3.1 Insights from bibliometric analysis3.1.1 Analysis of publications and research categories

Publication analysis indicates a growing interest in biochar and an expanding knowledge base in this research field. The number of scientific articles dedicated to biochar has consistently increased, reflecting its rising recognition for its importance in environmental remediation (Fig. 2). The continuous surge in scientific articles is indicative of the transition of biochar from being supported by mere anecdotal evidence to being recognized as a solution of critical importance in both academic and practical circles (Khan et al. 2021). A total of 7076 records related to biochar were identified by counting publications from 2007 to 2023. Research interest in biochar amendment began in 2007 with the first publication on biochar and has continued to increase over the years, reaching a peak of 1254 publications in 2022 (Fig. 2a).

Fig. 2

Publications and citations from 2007 to 2023 (a) and the top 10 Web of Science categories (b)

The growth can be divided into two stages: from 2007 to 2017, the total publications represented only 21% of the total number of publications on biochar amendment, while from 2018 to 2023, there was exponential growth with publications accounting for 71%. Furthermore, the annual growth rate has experienced a notable acceleration. In the initial stage, from 2007 to 2017, the growth rate amounted to 125% per year, while in the subsequent stage, spanning from 2018 to 2023, to an impressive 1099% per year (Fig. 2a). This remarkable increase highlights the growing popularity and significance of research on biochar. It underscores the growing interest among researchers and practitioners in discovering the potential of this sustainable solution as green clean-up method. Our examination revealed that research on biochar spans across 95 core categories recognized by the Web of Science, suggesting its interdisciplinary nature. The 10 primary categories include environmental sciences, soil sciences, engineering environmental, agronomy, plant science engineering chemical, biotechnology applied microbiology, water resources, energy fuels, and green sustainable science technology with at least 600 publications on biochar (Fig. 2b). Diverse applications of biochar are driving a notable shift in the approach to environmental management and sustainable development among researchers and practitioners. In agronomy, for instance, the ability of biochar to improve soil structure and nutrient retention directly influences agricultural productivity and food security (Lehmann and Joseph 2015). This evidence demonstrates the appeal of biochar research due to its wide range of applications and diverse sources.

3.1.2 Most influential sources and journals contribution analysis

The contribution of journals related to biochar research was analyzed based on the total publications. Indeed, our analysis revealed that the 7076 articles have been published in 566 journals. Figure 3a presents the most influential sources in the field counting at least 5 publications. This information is valuable for researchers, as it helps them to identify appropriate outlets for sharing their work. Based on the statistical results, the top 10 journals identified include Science of the Total Environment (423 counts) followed by Chemosphere (234 counts), Environmental Science and Pollution Research (211 counts), Agronomy-Basel (157 counts), Journal of Soil and Sediments (147 counts), Environmental Pollution (133 counts), Journal of Hazardous Materials (124 counts), Journal of Environmental Management (102 counts), Geoderma (106 counts) and Sustainability (106 counts) (Fig. 3b). This finding suggests that the journal Science of the Total Environment holds the highest level of prominence within the field, accounting for 7.6% of overall contributions (Li et al. 2020; Ahmad et al. 2021). However, many journals have started to explore the topic of biochar only recently, including Soil Biology and Biochemistry, Biochar and Frontier in Microbiology (Fig. 3b). Overall, regarding the multidisciplinary field of biochar application, many highly reputed journals are available (Fig. 3b).

Fig. 3

The overlap visualization of the most documents cited over time (a) and the 10 most influential journals (b)

In terms of citations, again Science of the Total Environment revealed to be the most cited source followed by Chemosphere. Beside the predominant appearance of this journal, it started the publication on biochar in 2011 with 1 article and progressively increased publications on this subject to reach a peak of 108 publications in 2021. However, the first journal on biochar application in 2007 was Plant and Soil with regular publications on the topic from 2007 to 2023 and a maximum of 8 articles published in 2015.

