Current Status and Prospects of Ecological Restoration and Brownfield Reuse Research Based on Bibliometric Analysis: A Literature Review

Abstract

Ecological restoration and brownfield reuse are important issues in the current field of urban sustainable development and environmental protection. This paper adopts bibliometric and network analysis methods, using more than 600 literature from Web of Science (WOS) and China National Knowledge Infrastructure (CNKI) databases as research objects, to analyze the research trends, main researchers, contributions of different disciplines, and research hotspots in ecological restoration and brownfield reuse. Based on the results of quantitative analysis, this paper reviews the main research theories, methods, and technologies of ecological restoration and brownfield reuse at home and abroad, as well as response strategies in different regions and future prospects under challenges. Among the top 10 disciplines in terms of disciplinary contribution, the discipline with the highest relevance in the research articles of CNKI is “Environmental Science and Technology”, accounting for approximately 47.24%. The discipline with the highest relevance in the research articles of WOS is “Building Science and Engineering”, accounting for approximately 61.21%. In terms of research theories and methods, emphasis is placed on the application of ecological engineering, landscape ecology, land economics, and sustainable development methods. At the same time, adaptive management methods are emphasized, aiming to achieve a balance between ecological protection and urban development. In terms of response strategies for different regions, the main strategies for ecological restoration and brownfield reuse are proposed from five main aspects: reducing soil erosion and controlling water pollution, restoring ecological communities and enhancing biodiversity, landscape reshaping and spatial transformation, tourism development, and leisure space design. Based on the current challenges in technology, law, funding, management, and society, research prospects for strengthening interdisciplinary integration, digital drive, interdisciplinary collaboration, and multi-party cooperation in the future are proposed. It can be seen that research in this field is no longer just a simple ecological issue, but a comprehensive social problem.

1. Introduction

Global climate change has exacerbated problems such as extreme weather, loss of biodiversity, and resource shortages. Meanwhile, the rapid urbanization process has forced a large amount of land to be developed for industrial or construction purposes, resulting in a decline in ecosystem functions and a continuous increase in the number of brownfield sites. The concept of sustainable development requires us to make rational use of land resources and reduce the damage to natural ecosystems. Ecological restoration and brownfield reuse are important ways to achieve this goal. Ecological restoration research originated in European and American countries in the early twentieth century, mainly due to environmental problems caused by mining and groundwater extraction [1,2]. With the increasing degradation of various ecosystems and worsening environmental problems, ecological restoration research has received more and more attention [3,4,5]. With the advancement of urbanization, a large amount of land has been abandoned or underutilized, forming brownfields. These sites are often contaminated, posing a threat to the surrounding environment and the health of residents [6,7]. The development background of brownfield reuse mainly stems from urban renewal, industrial relocation, military land use, etc. [8,9]. For instance, the researchers conducted a comparative study on the reconstruction of two former military barracks, namely Niš—Bubanjski Heroji and Filip Kljajić, demonstrating the different purposes of the two urban renewal projects. One is for the reconstruction for commercial purposes; the other is for the reconstruction led by heritage [10]. To address these issues, the restoration and reuse of brownfields have become a key focus of urban development. The reuse of brownfield not only solves the problems of idle land and pollution, but also transforms these sites into urban parks, sports stadiums, commercial areas, etc. through reasonable planning and design, enhancing urban functions and ecological environment.

