Cooperative Network of Countries

The results indicate the spatial distribution of cooperative networks of countries/regions related to biomass and energy conservation mapped in Fig. 2a). The visual representation of international networks consists of 138 nodes and 284 links, the size of a particular node, which represents the number of publications published in each country on a particular subject. The analysis revealed that the United States of America (USA) has published 345 cooperation articles and ranked first in the list of leading productive countries, followed by China, Germany, India, and England with 140, 126, 69, and 65 articles, respectively. The share of these top 20 countries in producing articles on the topic is around 73% of the whole world’s contribution to the dataset. Europe is at the top with 42%, North America contributed 31%, Asia 19%, Oceania 4%, and South American countries 4% (Table 1). Therefore, a substantial number of research articles are produced by European and North American countries, which means that these countries are working on an advanced level in this field of research. Regarding the cooperative centrality of countries, which shows those countries that have contributed substantially to enhancing international communication and cooperation associated with the research topic, the USA has the highest centrality of 0.36, followed by Australia and Japan with a centrality of 0.30 and 0.23, respectively (Table 1).

One of the reasons might be the source of GHG emissions, as research highlighted that developing nations, especially China and India, as well as developed nations (the USA and Europe), contributed almost equally to greenhouse gas emissions, which might be one of the aspects for intensifying research productivity related to bioenergy [28]. Asia produces almost 40% of its electricity from biomass [29]. However, in the current situation, the energy production from biomass is higher in the USA, followed by China, Germany, Japan, and India [30]. For example, thanks to recent advances in genetic engineering and biotechnology, the USA enhanced its biomass properties and production potential for bioenergy without compromising the country’s food security [31, 32]. Bioethanol production in the USA (from corn) and Brazil (from sugarcane) collectively makes both countries the world’s leading bioethanol producers [33], around 85% of production in the world [34]. Moreover, the majority of biofuel companies that are using microalgal biomass for the production of bioenergy are located in the USA (78%) and Europe (13%) [35, 36]. Success in new technologies, such as advanced AD of biomass, has become a commonly used technology in many developed nations to generate renewable energy, highlighting the relevance of research in this field [37]. The biogas industry in Europe is more industrialized and commercialized than in developing countries like China [38], accounting for 50% of the global biogas production [39]. Similarly, Iran has abundant biomass resources in agricultural waste (59%) and animal waste (28%); their biomass energy potential is about 13% of crude oil sales, which might be a promising reason for the upsurge in research related to the topic [40]. Finally, the African continent has the highest consumption of renewable energy (47.3%) due to traditional biomass burning for heating and cooking purposes, which accounts for 52% of the total carbon emissions, including 44% of CO₂ emissions and 36% of CH₄ emissions [41, 42]. This indicates huge potential in bioenergy sectors in African regions; however, the research productivity of these countries is greatly lacking.

Cooperative Network of Authors

The co-authorship network shown in Fig. 2b) consists of 621 nodes and 462 links, where nodes represent authors and links show their collaboration. The node size indicates how many articles a specific author has published, whereas the thickness of links represents the amount of collaboration between authors. The analysis found a few closed-loop circuits that showed a collaborative network between these authors. However, the overall number of links is less than the total number of nodes, which indicates that the research cooperation among authors on the topic is weak.

The leading, most productive authors in the field are those with higher node size, as shown in Fig. 2b). The majority of the leading authors are from the European, Asian, and North American regions. The analysis revealed that the topmost productive authors, each with 7 publications in the field of biomass and energy conservation, are Prof. Mehdi Bidabadi, Sadegh Sadeghi (Ph.D. student) (both are from the Mechanical Engineering Department, Iran University of Science and Technology), and Prof. Bernhard Peters (Faculty of Science, Technology, and Communication, the University of Luxembourg). These are followed by Alexander Popp from the Potsdam Institute for Climate Impact Research, Germany; Andrea Monti from the University of Bologna, Italy; J A Van Oijen from Mechanical Engineering at Eindhoven University of Technology, the Netherlands; and SC Bhattacharya and PA Salam, both from the Asian Institute of Technology, Thailand. As for the area of interest, leading authors are related to combustion technology, renewable energy, genetics/genome research, fluid mechanics, remote sensing, and conservation ecology, which highlights the multidisciplinary nature of the topic.

Cooperative Network of Institutions

The analysis results regarding the most productive universities/institutions in biomass and energy conservation topics found a cooperative network of organizations consisting of 509 nodes and 392 links (Fig. 2c)). The study found that the Chinese Academy of Sciences is the most productive institution globally. The superior position of Chinese institutions in the results might be because, in recent years, China has made remarkable academic and institutional improvements, particularly in science and technology [43]. Similarly, recent research found that Chinese funding for biomass energy research is second in the world after the USA [30], which might be the prime reason for the higher number of publications on this topic. The overall visualization shows that the majority of the leading institutions are from North America (with many from the USA and 1 from Canada), Europe, Asia, and South America.