3.1.3 Countries distribution and international collaboration analysis

The analysis, based on the average publication and citations per year, reveals that the 7076 original articles on biochar amendment originate from 113 countries (Fig. S2). The global distribution of research outputs on biochar, as depicted on the world map, highlights China, USA, and Germany as the primary contributors to biochar application research (Fig. S2a). China also leads in terms of single-country and multi-country publications (Fig. S2b). Factors such as funding availability, existing infrastructure, and public awareness have propelled research activities in these countries. In terms of publication and citation numbers, China stands out with 2163 articles and 69,882 citations, followed by the United States with 570 articles and 35,840 citations, and Australia with 312 articles and 23,455 citations (Fig. 4a). According to Leydesdorff and Wagner (Leydesdorff and Wagner 2009), while China leads in terms of volume, the qualitative impact and network influence of research from the United States of America (USA) and Australia hold substantial sway on the international stage. These findings indicate the significant interest of these countries in researching innovative remediation methods (Fig. 4a). However, among the top 10 countries, developed countries such as the USA, Australia, Germany, Canada, and Spain have a lower number of publications compared to developing countries like China, Pakistan, India, South Korea, and Egypt. Apparently, biochar research was prioritized by developing countries earlier than by developed countries.

Fig. 4

The overlap visualization of the most cited country over time (a) and most cited institution over time (b)

The analysis of co-authorship between countries reveals a significant and recent relationship between China and various nations, including Pakistan, India, Saudi Arabia, Egypt, Iran, and Poland. Previously, China primarily collaborated with the United States, Australia, Germany, England, South Korea, and Spain. China has the highest number of links (61) with other countries, indicating a strong co-authorship collaboration, with a total link strength of 1345. The USA follows China with 54 links and a total strength of 495 (Fig. 4a). Thus, the total link strength represents the overall high collaborative strength of China with other countries. Based on our analysis, we can conclude that China leads the area of biochar research in terms of the number of publications, citations, and international collaborations.

3.1.4 Analysis of institutes collaboration

The analysis of co-authorship in relation to institutions was focused solely on the institutions of the corresponding author. In the current study on biochar amendment, a total of 3085 institutions around the world participated in this area of research. However, only 6 institutions, all from China, contributed at least 100 articles (Fig. 4b). Among these institutions, the Chinese Academy of Science (CAS) stands out as the most influential, with 410 publications. It is followed by the University of CAS, Zhejiang University, and Nanjing Agricultural University. The prominence of CAS can be attributed to its status as the government-supported research institution with the highest citation count in China. CAS aims to facilitate collaboration among researchers from China and around the world to address identified problems. Financial support and government policies played a crucial role in promoting the leading position of the institute (Zhang et al. 2011). These factors contribute to the institute's ability to make substantial research investments and foster an environment conducive to groundbreaking discoveries. The funding not only helps to acquire advanced technological infrastructure but also attracts top-tier talent, ensuring that CAS maintains its position at the forefront of scientific endeavors (Jacob 2023).

The top 10 productive institutes, which account for 15.85% of the total publications in the field (Fig. 4b), demonstrate an imbalance in research output. Consequently, CAS is widely acknowledged as a highly influential institution in the field of biochar amendment. For instance, the industry collaborations of CAS have greatly facilitated the implementation of biochar technologies, ensuring a smooth transition from laboratory to practical real-world applications. These partnerships have not only advanced sustainable agricultural practices but have also made significant contributions to carbon sequestration efforts. Additionally, the efforts of CAS have established new standards within the academic community and have influenced the development of policies that prioritize sustainable practices. Through collaborations with government organizations and private companies, CAS ensures the integration of biochar technologies into national strategies for environmental sustainability.

3.1.5 Analysis of highly cited authors

Many influential authors have made significant contributions to research on biochar. An analysis of authorship using the"author(s)"unit of analysis and co-authorship as a type of analysis revealed that 1849 researchers have authored at least one document. Our analysis focuses on identifying the most influential authors who have published at least 5 articles (Fig. 5a). The author with the highest number of publications and citations is Ok Yong Sik, with 94 documents and 6056 citations, followed by Wang Hailong with 64 documents and 4177 citations, and Joerg Rinklebe with 48 documents and 2500 citations. Ok Yong Sik and Wang Hailong also have the highest number and strongest collaborations with other authors. Their total link strengths are 392 and 302, respectively (Fig. 5a), indicating their significant involvement in biochar research as a green clean-up solution. All of these authors share a focus on optimizing biochar production and their role in mitigating organic pollutants and heavy metals.

