Introduction

The effects derived from the conventional intensification of agriculture based on the high use of chemical inputs are clearly unsustainable. This intensification offers a set of practices based on the use of synthetic inputs for large areas of monoculture. The productivist model based on the high use of inputs had its origin in the industrial revolution; that is, at a time when the volume of the human population represented less than 15% of the current one and in which the unsustainability of productive systems was not so evident (^{Montoya et al., 2022}).

Currently, the great challenge of world agriculture is raising its economic, social, and environmental sustainability. Among the targets proposed in the Agenda of the Sustainable Development Goals (SDGs) for 2030, the purposes of ‘ending hunger, achieving food security, improving nutrition, and promoting sustainable agriculture’ can be highlighted (^{UN, 2022}). These goals have determined changes in the way intensification processes are implemented so that the priorities of the sector are met without overexploitation of the land (^{Creus, 2017}).

In this regard, ^{Lal (2019}) states that it is precisely in this context where the importance of eco-intensification can be insisted on for a much-needed paradigm shift, understanding that practices that improve soil quality and functionality will positively impact the activity and diversity of biota species, where microorganisms play a fundamental role; thanks to their ecosystem services, they are an essential part of a successful transition to sustainable agricultural systems.

This approach was developed since the beginning of the twentieth century in northern Europe under the concept of ecological agriculture, where they were already distancing themselves from the trends aimed at increasing yields in conventional agriculture (^{Calderón, 2004}).

By virtue of this ^{Ayan et al. (2021}) argue that the use of beneficial microorganisms constitutes a strategy to develop more sustainable agricultural systems and reduce the negative impact of chemicals for agronomic management (nutrition, pests, and diseases). In addition, plant-associated microbial communities improve plants’ tolerance to environmental stress, favor nutrient uptake and contribute to their growth (^{Santoyo et al., 2021}).

Based on these considerations, this work aimed to analyze the potential of using microorganisms as a basis for eco-intensification in agricultural systems. To this end, a meta-analysis was carried out considering the assumption that, in agricultural systems, eco-intensification does not weigh the benefits of managing microorganisms consciously to maximize productivity.

Materials and methods

The compilation of the scientific literature was carried out from January to March 2022, the period in which the research was conducted, documents published between 2015 and 2022 were used; in the case of eco-intensification concepts, publications that allowed understanding the construction of the concept were analyzed. Websites specialized in academic search engines were used, such as Google Scholar, ResearchGate, Science Direct, Scopus, and Web of Science, which allowed the search in: Agrosystems, Elsevier, Emerald, IOP, Nature, Oxford, Springer, and in the Mexican journals of Science and Technology of the National Council of Humanities, Sciences, and Technologies (CONAHCYT, for its acronym in Spanish), of free access.

Searches were conducted in Spanish and English. The main keywords used in the search were: 1) intensification/intensification; 2) ecological/ecological; 3) microorganisms/ microorganisms; 4) agroecology and 5) agriculture.

All publications found were managed and stored with the free access program (Zotero, 2018). The documents were exported in RIS format to a computer storage folder. The files were imported into the Nvivo trial version program, where a frequency analysis of words with at least nine characters was performed. This number of characters was selected since keywords or phrases related to the topics of the publications contain at least these characters.

The frequency analysis of words allowed the identification of the most representative keywords of each document, that is, all the words that were found within the first quartile (25%). These keywords allowed the selection of the publications that contained the highest percentage of information on the key research topics. The selected publications were subjected to a cluster analysis to identify the relationship between research topics through the Nvivo program through the classification parameters called nodes.

The nodes were coded with the first quartile (25%) of keywords. The cluster analysis allowed the determination of the frequency of studies for each topic and the relationship between them, the results of which generated three discussion approaches to analyze the potential of microorganisms in eco-intensification in agricultural systems.

Results and discussion

The frequency test of words with at least nine characters generated six words that represent 25% of the total words (in Spanish and English): intensification, ecological, microbial, agriculture, environmental, and agroecology, which are represented in the image of the most frequent words considering the 203 references analyzed (Figure 1).

Figure 1 Distribution of the most frequent words in publications and documents related to the eco-intensification of agricultural systems. 

Figure 1 shows the words of a length of at least nine characters most representative in the sample of scientific information selected, which shows the relationship that exists in the bibliographic collection with respect to the objective of this work. Of the total articles analyzed in the Nvivo software, only 12 (Table 1) relate the keywords: 1) intensification; 2) ecological; and 3) microorganisms.