Fig. 3.

Visualization of a) co-citation network of articles and b) scattered diagram of keyword co-occurrence analysis.

In this analysis, a larger number of nodes and fewer links suggest that research cooperative linkages between world institutions are weak. Even if such research collaboration exists, it mainly lies within different departments and scholars of the same institution. Therefore, those institutions need to increase their collaborative research work by adopting more internationalization and teamwork efforts in biomass and energy conservation research directions. This will enhance research output, researcher proficiency, and novel findings that can help resolve the research gaps in this field.

Journal Co-citation Analysis

The analysis revealed that 705 academic journals published articles related to biomass and energy conservation during the period 2000-2021, with 1652 cooperative links among journals (Fig. 2d)). This network visualization helps to identify the most relevant journals related to biomass and energy conservation and allows the authors to sort out an appropriate journal for their future publications. In our analysis, the journal Biomass and Bioenergy ranked first, followed by Science, Proceedings of the National Academy of Sciences, Nature, and Renewable and Sustainable Energy Reviews. Among the leading co-cited productive journals, the majority belong to Europe (UK, Netherlands, Germany, etc.) and North America (mainly from the USA). Similarly, “Forest Ecology and Management” shows the highest centrality, followed by Energy and Fuel Processing Technology, PNAS, USA, Agronomy Journal, and Energy. These results indicate that these journals are playing a vital role in highlighting the frontier topic of biomass and energy conservation research.

Research Hotspots Based on Reference Co-Citation Analysis

Research hotspots are the interconnection of articles or topics discussed during a certain period. Co-cited references, co-occurring keywords, and cluster keywords are the criteria for identifying research hotspots [44]. The visual distribution of highly cited papers and authors related to biomass and energy conservation consists of 900 nodes and 2520 links (Fig. 3a)). The research article entitled “Carbon-negative biofuels from low-input, high-diversity grassland biomass” published in “Science” by Tilman et al. received the most citations (54). He explains the importance of eco-friendly and energy-rich switchgrass for biofuel production compared to corn grain-producing ethanol or soybean for biodiesel [45]. A detailed list of the top 5 co-cited articles is shown in Table 2. These authors, their institutions, and journals played an exceptional role in this evolving field of research. Cluster analysis indicated the following themes in the domain: bioenergy production, ecosystem service, global economic long-term potential, gas fermentation, short-term bird response, and wildflowers-perennial wild plant mixture.

Research Hotspots Based on Keywords Co-Occurrence Analysis

This analysis consists of keywords obtained from the abstract, title, author keywords, and keywords plus to develop the keyword co-occurring network. The visual representation of the scientific knowledge of keywords consists of 559 nodes and 954 links in this network (Fig. 3b)). The highest quantity of nodes was noted in “bioma”, here representing biomass, which appeared 213 times and had a centrality of 0.18, followed by “conservation” (142, 0.16), “energy” (110, 0.14), “management” (94, 0.09), “impact” (70, 0.05), “biodiversity” (61, 0.09), “model” (58, 0.02), “growth” (55, 0.08), and “diversity” (52, 0.06).

Cluster analysis of the co-occurrence of keywords was divided into five clusters: Cluster-1 was “metabolic pathway” with major keywords “growth”, “energy conservation”, and “Escherichia coli”; Cluster-2 was “Clostridium ljungdahlii”, with major keywords “bioenergy”, “land use”, and “fuel”; Cluster-3 was “short-term harvesting” with major keywords “switchgrass”, “community”, and “soil”. Cluster-4 was “economic analyses” with major keywords “pattern”, “performance”, and “anaerobic digestion”; Cluster-5 was “environmental impacts” with major keywords “wood”, “temperature”, and “water”. According to cluster analysis, the hot topics of research related to biomass and energy conservation generally emphasized five aspects: advancements in bioengineering, land use and cover changes, alternate cropping systems, efficient technology, and impacts of deforestation.