Fig. 5

The overlap visualization of average most relevant sources over time (a) and the top 10 of the most relevant journal sources (b)

To gain a deeper understanding of the effectiveness of research conducted by these influential authors, we performed a threefold analysis based on the 20 most cited references, authors, and keywords (Fig. 5b). The results confirmed that Ok Yong Sik, Pan Genxing, and Joerg Rinklebe have the highest links with keywords and references. This indicates that these authors can be considered pioneers in this field of research, as their research is related to keywords such as biochar, soil remediation, bioavailability, soil amendment, carbon sequestration, heavy metals, soil fertility, compost, and charcoal (Fig. 5b). Despite their significant contributions, these authors are not among the authors of highly cited papers. This finding highlights that reliable authors are not always the fastest and most cited, as their research may be specialized within a specific field or they may have been engaged in interdisciplinary research at the time of publication.

3.1.6 Analysis of tree plot

Numerous publications have been retrieved on the topic of biochar amendment, resulting in a wide range of citation levels highlighting the link between cited references, authors and keywords (DE) (Fig. 6a), as well as countries, affiliation and cited sources (Fig. 6b). The number of citations a document receives can serve as an indicator of its reliability and relevance in the field. Generally, the more a document is cited, the more important, accurate and trustworthy the information it provides. Upon analyzing the most cited documents, several notable titles stand out (Fig. S3). Woolf et al. (Woolf et al. 2010) authored a document titled “Sustainable biochar to mitigate climate change,” which has gathered 1503 citations. Chan et al. (Chan et al. 2007) contributed “Agronomic value of green waste biochar as a soil amendment,” which has received 1192 citations. “Biochar impact on nutrient leaching from a midwestern agricultural soil” by Laird et al. (Laird et al. 2010) has obtained 997 citations. “Positive and negative carbon mineralization priming effects among a variety of biochar-amended soils” by Zimmerman et al. (Zimmerman et al. 2011) has been cited 966 times. Novak et al. (Novak et al. 2009) investigated the “Impact of biochar amendment on the fertility of a south earthen coastal plain soil” and has gathered 864 citations. Beesley et al. (Beesley et al. 2010) delved into the “Effects of biochar and green waste compost amendments on the mobility, bioavailability, and toxicity of inorganic and organic contaminants in a multi-element polluted soil,” receiving 845 citations. Lastly, Park et al. (Park et al. 2011) examined how “Biochar reduces the bioavailability and phytotoxicity of heavy metals” and has been cited 805 times. All of these documents were published during the initial research stage of biochar, suggesting that their findings have established a standard for future research in the field. Consequently, these documents have received the highest number of citations over time.

Fig. 6

Tree-field plot of the overlap visualization of interconnection between: a cited references (CR), authors (AU), and keywords (DE); and b between countries (AU_CO), Affiliation (AU_UN), and cited sources (DE)

3.1.7 Analysis of keywords and research progress

The analysis conducted in this section utilized all the collected keywords from the articles, including authors’ keywords and keywords plus (additional keywords that are identified and included in article metadata to enhance the discoverability of the article) (Fig. 7a). The purpose of this analysis was to identify research trends and establish future directions in order to enhance search precision. The primary keywords with the highest occurrences and citations associated to biochar were carbon followed by black carbon, absorption, impact, amendment, heavy metals, growth, organic matter, nitrogen and bioavailability (Fig. S4a). These keywords indicate that global research on biochar focuses on its sources, the role in pollution mitigation, and environmental impact of its production. Additionally, the keyword analysis revealed that during the earlier stages of research, studies were dominated by keywords such as charcoal, mineralization, sorption, adsorption, impact, nitrogen, water, and productivity (Fig. S4b), indicating a strong research interest in characterizing biochar materials and their sorption and adsorption capacities (Yadav et al. 2018; Barbhuiya et al. 2024). In later years, the terms black carbon and cadmium emerged, indicating a shift towards investigating the effects of biochar in heavy metal remediation, particularly cadmium (Fig. S4b). Overall, the analysis allowed us to identify three research hotspots in the field.