In this sense, it can be stated that despite the relationship that exists between ecological processes in agriculture and soil microorganisms, it is visualized that of the 203 publications, only 5.9% relate ecological intensification processes with the use of soil microorganisms. The rest had a majority focus on the importance for soil health, comparison with other technologies (conventional and sustainable intensification), crop association and rotation, biodiversity, and carbon sequestration, among others.

Ecological agriculture, according to ^{Trivedi et al. (2016}), is based on improving the ecosystem services provided by biodiversity, these depend on the degree of (agro)biodiversity at the field, farm, and landscape level, and they call it biodiversity-based agriculture, while other authors call it ‘ecologically intensive agriculture’. Its main objectives are to reduce negative environmental impacts and raise production limits of production-oriented agriculture, and it is known as ‘efficiency/substitution-based agriculture’, while others call it ‘ecological intensification’. Through the meta-analysis, this study shows the relationship between eco-intensification, as a synonym for ‘ecological intensification’, with the ecosystem processes of agroecosystems, specifically with soil microorganisms.

Figure 2 and Table 2 show the results of the multivariate analysis of clusters (the farther apart the elements, the less related they are) and Pearson’s correlation coefficients (coefficients close to one indicate a strong relationship between the variables and values close to zero indicate that there is no relationship between the two variables; values less than 0 indicate negative association), these indicate that there is a non-significant relationship between the studies that address the topic of intensification in agriculture with important components in agroecosystems, such as microorganisms present in the rhizosphere, according to the documents analyzed. This shows that there is a niche for research and intensive agricultural production based on maximizing ecological processes with soil microorganisms in agroecosystems.

Figure 2 Diagram of the cluster analysis of the topics analyzed level 3. 

The correlation according to the Pearson test (Table 2) indicates that the relationship between codes is not significant, with a degree of significance of p≤ 0.05; in the case of the relationship of the Rhizosphere code with Intensification, there is a significant relationship; nevertheless, it is negative, which allows us to understand that the publications address the topics with an inversely proportional relationship.

These results allow the publications to be separated into three thematic axes: 1) ecological intensification or eco-intensification (EI) of agricultural systems; 2) potential of soil microorganisms in eco-intensification; and 3) ecosystem services provided by microorganisms in eco-intensified agricultural systems.

Potential of soil microorganisms in eco-intensification

EI is a system based on agricultural production and sustainable management of soil quality and functionality. Nevertheless, to achieve this, it is vital to understand the microorganism-soil-plant relationships that are becoming more relevant every day since the functionality of agroecosystems is essential for human well-being as there are beneficial relationships that can be used to optimize agricultural production (^{Bajsa et al., 2013}; ^{Omotayo and Babalola, 2021}). These contributions that nature provides to human society are known as ecosystem services (ESs).

According to ^{Rojas and Hernández (2021}), ESs integrate tangible and intangible benefits derived from nature for the benefit of human beings and that, according to certain criteria, can be economically valued for their conservation and management. ^{Saccá et al. (2017}) established that ESs originate from the interaction of biotic and abiotic processes in the agroecosystem.

Therefore, it is essential to understand the ecosystem benefits that soil microbial activity can provide in agroecosystems. Soil microorganisms play several key regulatory roles in biogeochemical cycles, soil fertility, and resilience. They increase the density of nutrients by the decomposition of organic matter or by the solubilization of elements. Because of this, these microorganisms are called functional groups (^{Thiour et al., 2019}; ^{Ayan et al., 2021}).

It is recognized that the functional diversity of soil microorganisms can improve the stability of agroecosystems and the provision of ecosystem services, but there is a need to link existing knowledge on understanding the functionality of agroecosystems in the rhizosphere to fully explain the connection between diversity, stability, and function in agricultural systems (^{Santoyo et al., 2021}). In a general sense, Figure 3 specifies the main ecosystem services to which soil microorganisms contribute.

Figure 3 Main ecosystem services provided by soil microorganisms. Modified from ^{Saccá et al. (2017}); ^{Chen et al. (2019}). 

Conclusions

Only 5.9% of the total publications analyzed highlight the importance of soil microbiology in agricultural eco-intensification; this opens a new line of research, which is the study of the potential of soil microorganisms and the impact of ecosystem services on eco-intensified agricultural systems.

The strengthening of soil microorganisms as an element of eco-intensification undoubtedly represents a niche to improve agroecosystems’ ecology, productivity, and profitability, taking advantage of and maximizing the ecosystem services they offer.