There is a close relationship between ecological restoration and brownfield reuse. Ecological restoration is the prerequisite and foundation for brownfield reuse, and brownfield reuse helps to promote the process of ecological restoration [11,12]. The issue of ecological restoration has a history of hundreds of years and occupies an important position in human survival and development, but the academic community has conducted in-depth research on it for a relatively short period of time. Early research on ecological restoration was relatively scattered until 1980, when Cairns began systematic studies as a small branch of ecology. With the deepening of research, foreign scholars have conducted systematic assessments of ecological restoration issues and achieved important research results [13]. China’s research on ecological restoration started relatively late but has grown rapidly, accumulating rich theoretical achievements and practical experience in this field. The related research on brownfield reuse is also gradually being carried out along with the research on ecological restoration. Scholars have begun to pay attention to successful case studies of brownfield reuse both domestically and internationally. Developed countries in Europe and America began brownfield restoration and redevelopment in the 1980s, such as ecological restoration in the Ruhr region of Germany [14]. The main challenge of brownfield reuse lies in its pollution problem, which is an important obstacle to its reuse. Therefore, before reusing brownfield, ecological restoration must be carried out to eliminate or reduce the impact of pollution on the environment and human health. On this basis, brownfield management can be carried out, such as determining brownfield types, analyzing pollution information sources, integrating site resources, and confirming reasonable planning and design (Figure 1). In general, ecological restoration and brownfield reuse are mutually reinforcing. On the one hand, ecological restoration provides possibilities and conditions for brownfield reuse. On the other hand, the reuse of brownfield also provides funding and technical support for ecological restoration. Through ecological restoration and brownfield reuse, we can achieve a win-win situation of economic, social, and environmental benefits [15,16].

Based on this, through bibliometric analysis, this study systematically sorts out the current research status in the field of ecological restoration and brownfield reuse, including the main researchers, contributions from different disciplines, research hotspots, etc. It also compares WOS with China National Knowledge Infrastructure (CNKI). The main reason is that the core researchers are concentrated in China, the United States, and European countries. The complementary database coverage of the two can avoid regional research biases. WOS covers global core journals and reflects the latest international trends. CNKI mainly consists of Chinese literature, reflecting China’s policy orientation and practical demands. On this basis, summarize and generalize the main research theories and methods, strategies of ecological restoration and brownfield reuse, as well as the important fields for future development, with the aim of providing references and directions for the future research and practice of ecological restoration and brownfield reuse, promoting the development of this field, providing a scientific basis for responding to climate change, improving the urban ecological environment, and achieving the Sustainable Development Goals.

2. Research Methods and Data Sources

2.1. Research Methods

At present, bibliometric methods are widely used in different disciplinary fields. In the fields of ecology and environmental studies, some scholars use bibliometric analysis methods to analyze the evolution of ecological footprint research and the development trend of the impact of climate change on environmental governance [17,18]. Meanwhile, some scholars have also visually presented the current situation of diverse geographic information technologies in brownfield reuse by using bibliometric analysis methods. For example, in Zhang et al. research, the spatiotemporal distribution, knowledge groups, disciplinary structures, hot fields, and evolution trends of global brownfield research from 1980 to 2020 were sorted out using CiteSpace 6.4.R1 Advanced [19]. It can be seen that bibliometric analysis has been regarded as an important tool for summarizing historical experience, methodological techniques, and exploring future research hotspots. This study establishes a research process and framework and uses bibliometric analysis and network analysis to analyze the issues of ecological restoration and brownfield reuse (Figure 2) [20]. Bibliometric analysis is the process of mining and quantitatively analyzing the information contained in existing literature according to the analysis logic and methods of bibliometrics. Through mathematical and statistical analysis, it examines the external characteristics and interrelationships between literature [21]. Through the application of bibliometric analysis, it is possible to quantitatively reveal research hotspots and future development trends in this and related fields [22]. The limitation of quantitative research lies in its reliance on data quality. For instance, in bibliometric analysis, the source and quality of data affect the accuracy of the results. Moreover, it is difficult to deeply understand the essence of the research phenomenon. It is necessary to combine qualitative analysis, such as in-depth analysis of the literature content or field case analysis of ecological restoration and brownfield reuse to make up for the deficiencies.

2.2. Data Sources

This study searched for literature in both Chinese and English, covering the period from 2003 to 2024, and conducted the search on 12 November 2024. The Chinese literature data is sourced from China National Knowledge Infrastructure (CNKI), while the English literature data is sourced from the Web of Science (WOS) core collection database. The reason why this study chose the literature starting from 2003 is that the concept of sustainable development has been deeply rooted in the international community. Under the influence of policy promotion, economic demand, technological progress, and the improvement of social awareness, various countries have strengthened the protection and restoration of the ecological environment one after another [23]. For instance, during the period from 2003 to 2008, the EPA in the United States invested a significant amount of funds in brownfield governance and reuse, promoting the development and application of related technologies [24].