Fig. 7

The visualization of the most frequently cited keywords spanning from 2007 to 2023 (a); and keywords evolution including three periods: 2007–2012, 2013–2018, and 2019–2023 (b)

From 2007 to 2012, keywords such as straw, feedstock type, corn, zea mays, compost, pyrolysis, soil amendments, fertilizer manure, bioavailability, activated carbon, and biomass dominated the research (Fig. 7b). These keywords are related to biochar production, its application to soil, and its potential in soil remediation. During 2013 to 2018, the research focus shifted to keywords such as water retention, pH, salt stress, amendments, nitrous dioxide, indicating increasing interest in using biochar for water remediation (Fig. 7b). Research in this period of time was aimed to investigate the effectiveness of biochar in removing specific organic pollutants, such as polychlorinated biphenyls, which are classified as persistent organic pollutants. Also keywords such as metals, specifically Cd, Pb, Zn, Cu, also emerged during this period. These organic pollutants and metals are considered harmful to human health and the interest in studying the potential of biochar in removing them from water aligns with global research goals and sustainable development objectives related to improving access to clean drinking water (Majumder et al. 2023; Ahmed et al. 2016).

From 2019 to 2023, keywords such as CH4, N2O, and greenhouse gas emissions took prominence. These keywords are mainly related to air pollution, indicating that research aimed to investigate the effectiveness of biochar in improving air quality (Fig. 7b). Furthermore, keywords such as heavy metals, nanoparticles, temperature sensitivity, microbial community, and mineralization were highly used and cited. In this context, the implication of biochar in air pollution extends beyond its production through pyrolysis, which can produce greenhouse gases like CH4 and N2O. More importantly, researchers explored the use of biochar to mitigate air pollution through processes such as adsorption, desorption, and association with nanoparticles. This research has gained significant interest in recent years as a means of reducing environmental pollution through eco-friendly methods.

3.2 Current hotspots in biochar application3.2.1 Biochar use as soil amendment

The primary global application of biochar has been as a soil amendment, aligning with its definition as a carbon-enriched substance applied to the soil to enhance its functionality. The initial study investigating biochar as a soil amendment was published in 2007, and since then, over 250 articles have been recorded in this field by the Web of Science. The analysis of the yearly publication numbers over the past two decades reveals a clear upward trend in interest in biochar as a soil amendment since around 2010. This result suggests a potential shift from the traditional use of biochar as fuel and soil carbon sequestration strategy towards its adoption as a soil amendment in the past ten years. This paradigm shift is primarily driven by a growing body of field studies demonstrating the effectiveness of biochar in improving various soil properties, including water retention, nutrient cycling, and the promotion of beneficial microbial communities (Vijay et al. 2021; Qian et al. 2015).

Furthermore, biochar is widely acknowledged as a soil amendment for enhancing soil quality. However, there is currently no precise definition of biochar as a soil amendment product in the United States (Novak et al. 2014). This lack of clarity may present challenges for individuals seeking to utilize biochar in their soil amendment products. Despite this constrain, Jha et al. (Jha, et al. 2010) provided foundational evidence that biochar may extend beyond conventional soil amendment roles by potentially interacting with plant molecular genetics. Surprisingly, the plant genetics literature pays little attention to the use of biochar as a soil amendment. This highlights a gap between the fields of soil science and plant genetics, particularly regarding the understanding of how biochar addition activates specific genes in roots. This knowledge could contribute to the development of plants capable of adapting to harsher environmental conditions.

On the other hand, soil science predominantly focuses on the nutrient effects of biochar and its potential impact on greenhouse gas or ammonia emissions from manure (Lyu et al. 2022). In terms of geochemistry, there are relatively few articles that specifically address biochar as a soil amendment. However, the application of biochar was reported to significantly enhance soil geochemistry, particularly in terms of nutrient retention, water holding capacity, and microbial activity (Yu et al. 2013; Giagnoni and Renella 2022) with particular emphasis on the effects of biochar amendments on soil nutrient composition and microbiology. However, the geochemical literature seems to have overlooked the long-term effects of the soil amendment efficiency of biochar in recent years, which presents an opportunity for new research ideas. Overall, biochar soil amendment research has the potential to greatly benefit those working in the field of environmental science, as it has the potential to integrate genomics, geochemistry, and plant nutrition related to biochar amendment research into a comprehensive system in future.