The world population is increasing at an alarming rate and requires huge amounts of energy resources to fulfill their daily requirements. Currently, the majority of energy is derived from fossil fuels, which need to be supplemented with sustainable energy sources. This can be achieved by utilizing renewable resources, such as microbiota and green plants, to address the growing energy demands driven by industrialization and civilization [46]. Anaerobic digestion technologies are widely used worldwide to produce bioethanol, biomethane, and biofertilizers. High-solid anaerobic digestion has become the mainstream technology for treating organic wastes [6]. All these techniques have certain advantages, and comprehensive and advanced research is needed to cope with the difficulties and challenges in the biomass energy research field. For instance, Escherichia coli is widely used in biotechnology and industrial applications to produce biofuels [47, 48]. However, an in-depth understanding of the molecular mechanisms underlying these processes through advanced technologies such as metagenomics, metatranscriptomics, and proteomics could help to optimize the process by developing innovative novel bioenergy strategies [49]. Another key technology for bioenergy production is the microbial fuel cell (MFC), which uses microbial metabolism to transform biomass energy into electricity [50]. Microbes heavily rely on synthetic biology to produce biofuel. Researchers have also found that certain bacterial and archaeal strains, such as Clostridium ljungdahlii sp., Fastidiosipila, Paeniclostridium, Ruminofilibacter, Cloacimonetes, Smithella, Methanosarcina, Methanosaeta, Methanoculleus, and Methanomassiliicoccales, etc., have great potential to produce biofuel [48]. For example, Methanosarcina and Methanosaeta are two well-known acetoclastic methanogens dominating anaerobic digesters. Methanomassiliicoccus can reduce methanol with H₂ and utilize mono-, di-, tri-methylamine, and dimethylsulphide as a substrate in high-solid digestion [51]. Moreover, during gas fermentation with pure carbon monoxide, Clostridium ljungdahlii produces ethanol more quickly and with greater specificity when acetate is added [52].

The measurement of switchgrass (Panicum virgatum L.) biomass yield is important to predict the future of bioenergy production and its impacts on biodiversity with changing climate scenarios [53]. Mixed-species grasslands are one potential bioenergy system since they may produce biomass for energy while also providing other ecosystem services, such as biodiversity conservation [54]. The extensive use of certain keywords on biomass energy conservation can lead the researcher to the latest and most advanced research directions, as shown in Fig. 3b). The overlapping of these keywords can lead the researcher to find hot topics and research gaps by analyzing past research findings.

Global Trends in Research Based on Keyword Burst-Citation Analysis

The global trends for biomass and energy conservation research were investigated using the burst-citation of keywords. Keyword bursts can be used to ascertain frontier topics or emerging trends [55]. The top three keywords with higher burst-citations are “cover-crop”, “climate change”, and “performance” (Table 3). In recent years, the use of cover crops has been gaining more and more attention for the sustainable production of bioenergy due to the reliance on balancing carbon (C) and nitrogen (N) levels in the soil [56]. A systematic review specified the prime role of cover crops in enhancing soil physical properties and offsetting crop residue removal effects on soil [57]. Introducing cover crops after harvesting bioenergy crops is a critical management practice because cover crops can enhance N and C levels and reduce the N-leaching process [58]. For example, an investigation by Austin et al. revealed that the induction of cover crops in a corn bioenergy system could be helpful for an increase in biomass production, enhancing the soil carbon pool and minimizing the losses after corn residue removal [59]. Nevertheless, it is still unclear whether the cover cropping system is a viable long-term solution for bioenergy crop production [60]. Therefore, more advancements in crop management systems can help us understand the future potential of bioenergy.

The performance of various technologies and the role of bioenergy are gaining special attention to achieve more productivity, energy conservation, and eco-friendly objectives. Ronzon and Sanjuán emphasize that the change in practices, technical and technological innovations, and application are the key areas of action for increasing the performance of the bio-economy [61]. The researchers highlighted that the energy conservation objective had been achieved through the efficient performance of various machinery using bioenergy products [62]. However, inadequate energy resources need to be conserved for future generations. Thus, there is a dire need to focus on high-performance, energy-efficient devices and sustainable alternatives and minimize the dependency on non-renewable resources to achieve a healthier society and clean environmental objectives.

As far as the link between biomass-derived energy and climate change is concerned, many research gaps need to be addressed. A systematic review of 17 biomass studies highlighted that it is still difficult to answer to what extent bioenergy has the potential for climate change mitigation [63] because of many uncertainties about future agricultural yield advancements and the availability of land for plantations. Land use and cover changes to replace forests with agricultural land for biofuel crop production might backfire, as land conversion may cause higher carbon and water resource losses [64]. Moreover, excessive fertilizer use for biofuels will significantly increase the atmosphere’s heat-trapping molecules, i.e., nitrous oxide [65]. Another agricultural model in a USA-based study simulated that corn for ethanol production nearly doubled GHG emissions over thirty years [66]. These significant variations in results highlight the uncertainties about the future of bioenergy. Therefore, more integrated efforts from various disciplines are required to fill existing research gaps and balance forestry, agriculture, and bioenergy production. This could be very helpful for developing a sound climate change mitigation strategy along with maximum socio-economic benefits for society.

Future Research Prospects