By analyzing the meanings of objects, the concepts of “ecological restoration” and “brownfield reuse” have been widely recognized, but there may be multiple ways of expressing them in writing, such as “ecological restoration” being similar in meaning to “ecological governance” and “ecological restoration” and “brownfield reuse” being similar in meaning to “abandoned land renewal” and “polluted land reconstruction”. To ensure accurate and comprehensive retrieval of all relevant literature, this article breaks down the research object into keyword combinations for retrieval, such as “ecological restoration + brownfield + reuse” and “ecological restoration + polluted land + regeneration.” The specific retrieval method is as follows: In the CNKI database, the limited literature belongs to the main database of Chinese journals, and the search method is professional search. The search formula is SU = (“brownfield” + “industrial abandoned land” + “polluted land” + “polluted land” + “polluted land plot”) * (“ecological restoration” + “ecological restoration” + “ecological governance” + “ecological reconstruction” + “ecological improvement”) * (“reuse” + “regeneration” + “update” + “development” + “reclamation” + “restoration” + “governance” + “reshaping” + “revival” + “activation”). A total of 266 literature were obtained, and after excluding 2 literature types as information, 264 valid Chinese literature were obtained. In the WOS database, set the search databases as SCIE and SSCI, the article type as “Article”, and use advanced search. A total of three search equations are set, namely reuse (#1), ecological restoration (#2), and brownfield (#3). Topic #1 is TS = (reuse) OR TS = (regeneration) OR TS = (restoration) OR TS = (recovery) OR TS = (rehabilitation). Theme #2 is TS = (ecological restoration) OR TS = (ecological recovery) OR TS = (ecological rehabilitation) OR TS = (ecological governance). Topic #3 is TS = (brownfield site) OR TS = (brownfield) OR TS = (contaminated site) OR TS = (derelict land) OR TS = (abandoned industrial site). The logical relationship between the search equations is set to AND, resulting in 320 valid English literature.

2.3. Data Processing

Export the Chinese and English literature obtained from CNKI and WOS in Refworks format, use CiteSpace knowledge graph software (version 6.4) to identify the key information of the two data source literature, and draw clustering knowledge graphs for visual analysis. The key information analyzed includes the publishing institution, discipline, and keywords of the literature.

According to the information processing performed by the software, a total of 273 keywords were identified in the CNKI data source literature, and 470 keywords were identified in the WOS data source. Due to the presence of a large number of synonyms in key information, keywords with similar or identical meanings were manually merged, such as “brownfield restoration” and “brownfield recovery”, which express the same meaning. Therefore, they are uniformly represented as “brownfield restoration.” In addition, all meaningless high-frequency words such as “measures”, “strategies”, and “scales” were manually identified and removed. Finally, 140 keywords from CNKI data source literature and 398 keywords from WOS data source literature were determined as the clustering analysis objects.

3. Comprehensive Analysis of Ecological Restoration and Brownfield Reuse

3.1. The Publishing Trends of Publications from Different Countries

The research on ecological restoration and brownfield reuse in foreign countries all originated in the 1980s, and the earliest English literature search result was a Canadian thesis in 1980 [25]. In the same year, the U.S. Congress made the earliest and most authoritative definition of “brownfield” [26]. The research on this direction in English literature began with the abandoned land left over from the “deindustrialization” process initiated by various countries due to economic reasons and the subsequent long-term strategies formulated by each country for the restoration of abandoned land for environmental protection. China’s research in this field was about 30 years later than foreign countries, and the earliest Chinese literature search result was in 2004 [27]. In this year, China proposed a strategic layout for revitalizing the old industrial zone in Northeast China. China’s research in this direction began with this opportunity, with the core goal of reversing the economic situation of prosperous industrial cities that declined more than half a century ago. In summary, foreign research in this field began with an emphasis on environmental protection, while China actually started with the goal of economic development or revitalization. In recent years, ecological restoration and brownfield reuse have gradually received attention from the academic community, and the number of publications has been increasing year by year. Through statistical analysis of research literature published in the WOS database since 2003, it was found that over time, this field has received increasing attention from scholars, and the number of publications has gradually increased (Figure 3).