3.2.2 Biochar use for water treatment

The utilization of biochar for wastewater treatment is a recent and evolving area of research. The application of biochar has demonstrated the potential to enhance water quality and reduce treatment costs by providing a suitable medium for microorganisms to degrade organic matter and pollutants present in wastewater (Novak et al. 2016). There are currently two primary methods of biochar application in wastewater treatment: the suspended biochar system and the fixed-bed biochar system. In the suspended biochar system, biochar is finely ground and mixed into a slurry, which is then utilized as a filter medium in the treatment of wastewater (Fig. S5). Conversely, in the fixed-bed biochar system, biochar is packed into a filter bed and wastewater is allowed to percolate through it (Inyang and Dickenson 2015; Enaime et al. 2020).

Randomized trials have consistently demonstrated that the inclusion of biochar in wastewater treatment processes leads to improved removal rates of pathogens and organic compounds, surpassing those achieved through traditional treatment methods. The effectiveness of biochar can be attributed to its porous nature, which provides an increased surface area for the adsorption of pollutants. As a result, what was once regarded as waste biomass now presents itself as a valuable resource for initiatives focused on ecological sanitation (Enaime et al. 2020). For example, Gupta et al. (Gupta et al. 2022) elucidated that biochar fosters a favorable environment for microbial communities within wastewater systems, thereby expediting the breakdown of volatile fatty acids, a problematic pollutant in untreated water. Future research efforts in this area may concentrate on optimizing the design of biochar systems and enhancing treatment efficiency.

The establishment of a detailed range of optimal operating conditions for different wastewater parameters is crucial, particularly considering the advancements in characterization techniques and numerical simulation (Faisal et al. 2023). These contemporary methodologies have revolutionized the manner in which engineers determine the ideal operating conditions required to meet regulatory standards and achieve sustainable waste treatment goals.

3.2.3 Biochar use in air pollution control

In air pollution control, biochar can be used to remove organic pollutants from industrial and domestic wastewater and in controlling gaseous pollution. Current research has been focusing on defining the suitability of different biochar types in adsorbing organic gases and particulate matter from air. Previous studies have reported the effectiveness of biochar in absorbing gases such as methane, ammonia, and hydrogen sulfide. Gwenzi et al. (Gwenzi et al. 2021) delineated how specific variants of biochar demonstrate exceptional proficiency in adsorbing organic gases and particulate matter, thereby purifying air to a remarkable extent. Chandra and Bhattacharya (Chandra and Bhattacharya 2019) indicated that when biomass undergoes pyrolysis at carefully calibrated temperatures, the resulting biochar exhibits increased porosity along with a greater surface area–characteristics essential for maximizing adsorption efficiency.

On the other hand, it was highlighted that the surface acidity of biochar can act as a key factor in the removal of ammonia gas, with the biochars containing the highest acidic functional groups showing the highest ammonia retention over time. According to Chen et al. (Chen et al. 2021a), these functional groups comprising mainly carboxyl and hydroxyl groups engaged in hydrogen bonding and electrostatic interactions with ammonia molecules. This engagement not only facilitates the initial adsorption of ammonia onto the biochar surface but also contributes to its long-term sequestration. As a result, biochar may present a low cost and environmentally friendly alternative to waste management since it can be produced from waste biomass and has the potential to be used in gas purification, with the added benefit of reducing biomass waste through pyrolysis. The increasing number of patents relating to the use of biochar. includes methods for producing biochar and bio-oils, as well as apparatus for gasifying biomass to produce biochar. The number of patents for the use of biochar in pollution and emissions control is also increasing, e.g., for reducing and preventing air pollution and treating food processing wastewater using biochar (Kumar and Bhattacharya 2021).

