data were eliminated. The filtered information was then organised into different tables and figures and the data were analysed. The different tools used were Excel and SciMAT. To create the network maps, VOSviewer was selected, which is widely used in this kind of study [67]. Lastly, in order to extract the main trends in the research, a keyword analysis was conducted [68]. VOSviewer was selected, which is widely used in this kind of study [67]. Lastly, in order to extract the main trends in the research, a keyword analysis was conducted [68].

Figure 1.
Overview of the methodology. Table 1 presents data on the general variables concerning research on Sustainable Agriculture in Mexico (SAM) during 2000-2020. The number of documents published during the entire period on this topic amounted to 867, while in the case of research on Agriculture in Mexico (AM), there were 10,338. This indicates that the research on sustainability accounted for 8.4% of total research on Mexican agriculture. We searched for this information for other countries in order to compare this ratio. We have verified that this percentage is lower than countries such as China (11.6%), Spain (10.4%) or Australia (9.7%); similar to others, such as the United Kingdom (8.6%); and higher than others, such as the United States (6.8%). The number of papers on SAM increased from 12 in 2000 to 109 in 2020.

Sustainable Agriculture in Mexico Research
The rate of annual variation in the amount of documents on research in AM and SAM is shown in Figure 2. The average annual variation of SAM papers was 11.7% while that on AM was 8.4%. As we can see in the figure, the research on AM had a higher growth in almost the whole period analysed until the year 2010, when the trend reversed. This leads us to state that, in recent years, the research on SAM has been increasing in prominence within the research on AM.
In the 867 articles analysed, a total number of 3167 authors were involved. Over the years, this variable has shown a continuous growth trend. The amount of authors has increased from 33 in 2000 to 540 in 2020. The average amount of researchers per document has grown from 2.7 to 5.1. Therefore, the annual average growth of the authors was 15.1%. Of the total 3167 authors, 88.3% only participated in one of the articles of the sample analysed, while less than 1% participated in five or more articles. These data show that there is a high concentration of research on SA in a small group of researchers, who constitute the main drivers of this research topic.  Table 1 presents data on the general variables concerning research on Sustainable Agriculture in Mexico (SAM) during 2000-2020. The number of documents published during the entire period on this topic amounted to 867, while in the case of research on Agriculture in Mexico (AM), there were 10,338. This indicates that the research on sustainability accounted for 8.4% of total research on Mexican agriculture. We searched for this information for other countries in order to compare this ratio. We have verified that this percentage is lower than countries such as China (11.6%), Spain (10.4%) or Australia (9.7%); similar to others, such as the United Kingdom (8.6%); and higher than others, such as the United States (6.8%). The number of papers on SAM increased from 12 in 2000 to 109 in 2020.

Sustainable Agriculture in Mexico Research
The rate of annual variation in the amount of documents on research in AM and SAM is shown in Figure 2. The average annual variation of SAM papers was 11.7% while that on AM was 8.4%. As we can see in the figure, the research on AM had a higher growth in almost the whole period analysed until the year 2010, when the trend reversed. This leads us to state that, in recent years, the research on SAM has been increasing in prominence within the research on AM.
In the 867 articles analysed, a total number of 3167 authors were involved. Over the years, this variable has shown a continuous growth trend. The amount of authors has increased from 33 in 2000 to 540 in 2020. The average amount of researchers per document has grown from 2.7 to 5.1. Therefore, the annual average growth of the authors was 15.1%. Of the total 3167 authors, 88.3% only participated in one of the articles of the sample analysed, while less than 1% participated in five or more articles. These data show that there is a high concentration of research on SA in a small group of researchers, who constitute the main drivers of this research topic.