The number of publications per year can, to some extent, represent the research strength and popularity of a country in a specific research field. Statistics on the annual publication numbers of the top 20 countries in the past 20 years, including the total number of Chinese literature, including WOS and CNKI data sources. The results show that there are significant differences in the earliest publication time and annual publication number among countries, indicating significant disparities in research in this field. In terms of the earliest publication time, five countries, including the United States, China, Canada, Italy, and Portugal, had publication records at the beginning of the statistical period, in 2003, indicating that these countries started research in this direction earlier. Compared to the annual number of publications, China and the United States have tremendous dominance in this direction, with annual publications consistently exceeding more than half of the world’s annual publications [28,29,30]. Next is France. Although there have been publication records since 2008, the annual publication number has maintained a growth trend, indicating that the country is paying more and more attention to research in this area, and its research strength is gradually improving [31,32,33]. China, the United States, and Italy will rank among the top three countries in terms of publication number in 2023, indicating that these three countries currently have the most research strength and popularity [34,35,36]. The reason for the continuous increase in research on ecological restoration and brownfield reuse in China is that after developing its economy, China has gradually realized the importance of environmental protection.

3.2. Research Dynamic Trends Between WOS and CNKI

From the annual publication numbers both domestically and internationally, both have shown an overall upward trend in their research direction, but both have significant fluctuations (Figure 4). Since 2018, the annual publication number and research enthusiasm of English literature have significantly increased, mainly due to the drive of policies and regulations, the promotion of social demand, the promotion of academic research and technological innovation, and the strengthening of international cooperation and exchange [34,37]. These factors work together to promote the in-depth and development of research on ecological restoration and brownfield reuse abroad. For example, in terms of policy and regulatory drivers, governments around the world have introduced or improved environmental regulations to address environmental issues and strictly control brownfield pollution and ecological damage. These regulations not only stipulate the standards and procedures for brownfield management but also provide corresponding financial support and tax incentives, thus stimulating the research enthusiasm of academia and practice for ecological restoration and brownfield reuse [38,39]. It needs to be emphasized that there was a significant increase in relevant research in China in the two consecutive years after 2015. In 2016, China hosted the International Brownfield Governance Conference for the first time. The theme of this conference was “China’s Palm Land Pollution and Environmental Governance”, which explored the policy orientation and industrial upgrading pattern, risk management and governance approaches, as well as cutting-edge technologies and typical cases of remediation and governance facing China’s pollution site governance. This conference had a strong influence at that time [40,41,42]. However, in the following years, scholars’ enthusiasm for this research significantly declined.

3.3. Network Characteristics of Cooperative Institutions Between WOS and CNKI

The collaborative network knowledge graph of literature publishing institutions is shown in Figure 5 and Figure 6. Among them, Figure 5 represents the collaborative network of WOS literature publishing institutions, and Figure 6 represents the collaborative network of CNKI literature publishing institutions.

The results show that there is a small gap between top domestic and foreign research teams in this research direction. Compared with domestic and foreign data, Rutgers University System (RUS), which has published the most papers in this field, has published a total of 10 papers in the past 20 years, while Beijing Forestry University, which has published the most papers in China, has published 11 papers in the past 20 years. As for the Rutgers University System (RUS), the main reasons lie in its abundant brownfield resources and the demand for urban renewal. Many cities in New Jersey are confronted with the challenges of urban renewal and sustainable development, and brownfield reuse has become an important way to address these issues. Meanwhile, RUS has strong research capabilities in multiple disciplines, such as environmental science, ecology, engineering, and sociology. The advantage of this multidisciplinary intersection enables it to study ecological restoration and brownfield reuse from different perspectives, such as pollution control from the perspective of environmental science and community participation and stakeholder relationships from the perspective of sociology [43,44]. The relevant research in China is mainly concentrated in universities. The reason for this is that universities have a complete disciplinary system, covering multiple disciplines related to ecological restoration and brownfield reuse, such as ecology, environmental science, forestry, and soil and water conservation. As a comprehensive university with forestry and ecological environment as its characteristics, Beijing Forestry University’s systematic discipline construction provides a solid disciplinary foundation for ecological restoration research. Meanwhile, Beijing Forestry University has high-level scientific research platforms, such as the Beijing Municipal Education Commission Engineering Research Center for “Source Control and Ecological Restoration Technology of Polluted Water Bodies”, etc. These platforms provide advanced experimental equipment and technical support for ecological restoration and brownfield reuse research [45,46]. However, apart from top teams, there is a significant gap between Chinese research institutions and foreign research institutions. The top 10 teams in terms of the number of publications on CNKI have published an average of 6.1 papers over the past 20 years, while the corresponding criteria on WOS are 8 papers. Expanding the calculation to the top 30 teams for verification, the average number of papers published by the top 30 teams on CNKI over the past 20 years is 3.6, while the corresponding condition on WOS is 4.63. There is also a gap between domestic and foreign teams.

