This study adopted a structured bibliometric review methodology to map global research trends at the intersection of biochar, urban climate resilience, and nature-based solutions (NbS). Bibliometric analysis enables quantitative examination of publication patterns, thematic evolution, and scholarly networks.

The primary data source was the Lens.org database, selected due to its open-access coverage of scholarly publications, citation metrics, and interdisciplinary indexing. The database search was conducted in March 2025. The search strategy combined keywords related to biochar and urban climate resilience using Boolean operators. The core search string applied was: (“biochar”) AND (“urban” OR “city” OR “green infrastructure”) AND (“nature-based solutions” OR “NbS” OR “climate resilience” OR “climate adaptation”).

In it, the search period was limited to 2010–2025 to capture contemporary developments aligned with the expansion of NbS research.

The screening process involved several stages:

Metadata extracted included publication year, authorship, institutional affiliation, country of origin, keywords, and citation counts.

Bibliometric analyses included:

Network visualizations and thematic clustering were conducted using bibliometric mapping software (example; VOSviewer), enabling visualization of research hotspots and interdisciplinary linkages.

To strengthen interpretation, a complementary qualitative literature review of peer-reviewed articles published between 2020 and 2025 in Scopus- and Web of Science-indexed journals was undertaken. These studies provided contextual insights into empirical applications of biochar in urban NbS frameworks.

To enhance transparency and replicability, the bibliometric search strategy, inclusion criteria, and analytical procedures are explicitly documented. The dataset was derived from the Lens.org platform using predefined search strings and screening procedures. Bibliometric network analyses were conducted using dedicated mapping software to ensure methodological consistency. The compiled dataset and analytical workflow are available from the author upon reasonable request to support verification and future comparative studies.

Growth trajectory

Bibliometric analysis of Lens.org data reveals that research on biochar has grown markedly since 2010, with particularly sharp increases after 2015. This trajectory reflects the rising global recognition of biochar as both a climate mitigation tool and a versatile material for sustainable development [7,11]. By 2024, annual publications exceeded 4,000, with urban-related applications constituting an expanding but still minority share of the total. The trend parallels broader shifts toward NbS in climate policy, with research increasingly exploring synergies between engineered and ecological solutions [1].

Between 2010 and 2015, publication output remained relatively modest, reflecting the early experimental phase of biochar research. A marked acceleration occurred after 2015, coinciding with the rise of urban sustainability agendas and NbS policy frameworks. By 2024, annual publications exceeded several thousand globally, with urban-focused studies representing a growing but still emerging subfield [7]. The observed increase corresponds to a significant percentage growth across the study period, highlighting expanding interdisciplinary engagement and policy-driven research priorities.

Geographic distribution

Lens.org mapping indicates that China and the United States dominate biochar research output, together accounting for nearly half of global publications. European countries such as Germany, the United Kingdom, and Italy also contribute substantially, often with a stronger policy and NbS framing. Emerging research hubs in the Global South particularly India, Brazil, and South Africa are beginning to address context-specific urban challenges, such as waste valorization and heat island mitigation [10,12]. However, the bibliometric footprint of African and Latin American cities remains relatively modest, signaling a need for greater geographic diversification of case studies.

Disciplinary evolution

Early biochar studies were dominated by agronomy and soil science, with primary focus on crop productivity and soil fertility. Since 2015, bibliometric keyword clusters reveal diversification into environmental engineering, urban planning, and materials science [13]. The rise of terms such as “stormwater”, “bioretention”, “green roofs” and “urban soils” since 2020 underscores a shift toward urban-focused applications. Interdisciplinary collaborations between soil scientists, hydrologists, ecologists, and urban planners are increasingly evident, indicating that biochar research is moving beyond sectoral silos into holistic NbS frameworks [7].

Citation patterns

Citation analysis highlights several influential clusters. Highly cited works emphasize biochar’s capacity for carbon sequestration pollutant adsorption, and stormwater management [14-15,16-17]. Notably, reviews published in the last five years tend to integrate biochar into broader NbS or circular economy discourses, reflecting its growing relevance in sustainability transitions [3]. The convergence of citations around resilience, urban sustainability, and circularity illustrates the field’s alignment with global climate policy narratives.

Bibliometric analysis of publications retrieved from Lens.org indicates a rapid expansion of scholarly interest in biochar, particularly as part of nature-based solutions (NbS) in urban environments. The publication trajectory since 2018 shows exponential growth, reflecting rising global awareness of climate change, urban vulnerability, and the urgent search for scalable interventions [18]. Early studies were largely experimental, focusing on biochar’s soil-enhancing properties, but over time, the research scope has broadened to encompass urban resilience, climate policy, and sustainable infrastructure.

A co-occurrence keyword analysis reveals three major thematic clusters:

These clusters demonstrate that biochar is no longer viewed solely as a soil amendment but is increasingly framed as part of urban sustainability transitions. Studies stress that incorporating biochar into NbS strengthens cities’ capacity to manage multiple stressors such as heat islands, flooding, and air pollution [19-20].

Geographic and Institutional Patterns

Geographically, China, the United States, and European Union countries dominate biochar research output, together accounting for over 60% of indexed publications. China leads in experimental research, particularly in large-scale pyrolysis and soil restoration projects, while Europe emphasizes policy frameworks and NbS integration, especially within the European Green Deal and climate-neutral cities agenda [21]. African and Latin American contributions remain limited but are growing, with studies exploring biochar’s potential in waste-to-energy initiatives and urban agriculture.