In the year 2000, no two papers were published in the same journal. However, in 2020, the 109 papers on SAM were published in a total of 71 journals. The average volume of documents per journal has stayed constant over the entire period. In total, the 867 documents on SAM were published in 432 different journals. The average growth of the number of journals was 9.3% per year. Of the total sample of journals, 70.4% have published just one article on SAM, while 9.1% have published five or more. Again, we can confirm that the publication of the papers was concentrated in a small group of journals.   Table 2 shows the data on the classification into thematic categories established b Scopus. It is important to note that the same paper can be simultaneously classified several categories. In both the SAM and the AM documents, the most popular category Agricultural and Biological Science with around 54% of the total in both cases. In these tw lines of research, the next two prominent topics are Environmental Sciences and Soci Sciences. Considering the different fields included within the concept of sustainabili The studies on SAM as a whole obtained a total amount of 12,787 citations from 2000 to 2020. The research on AM as a whole accumulated a total of 166,356 citations. Therefore, research on SAM accounts for 7.7% of the total citations of the research on Mexican agriculture. This is due to the later development of research on SAM with respect to AM. The amount of citations has risen from one in 2000 to 2504 in 2020, which represents an average annual increase of 47.9%. The average of citations achieved per paper has grown from 0.1 in 2000 to 17.6 in 2020. In the case of research on AM, the average number of citations per document is 16.1. These data show that the research on SAM has been growing importance within the research on AM in recent years, not only with respect to the number of papers, but also in terms of its prominence measured through the cipher of citations. Table 2 shows the data on the classification into thematic categories established by Scopus. It is important to note that the same paper can be simultaneously classified in several categories. In both the SAM and the AM documents, the most popular category is Agricultural and Biological Science with around 54% of the total in both cases. In these two lines of research, the next two prominent topics are Environmental Sciences and Social Sciences. Considering the different fields included within the concept of sustainability (environmental, economic and social), we can observe certain notable differences. If we talk about SAM research, higher values are observed in the environmental, social and economic categories, showing the greater significance of these areas in this line of research. Specifically, the Environmental Sciences category represents 49.3% of SAM papers, whereas it only accounts for 32.6% in AM papers. Similarly, the Social Sciences category accumulates 25.5% in SAM line, whereas it only accounts for 17.3% in AM research. The economic aspects (Economics, Econometrics and Finance and Business, Management and Accounting) represent 4.7% and 3.5%, respectively, in the case of SAM, and only 3.4% and 1.7% in the case of research on AM. These data highlight two realities. On the one hand, as we would expect, the research on SAM pays greater attention to the different aspects related to the environmental, social and economic fields, while the research on AM focuses on different technical and biological aspects. On the other hand, they also reveal that in the research on sustainability, topics focused on environmental issues are still predominant and those themes related to social and economic aspects still need to be developed. Therefore, there is a need to broaden research from these perspectives and to conduct holistic studies that consider the three dimensions of sustainability.  Table 3 includes data on the most prolific journals in SAM during the period 2000-2020, in relation to the documents that form part of the analysed sample. Publication in these journals is much broader and covers a wide range of topics. However, in this section, we want to focus on the relevance of SAM publications for these journals, as measured by the different indicators shown in the table. This group is composed of Mexican and European journals but none from the United States stand out. They are highly diverse in terms of field of specialisation and level of relevance measured through the Scimago Journal Rank (SJR 2020) impact. Furthermore, we can observe large differences in terms of their incorporation in this line of publication and the date of publications of the last article on SAM. Together, this set of journals has contributed 177 papers to the sample, accounting for 20.4% of the total amount of documents published. Tropical and Subtropical Agroecosystems, with 39 papers, is the journal that has contributed the most to SAM publishing. This journal has an H index of 4 (this refers to the documents included in the SAM sample, and not to the total number of documents published by the journal), a total amount of 50 citations and an average of 1.3 citations per paper. Furthermore, it has an SJR factor of 0.249 and began publishing in this field in the last decade, with its first article on SAM in 2011. This is followed by Sustainability with 29 articles. This journal has an H index of 5, a total amount of 82 citations and 2.8 citations per paper, and its SJR factor is 0.612. This journal was one of the latecomers to this field, with its first On the contrary, the journal that has most recently begun to publish in this field is Terra Latinoamericana, with its first article on this topic being published in 2016. This journal has published a total number of 10 documents. It was indexed in the SJR for the first time in 2020. It has an H index of 3, a total amount of 16 citations and an average number of 1.6 citations per paper. Finally, the journal with the highest SJR factor is Journal of Cleaner Production, which has published a total of nine articles on SAM.   