More productive scholars represent more innovation and in-depth research in this field, and their focus can, to some extent, represent the research hotspots of this research direction (

Table 1. Top 10 high-yield authors in the field of ecological restoration and brownfield reuse.

Author	Number of Publications (Articles)	Average Citation Frequency (Per Article)	Country	Research Keywords	References
Gochfeld, Michael	11	13.64	USA	Remediation, Department of Energy, Ecological resources, Risk evaluation.	[12,47,48]
Burger, Joanna	11	11.73	USA	Assessment, Remediation, Polluted sites, Department of Energy.	[12,47,48]
Jeitner, Christian	6	5.33	USA	Ecological resources, Ecological risk, Restoration, Assessment.	[48]
Mench, Michel	4	22.00	France	Biodiversity, Phytoremediation, Metal pollution, Soil functions, Organic amendments.	[49]
Clements, William	4	17.75	USA	Future land use designations, Resource evaluations, Ecological risk, Contaminated sites.	[30]
Prudent, Pascale	4	17.75	France	Passive ecological restoration, Native plant selection, Phytoremediation strategy, Metal tolerance, Heavy metal pollution.	[33]
Musco, Luigi	4	13.75	Italy	Benthic biodiversity, Food web, Industrial contamination, Mediterranean Sea, Sediment pollution.	[34]
Brown, Kevin G.	4	9.25	USA	Biodiversity, Contaminated sites, Ecosystem functions and services, Heavy metals, Stream restoration.	[47]
Kosson, David	4	9.25	USA	Ecological resources, Ecological risk, Human consequences, Ecological and eco-cultural consequences.	[12]
Becerril, Jose	3	34.67	Spain	Phytostabilization, Heavy metals, Mine tailing, Revegetation, Metal pollution.	[49]
Author	Number of Publications (Articles)	Average Citation Frequency (Per Article)	Research Keywords		References
Liu, F.Y.	4	49.50	Collaborative regeneration, Optimization of industrial structure, Maintaining social interests, Industrial abandoned land, Activation regeneration.		[50]
Feng, Y.X.	4	17.25	Mining brownfield, Ecological design, Ecological restoration, Landscape urbanism, Mining Park.		[51]
Zheng, X.D.	4	7.00	Soil pollution management, Sponge city construction, Ecological restoration, Brownfield regeneration, Urban renewal.		[52]
Li, H.Y.	3	31.67	Ecological restoration, Urban green space system, Natural reserves, Industrial abandoned land, Landscape renewal.		[53]
Cheng, W.	3	15.67	Industrial abandoned land, Ecological restoration, Landscape renewal, Old cement factory, Reform.		[54]
Zheng, X.	3	5.00	Landfill sites, Industrial abandoned land, Ecological restoration, Regeneration planning and design, Urban renewal.		[55]
Feng, S.S.	2	15.50	Industrial abandoned land, Ecological restoration, Landscape renewal, Mining site, Green infrastructure, Landscape connectivity.		[56]
Chen, X. K.	2	10.00	Ecological restoration, Urban green space system, Ecopark, Natural reserves.		[57]
Li, Q.	2	7.00	Industrial abandoned land, Ecological restoration, Urban double repair, Landscape design, Landscape regeneration.		[58]
Di, J.	2	5.00	Ecological restoration, Urban restoration, Urban renewal, Industrial abandoned land, Landscape renovation.		[59]

). By presenting specific data on the top 10 authors with the highest number of publications in this field over the past 20 years, one can gain a certain understanding of the research situation in this field. This paper separately counted the names of the top 10 scholars with the highest productivity displayed on the WOS and CNKI data sources, the number of papers published, the number of citations per paper, and research keywords. The WOS data source also included the author’s country of origin.