Institutional collaboration networks suggest that cross-disciplinary and cross-regional partnerships are key drivers of innovation. Universities and climate research centers frequently collaborate with municipal governments and NGOs, reflecting the applied nature of NbS research. This trend is consistent with bibliometric evidence that policy-oriented journals are publishing more studies on urban biochar applications than traditional soil science journals [22].

Citation and Influence Metrics

Citation analyses reveal that high-impact papers are those bridging technical properties of biochar with urban policy discussions. For example, life-cycle assessment studies evaluating carbon savings of biochar-integrated NbS attract more citations than purely laboratory-based investigations [23]. Similarly, interdisciplinary reviews that link biochar with circular economy models in cities demonstrate strong citation networks, signaling their importance in shaping research directions.

The bibliometric evidence indicates that theory-driven studies remain underrepresented. Although empirical and applied works dominate, conceptual frameworks situating biochar within NbS for resilience, justice, and governance are scarce. This signals a research gap with important implications for advancing holistic climate solutions.

Biochar and Carbon Sequestration in Urban Contexts

One of the most frequently cited themes in the bibliometric review is the role of biochar in carbon sequestration and emission reduction. As a stable carbon-rich material, biochar resists microbial decomposition and remains in soils for centuries, making it a vital tool for long-term carbon storage [24]. In urban NbS, biochar-enhanced soils and green spaces can provide dual benefits: improved soil fertility for urban agriculture and net-negative carbon emissions for climate targets.

Recent life-cycle assessments suggest that applying biochar to green roofs, roadside vegetation, and urban gardens could offset substantial carbon footprints in cities [25-26]. Moreover, integrating biochar with organic municipal waste management offers a circular solution, reducing landfill emissions while creating valuable soil enhancers for NbS projects.

The network diagram illustrates the interconnectedness of policy, governance, and industrial ecology concepts that are crucial for effective biochar integration. Central nodes such as industrial symbiosis and industrial ecology highlight the systemic approach required to link biochar production with resource efficiency and waste management. Surrounding concepts such as carbon trading, regional synergies, and property rights emphasize the governance instruments that can incentivize biochar adoption through market mechanisms, land-use regulations, and collective action. The inclusion of backcasting and regional planning suggests that long-term visioning and coordinated development are necessary to integrate biochar into sustainable regional transitions. Furthermore, links to biomass, coal, and life cycle reflect the importance of aligning biochar policies with energy strategies and environmental performance assessments. Overall, the diagram shows that biochar governance requires a multi-dimensional policy framework that combines regulatory, economic, and planning tools to support its scaling and mainstreaming.

Emerging Discussions and Future Directions

The bibliometric analysis demonstrates rapid growth in biochar research within urban NbS frameworks, yet several structural gaps remain. First, most studies are short-term or laboratory-based, limiting understanding of long-term performance under real urban conditions. Large-scale field trials and longitudinal monitoring remain scarce.

Second, governance and policy integration are underdeveloped. Although biochar is increasingly discussed in sustainability discourse, standardized urban planning frameworks and financing mechanisms remain limited. This restricts replication and mainstream adoption.

Third, socio-economic and justice dimensions are underrepresented. Few studies examine stakeholder engagement, equity implications, or distribution of benefits across urban communities. Integrating social science perspectives will be essential for inclusive climate resilience planning.

Fourth, economic feasibility and business models require greater scholarly attention. While technical feasibility is widely explored, cost–benefit analysis, supply chain logistics, and scalable production pathways remain emerging areas.

Future research is increasingly interdisciplinary, integrating digital monitoring tools, circular economy frameworks, and urban governance models. Strengthening collaboration between engineers, urban planners, ecologists, and policymakers will be central to advancing biochar as a practical NbS for climate-resilient cities.

Key Research Gaps

Several critical gaps emerge from the bibliometric mapping. First, there is a lack of longitudinal and large-scale field studies. Most research remains confined to pilot projects or laboratory-scale experiments, limiting the ability to generalize results for urban systems. While biochar has demonstrated potential for carbon sequestration and soil remediation, long-term monitoring of its ecological and socio-economic impacts is scarce [24].

Second, policy and governance frameworks integrating biochar into NbS remain underdeveloped. Cities adopting biochar projects often do so in isolation, without standardized guidelines or financing mechanisms. This limits replication across regions and undermines scalability [33]. Bibliometric trends suggest that while biochar is discussed in climate adaptation policy circles, its institutionalization in urban planning processes is still nascent.

Third, there is limited exploration of equity and justice dimensions. Emerging scholarship points to the risk that biochar-based interventions could reinforce existing inequalities if benefits such as improved green spaces or carbon credits are unevenly distributed across urban populations [19]. Yet only a small share of reviewed publications explicitly addresses inclusivity, stakeholder engagement, or community participation.

Finally, although technical innovations such as integrating biochar with digital monitoring tools are gaining traction, research on economic viability and business models remains scarce. Questions of cost-effectiveness, supply chain logistics, and potential markets for biochar-enhanced products require greater scholarly and policy attention [23].