Table 4 includes information on the collaborative networks that Mexico has established in SAM and AM research. The average percentage of projects developed through international collaboration is more important in SAM than in AM research, with 41.4% and 34.3%, respectively. However, the total amount of international collaborators is significantly higher in AM (103) than in SAM (70) research. As sustainability research requires a multidisciplinary approach, this field of study is also considered to be more collaborative. However, the data presented here are explained by the incipient state of this line of research and the unequal number of publications with respect to the general line of research. As for the most relevant collaborators in each of the research lines, many similarities can be found. However, in terms of AM research, we find Australia and Brazil in the group of principal collaborators, whereas Belgium and Saudi Arabia are more important in the case of SAM. In accordance with the number of citations, the papers developed through international collaboration accumulate a higher amount in both lines of study, on average, than the documents without this international dimension. The average number of citations per paper written in collaboration is higher in the case of research on SAM than for AM papers (25.9 and 20.3, respectively).  Table 5 provides information on the institutions that have contributed the most to SAM research during the period 2000-2020. It should be noted that all these institutions may have a much longer publication history. However, as explained above, in this paper, we focus on research carried out in the current century. For example, some publications from 1997 by El Colegio de la Frontera Sur (ECOSUR, San Cristóbal de las Casas, Mexico) can be found. All of them are institutions with Mexican nationality. With 86 contributions, the National Autonomous University of Mexico is at the top of the table. This institution has the highest H index, which is 19. Furthermore, it has a total amount of 1321 citations, and an average of 15.4 citations per document. The institution in second position, with the most contributions, is the Instituto Politécnico Nacional with 38 papers. This center has achieved a total amount of 261 citations, an average of 6.9 citations per paper and an H index of 10. The Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias, with a final cipher of 35 documents, is the affiliation in third place in the table. This institution has a total of 571 citations, an average of 16.3 citations per document and an H index of 12. The International Maize and Wheat Improvement Center is the institution that has obtained the highest recognition for its research, using citation accumulation as an indicator, as it has achieved a total of 2020 citations and an average number of 59.4 citations per document. This institution, which is in fourth position in terms of the quantity of documents, with 34, has the highest H index. It should be noted that this centre is an international institution that is part of the Consultative Group on International Agricultural Research (CGIAR). However, given that it is based in Texcoco, and that it appears in Scopus as a Mexican-affiliated centre for all purposes, it has been considered appropriate to maintain it as such. With reference to international collaboration, the average percentage of documents on SAM developed by this cluster of affiliations is 42.6%. In the table, we can see that in all cases a higher average amount of citations is obtained in projects implemented in collaboration, except for the Instituto Politécnico Superior. Those above the average are the Centro Internacional de Mejoramiento de Maíz y Trigo with 73.5% of its publications carried out through collaboration, followed by the Colegio de la Frontera Sur (ECOSUR, San Cristóbal de las Casas, Mexico) with 63.6%, and the Universidad Michoacana de San Nicolás de Hidalgo with 58.3%. It is worth pointing out that the three institutions with the highest average cipher of citations per document coincide with those that have the greatest difference in citations obtained in papers developed through international collaboration. The Centro Internacional de Mejoramiento de Maíz y Trigo, on average, obtains 61.5 citations more in studies carried out in collaboration, and is the institution with the highest average amount of citations per paper in the table. The Colegio de la Frontera Sur (ECOSUR, San Cristóbal de las Casas, Mexico) is the second institution in relation to the average amount of citations per document and, on average, obtains 31.7 citations more in its collaborative studies. The Instituto de Investigaciones en Ecosistemas y Sustentabilidad is the third institution in terms of citations per article. In this case, it obtains 59.9 more citations in studies implemented based on collaboration. Therefore, we can conclude that establishing quality collaboration relationships has a positive influence on the number of citations achieved by the studies published. However, the amount of studies carried out through collaboration is not directly related to the number of citations obtained. Table 6 includes the most salient authors on SAM research considering the amount of contributions. Overall, the researchers in this group come from eight Mexican and three international institutions, two of them in the USA and one in Belgium. Six of the eight Mexican institutions are also on the list of most prolific institutions. The reason explaining the appearance of new institutions in this section is the establishment of collaboration networks between authors, which helps to position some of them in prominent positions.   