In the WOS data source, American scholar Gochfeld is the most prolific author, with a total of 11 papers [47,48]. They mainly focus on the ecological resources and risks related to brownfields and have repeatedly mentioned the role of the energy sector in this regard. Spanish scholar Becerril is the most cited scholar, with an average of 34.67 citations per paper. In his research, at a former wood preservation site contaminated with Cu, various phytomanagement options have been assessed in the last decade through biological and other technological means. Specifically, based on crop rotation management of tobacco and sunflowers, combined with the incorporation of compost and dolomitic limestone, it has proven to be efficient in Cu-associated risk mitigation, ecological soil function recovery, and net gain of economic and social benefits [49]. In summary, they mainly focus on mining wastelands and metal pollution and conduct in-depth research on the application of plant restoration in this field.

In the CNKI data source, scholar Liu from Northeastern University has both the highest number of publications and the highest number of citations per paper, with 4 publications and 13.64 citations per paper [50]. His research focuses on industrial wastelands and proposes a theoretical system for using collaborative methods to manage industrial wastelands. His palm land restoration method, which combines “ecosystem evolution”, “industrial structure optimization”, and “social interest maintenance” is widely recognized. Other scholars’ research directions include Feng’s use of ecological restoration methods for the transformation of mining wastelands [51] and Zheng’s research on brownfield and ecological city construction from the perspective of urban renewal. In her research, taking brownfield in Huangshi City, Hubei Province, as the research object, the unique “city-brown-green” spatial pattern characteristics of Huangshi City were analyzed from the spatial relationships between brownfield and important ecological patches, ecological corridors, and planned green spaces. This study found that a considerable proportion of brownfields occupy multiple key ecological nodes in Huangshi City. The research incorporates brownfield sites as important potential spaces into the green space system planning, which is conducive to the construction and optimization of the urban green space network [52].

3.4. The Contribution of Different Disciplines in Ecological Restoration and Brownfield Reuse

The clustering line links in Section 3.3 can visually analyze the level of cooperation between publishing institutions. The denser lines represent closer cooperation between research institutions abroad, and the combination of dark and light lines indicates that these collaborations not only started early but also indicate that many institutions have maintained high vitality in the past five years. Compared to foreign countries, there is much less cooperation between various institutions in China. Although there are also early collaborations, domestic research on this research direction has maintained a low level of connection throughout. This may indicate weak interdisciplinary communication in domestic research. Although there is also a possibility of gathering talents from various disciplines within the team, it is more likely to show that each research team only focuses on the limitations of professional fields. Overall, (1) top domestic research teams are able to reach a level comparable to top international teams, but there is still a gap in the research achievements of other research teams in this field; and (2) compared to foreign countries, domestic research teams may have fewer professional collaborations or interdisciplinary collaborations.

The graphical results of literature discipline statistics provided on WOS and CNKI also confirm this possibility, as shown in Figure 7 and Figure 8. Among the top 10 disciplines that contribute to this field, “environment science technology” has the highest correlation, with foreign research articles in this field [60,61,62], accounting for about 47.24% of all disciplines. The discipline with the highest correlation among Chinese research articles in this field is “architecture science and engineering” [35,63,64], accounting for approximately 61.21%, indicating that a single discipline in China has a greater contribution in this field. The comprehensive evaluation of the top 10 disciplines, both domestically and internationally, will further strengthen this result. The proportion of foreign disciplines is more even, with each discipline accounting for more than 2%, while the top two disciplines in China overwhelmingly, with only the top four accounting for 2%. This indicates that there is still a significant gap in the introduction of interdisciplinary perspectives in China’s research in this field compared to foreign countries.

