3.1. Sustainable Agriculture in Mexico Research
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.
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.
A total amount of 71 countries participated in the elaboration of the studies. These countries are Mexico and 70 collaborating countries in research on SAM. The number of these countries has grown from four in 2000 to 35 in 2020. Specifically, the average growth of the number of countries participating in research on SAM was 11.5% per year.
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.
3.3. Journals
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 article published in 2015. It continues to publish on the subject today. Meanwhile,
Wit Transactions on Ecology and the Environment is in third place with 18 documents. This British journal show an H index of 2, a total amount of 12 citations, an average of 0.7 citations per document, and an SJR factor of 0.180. With only 12 papers on SAM,
Soil and Tillage Research has the most prominent H index with a value of 12. It additionally has the highest total citations and average amount of citations per document, with 874 and 72.8 respectively. Moreover,
Soil and Tillage Research is the longest-established journal in this field, publishing its first contribution on SAM in 2000. 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.
3.5. Institutions
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.
3.7. Keywords Analysis
Figure 3 and
Figure 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.
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 productivity–evapotranspiration 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.