Figures 3 and 4 illustrate the network maps of keywords used in the documents published on SAM based on their cluster groups and evolution over time, respectively. The size of the labels corresponding to each term (represented by a circle) varies according to the number of times it is repeated. Thus, larger circles represent terms that have been used in a greater number of articles and vice versa. In Figure 3, the terms have been grouped according to the number of co-occurrences in the sample documents. The resulting clusters have been represented in different colours in order to differentiate between them. In Figure 4, the colour varies depending on the moment of greatest use of each keyword, with the dark blue tones corresponding to earlier periods and the yellow tones to more recent moments.   In Figure 3, we can observe six different clusters, representing the different lines of research that dominate this field. These groups have been obtained using algorithms available in the software application, based on the application of the strength of association similarity index for the normalisation of co-occurrence values. The red cluster focuses on the development of sustainable agricultural models in vulnerable rural areas. The overall objective of this line of research is the development of agricultural models that provide a base for the rural development of the most disadvantaged areas, where other alternatives do not exist. The priorities are to ensure food supply and to maintain the rural population. To do this, this line is based on the local traditional knowledge and the rich biodiversity of Mexico. In this line, Ubiergo-Corvalán et al. [69] documented the edible plant agrobiodiversity of the agroecosystems in the indigenous area of maya-ch'ol in Chiapas. Castro-Sánchez et al. [70] investigated the relationships of native Purépecha communities with edible mushrooms and their environment, the place of mushrooms in the indigenous cosmovision and classification structure, the forms of management and the social and environmental issues associated with their usage. Moreno-Calles et al. [71] analysed the contribution of ethnoagroforestry to support biodiversity, including plants and animals, ecosystems and landscapes, as a basis for food sufficiency and sovereignty for communities, regions and the whole of the Mexican nation. In Figure 4, we can see that this cluster corresponds to a recent line of research, given that the yellow and green tones are predominant among the keywords. The most prominent new concepts are food security, traditional knowledge and local knowledge. Given its recent incorporation, the number of studies on these aspects is even smaller and, therefore, the circles are smaller. If the shades turn darker without an increase in the number of publications, we can consider them to be terms that are in vogue.  The blue cluster refers to the sustainable exploitation of agroforestry systems. Its objective is to obtain a series of products, other than wood, through the use of traditional mixed management system practices with grazing and livestock. Furthermore, all of this is carried out under the premise of conservation, based on the use of sustainability indicators. An example of these systems is bovine farming. Prospero-Bernal et al. [72] analysed the sustainability of small-scale dairy systems that are based on conventional irrigated cut-and-carry pastures and cereal straw and commercial feed concentrates in the highlands of central Mexico. Espinoza-Guzmán et al. [73] evaluated the dynamics of changes in the agroecosystem of a shade-grown coffee plantation in the upper La Antigua river basin, Veracruz, considered as one of the principal systems for the conservation of biodiversity. Albarrán-Portillo et al. [74] analysed the socioeconomic and productive characteristics of agrosilvopastoral systems that comprise different elements such as crops, pastures, trees and shrubs, and are seen as a way forward to satisfy future necessities for different commodities such as food, feed, fuel, and other products, as well as for providing environmental and social benefits. García-Pérez et al. [75] studied Chamaedorea hooperiana as an alternative crop in primary forest, capable of providing an economic return while contributing to forest sustainability, in Los Tuxtlas Biosphere Reserve in Veracruz. Ferguson et al. [76] compared two systems of cattle ranching through the use of system indicators. While one used extensive grazing, annual burning of pastures and regular application of agrochemicals, jeopardising biodiversity and long-term productivity, the other employed holistic management with careful land-use planning, rotational grazing, diversified forage, and a lower amount of purchased inputs. With respect to the use of keywords over time, in Figure 4, we can see how within this line of research, there has been a replacement of terms over the years. During the first decade, the dominant terms were conservation, agroforestry and coffee, and today they have evolved towards the concepts of conservation of biodiversity, agroforestry systems, cattle and grazing. In this way, we can appreciate greater precision in the terms and a replacement of others, fruit of the development of the research.