3.2.4 Biochar use in agricultural applications

The use of biochar in agriculture is one of the most widely researched areas, and many review articles focus on this topic (Dwibedi et al. 2023; Rombola et al. 2022; Wang et al. 2022). Biochar can be used as a soil amendment, and in recent years, there has been increasing interest in integrating biochar with fertilizer application to improve the nutrient retention capacity of the soil. The impacts of biochar on soil physical properties, chemical properties, microbial activities, and greenhouse gas emissions from soil, as well as plant productivity and the economics of biochar in agricultural practices, have been extensively studied (Wang et al. 2022). These studies showed that biochar amendment improves the water holding capacity, and increases the cation exchange capacity of the soil, which is crucial for nutrient retention in the soil and nutrient supply to plant roots. Furthermore, Rombel et al. (Rombel et al. 2022) conducted an experiment that supports the idea that combining biochar with conventional fertilizer formulations not only enhances soil fertility, but also improves crop yield through better nutrient management practices. This finding highlights the important role of biochar in advancing sustainable agricultural methods that balance productivity gains with ecological stewardship, aligning perfectly with global efforts to reduce the environmental impact of agriculture while meeting the food demand of an increasing world population. Both field and laboratory experiments have been conducted in this area, and the physico-chemical properties of the applied biochars are usually thoroughly characterized. Research on the water retention capacity of soil demonstrated that biochar addition can significantly increase the ability of soil to retain water (Kumar and Bhattacharya 2021). This may be due to the high porosity of biochar, which increases soil water holding capacity, while the hygroscopic nature of biochar allows water absorption and slow release to the surrounding soil due to the water-repellent surfaces of biochar particles, and the good connectivity of pore spaces in biochar (Chen et al. 2021b; Verheijen et al. 2022).

3.2.5 Biochar for waste management and bioenergy production

Typical waste in modern society, such as municipal solid waste, industrial waste, and hazardous waste, can be classified as organic or inorganic. Organic waste contains carbon, while inorganic waste mainly consists of minerals such as metals, glass, and plastics. Over 60% of the annual waste in the world is organic (Wilson et al. 2015). When organic waste is buried in landfills, it undergoes anaerobic digestion by microorganisms, leading to the release of methane, a harmful greenhouse gas (Ragazzi et al. 2017; Zulkepli et al. 2017). This process contributes to global warming and causes water pollution due to the production of leachate. Biochar provides a practical solution for managing organic waste. As a form of carbon produced by heating organic material at high temperatures without air (pyrolysis) it can be used in conjunction with anaerobic digestion processes, thereby increase the biogas yield by up to 30% (Zhao et al. 2021). This improvement directly enhances the efficiency and viability of biogas as a renewable energy source, addressing environmental concerns and the need for reliable alternatives to fossil fuels. The increased efficiency of biogas production could result in more operational anaerobic digestion plants, leading to better management of organic waste that is currently not economically viable to treat, thereby also introducing new revenue streams.

In addition to its environmental benefits, the byproduct of biochar production, referred to as bio-oil, can serve as renewable fuel. Sharma et al. (Sharma, et al. 2019) emphasize the importance of adopting thermal cracking and esterification processes, which effectively minimize the presence of free radicals and unwanted acidic compounds in bio-oil. This transformation not only enhances the stability and combustion efficiency of bio-oil, but also expands its applicability across various industrial sectors. Zacher et al. (Zacher et al. 2014) outlined a range of feasible approaches for refining bio-oil, not only to alleviate its inherent drawbacks, but also to maximize its usefulness as a sustainable energy source. Furthermore, biochar serves as a co-product alongside bio-oil and syngas, providing an additional source of renewable energy (Iwuozor et al. 2023). Such endeavors not only hold the potential for a decrease in waste generated through biochar production but also signify efforts towards diversification of fuel supply by environmentally sustainable alternatives.

3.2.6 Integration of biochar with other green clean-up methods

The integration of biochar with other green clean-up methods can address environmental contamination and promote sustainable remediation practices. This approach combines biochar with techniques such as phytoremediation or microbial degradation to maximize contaminant removal efficiency while minimizing negative environmental impacts (Lyu et al. 2022; Inyang and Dickenson