Keywords Analysis
The yellow cluster shows a very recent line of research, which emerged in approximately 2013. It specialises in the development of energy crops for different uses. Together with food supply, the availability of energy resources for the rural communities is a pressing challenge in Mexico. In accordance with the Law on the Use of Renewable Energies and the Financing of the Energy Transition, Mexico aims to achieve the use of 35% renewable energy for 2024. The development of agricultural models that include in their objectives the production of energy through energy crops or through the use of surplus biomass has emerged as a sustainable alternative. Within this field, we can find studies such as those by Molina-Guerrero et al. [77], who analysed the potentiality of agricultural residues generated by Mexico's principal crops to produce energy (including sorghum, sugar cane, corn, wheat, barley, beans and coffee). Similarly, di Bitonto et al. [78] analysed and characterised another group of Mexican biomass wastes (including different seeds and fruits such as jatropha, avocado, palm, peppers, flamboyant, coconut and nance) to obtain a complete exploitation of their energy potential. Medina-Santana et al. [79] used a water-energy-food nexus approach to evaluate the sustainability of a multi-objective agricultural model in a community in Michoacan. The findings indicate that the sale of bioethanol as an economic activity could be considered attractive by slightly increasing the price of biofuel and the yield of sugar cane.
The violet cluster combines two priority themes: water resource management and land use and its changes. Given that a large part of Mexican agriculture is developed in arid and semi-arid climates where the availability of water resources is the principal limiting factor, the sustainable management of this resource is an urgent need [38]. Mexican agriculture has advanced greatly in terms of adopting technology to improve efficiency in water use. Reyes-González et al. [80] created evapotranspiration maps using multispectral remote sensing vegetation indices in order to quantify crop water consumption in line with their physiological phases. López-Hernández et al. [81] analysed the productivityevapotranspiration relationship, concluding that the determination of productivity through evapotranspiration has a direct relationship with crop yields, as it improves irrigation efficiency. The modifications experienced in land use have been related to the different environmental impacts but particularly the degradation of water masses [38]. Vanderplank et al. [82] reported that seawater intrusion into aquifers as a result of unsustainable extraction, mainly for agricultural irrigation, causes impacts on adjacent ecosystems, resulting in the loss of more than twenty native plants in the San Quintín valley. Furthermore, groundwater quality is also affected by salinisation and pollution as a consequence of wastewater use for agricultural irrigation and the fertilization [83]. In order to resolve these problems, different alternatives have been proposed. First, the search for new safe sources of water. González-Bravo et al. [84], for example, proposed the development of seawater desalination plants to contribute to supplying the growing water needs and to fight against the degradation of the over-exploited water masses. Studies such as those by Fernández et al. [85] proposed the development of agricultural models that contribute to supplying food, without modifying the natural environment and reusing resources, such as the urban agricultural systems.
The light blue cluster corresponds to the more consolidated line, given that in the map of the temporal scale, we can see how the dark shades dominate in the keywords.
The central theme of this cluster is conservation agriculture. These types of system are made up of a set of techniques including minimum tillage, permanent soil cover and the diversification of crops, which have the basic purpose of conserving the implementation of a more efficient system based on an integrated management of the soil, water, biological agents and external inputs [86]. The implementation of conservation agriculture jointly with an efficient use of fertilizers can improve the yields and quality of the production of the crops [87]. Fuentes et al. [88] studied maize and found that the application of conservation agriculture in this crop can increase soil carbon content and reduce carbon dioxide (CO 2) emissions. Rivers et al. [89] found that conservation agriculture can help in pest control by improving soil characteristics and reducing erosion, creating a better habitat for beneficial organisms.
Finally, the green cluster focuses on climate change and the impact of its consequences on the flow of ecosystem services derived from the agroecosystems. This cluster exemplifies how a new topic can become dominant within a field of study. In Figure 4, we can observe that the term climate change is not relevant until 2016; however, this concept is represented with one of the largest circles. This indicates that in a short time, it has attracted great interest in this field of study. One of the clearest impacts of climate change is the availability of water. Hernández-Bedolla et al. [90] estimated water availability under different baseline scenarios, concluding that the main factors affecting water availability are decreasing precipitation and high temperatures. Molina-Navarro et al. [91] concluded that in the Guadalupe basin, the run-off can be reduced by between 45% and 60%, while the recharging of the groundwater can fall by up to 74% as a consequence of climate change. There is a group of studies that analyse the characteristics of the crops and the soil under different scenarios of climate change. Díaz et al. [92] studied the bacterial community linked to the roots of three crops grown in semi-arid environments, under different growth cycles, to provide knowledge on the composition of their microbial community during the warm season in Northeastern Mexico. Baez-Gonzalez et al. [93] aimed to develop eco-efficient bean cultivars to be planted at high densities to sustain bean production in a changing climate. However, the impact of climate change can also give rise to economic and social consequences, particularly for more vulnerable countries such as Mexico, which represents important challenges for their development and agricultural well-being [94]. Another group of studies focuses on the impact on living conditions and the perceptions of the farmers. Orduño-Torres et al. [94] analysed the farmers' environmental perceptions and preferences in relation to climate change adaptation and mitigation actions. Shinbrot et al. [95] analysed the importance of vulnerability context, livelihood assets and climate perceptions of the farmers to adopt climate-related adaptation strategies.

Conclusions
The purpose of this study was to offer an overview of the development of the research on sustainable agriculture in Mexico during the 21st century. With this aim, a bibliometric analysis was conducted over a sample of 867 documents. Furthermore, in order to verify the magnitude of the evolution of this field of study, a parallel search of the general research on agriculture in Mexico has also been conducted. Subsequently, we performed a productivity analysis on the basis of the amount of documents, journals, subject categories, authors, affiliations and collaborations, and also a study of the principal topics developed based on the keywords used.
The findings indicate that both lines of research have increased in significance in recent years. However, even though the research on sustainability in agriculture in Mexico has emerged fairly recently, it has become a priority line over the last decade. This outcome is coherent with the trend observed worldwide in research in this area, especially in relation to the achievement of the Sustainable Development Goals of the 2030 Agenda promoted by the United Nations. Therefore, both in the general and in the sustainability lines of research, the dominant thematic categories were Agricultural and Biological Science, Environmental Science, and Social Science. In the case of the research on SAM, the series of topics related to the concept of sustainability was given more emphasis than in the research on AM. It is, therefore, necessary to foster studies about social and economic perspectives and to conduct research that considers the three dimensions of sustainability.
Examination of collaborative networks has found that the quantity of international collaborative studies was higher for SAM than for AM research. Thus, we can observe that, compared to other areas of study, sustainability is more multidisciplinary, and also more widely studied through international collaboration between institutions. Among the different reasons for the increase in international collaboration between Mexican institutions and foreign centres is the large number of international initiatives that promote the creation of global networks for sustainable development in different areas, especially from the United Nations. Furthermore, we are able to verify that there is a direct relationship between the quality of the collaborative relationships and the average amount of citations achieved by the studies. However, we cannot determine a direct relationship between the quantity of the studies performed through collaboration and the number of citations.
The analysis of the keywords shows six clusters in the research on SAM, focused on topics such as the development of sustainable agricultural models in vulnerable rural areas, the sustainable exploitation of agroforestry systems, the development of energy crops for different uses, water resource management and land uses and their changes, conservation farming and climate change. Within these topics, we have found new concepts that can consolidate and become the central themes, as in the case of climate change, food security, agricultural intensification or land use; and others such as agriculture, coffee, productivity or agroforestry.
The results of this study show that, during the last decades, Mexican agriculture has undergone a strong modernisation process, placing it among the world's leading producers and exporters. This process has been mainly based on the adoption of new models of agricultural management, the use of technological innovations, and the diversification of crops to access new markets. These advances have allowed Mexico to move from traditional subsistence agriculture to highly professionalised and productive agriculture. However, this process has not developed in a balanced way. Once the goals of production and commercialisation have been achieved, Mexico must face new challenges in order to achieve a fully sustainable agriculture.
The main areas to work on, which constitute the main lines of research proposed, are the following: (i) from an environmental perspective, production models must be developed that are more respectful of the surrounding ecosystems as a whole, especially with regard to water bodies that are currently overexploited; (ii) from the perspective of the economic sphere, the development of ancillary industries should be promoted, as well as processing industries, capable of generating added value for the country; and (iii) with regard to the social sphere, it is here that the greatest efforts must be made, given that the current model is increasing inequality between northern and southern territories, between marginal rural areas and urban centres, and between employers and employees. Funding: This research received no external funding.

Conflicts of Interest:
The authors declare no conflict of interest.