In summary, compared to the top 10 disciplines in this field, both WOS and CNKI, environmental science, engineering, science and technology, and agriculture have all made significant contributions, demonstrating their attention to this field [65,66,67]. In addition, some disciplines have different definitions at home and abroad, which can lead to different names and blurred research boundaries in the same field [68,69]. After excluding several ambiguous disciplines, there are obvious differences in the research of this direction at home and abroad in some fields. Literature on WOS shows that this direction has received particular attention in the field of basic disciplines abroad, while it has aroused interest in the field of applied disciplines in China [70,71,72].

However, what we need to be clear about is that for the data sources of WOS and CNKI, we have set nearly the same search conditions. Taking agriculture, toxicology, and chemistry as examples, we also conducted literature searches in these disciplines for CNKI. However, the results show that Chinese scholars have conducted relatively few studies on such disciplines and thus have not been reflected in the relevant results. However, the WOS data source is different. Among the top 10 disciplines contributing to this field, these disciplines show more activity. We need to be clear that in the WOS data source, fields such as building science, urban studies, and low politics have not been excluded. The reason why they are not as clearly displayed as in the CNKI data source is simply that in the context of more complex disciplinary research, the number of literature contributions from these disciplines in this field is not as large as that from a large number of basic disciplines. This does not mean that the contribution value of these disciplines is low or that their popularity is small. Overall, it is because WOS has a larger base of basic research, while CNKI has less research in the corresponding field that there are differences in the disciplinary analysis results.

3.5. Analysis of the Main Research Theories and Methods of Ecological Restoration and Urban Brownfield Reuse

The co-occurrence network graph of keywords can intuitively represent the frequency and centrality of keyword occurrence (Figure 9 and Figure 10). The results show that most high-frequency keywords in Chinese and English literature are highly overlapping or have similar meanings, including “ecological restoration”, “contaminated sites/brownfield”, and “ecosystem services/ecological design”, demonstrating the same core goal of using ecological methods to remediate polluted land. However, apart from a few key keywords, English literature focuses more on exploring targeted solutions, such as “heavy metals”, “microbial community”, and “toxic elements.” For example, in Santala et al. study, the contaminated copper–nickel smelter in Sudbury, Canada, was taken as the research object. By transplanting native understory vegetation, the factors affecting the successful transplantation of native understory vegetation on metal contaminated soil were explored, and the community-level characteristics of the transplant site and short-term indicators expressing the performance of selected species were evaluated [73]. In Fuentes et al. study, the response of four commonly used plant species for ecological restoration was evaluated, namely Pinus halepensis, Pistacia lentiscus, Juniperus oxycedrus, and Rhamnus alaternus, which grew in nutrient solutions containing different concentrations of copper, nickel, and zinc. The results indicate that Rhamnus alaternus and J. oxycedrus have higher sensitivity to Cu and Zn compared to P. halensis, especially P. lentiscus. Therefore, when using Mediterranean woody species to restore heavy metal-contaminated sites, it is necessary to consider the comparative response of local plant species to these pollutants [74]. However, Chinese researchers focus more on how to restore the use value of various brownfields, such as “mine park”, “artistic design”, “theme regeneration”, and “cultural landscape.” For example, in Fang research, based on in-depth analysis and research of classic cases at home and abroad, such as Shanghai Chenshan Botanical Garden, Tangshan Mine Park, and Byxbee Park, the application principles and methods of land art in industrial wasteland landscape redesign are summarized. In the case of Shanghai Chenshan Botanical Garden, researchers analyzed the design strategy and techniques of designer Zhu Yufan, which adopted the principle of minimal intervention. Through the setting of landscape elements, such as waterfalls, boardwalks, and water curtain holes, the landscape elements were perfectly integrated with the terrain while maintaining the natural scenery of the site and endowing it with more landscape functions and aesthetic value. At the same time, researchers also combined the practical project of Mulongshan in Zibo City, Shandong Province, to explore the successful experience of combining land art with industrial abandoned land in the entire project design and planning [75]. In Zhu research, taking the industrial abandoned land in Ma’anshan City, Anhui Province, as an example, combining the regional characteristics of Ma’anshan Old Mining Institute, starting from the functional zoning of the site, road system planning, restored plant landscape design, characteristic landscape ornaments, etc., the industrial abandoned land in Ma’anshan City was transformed into Ma’anshan Steel Culture and Creative Industry Park [76]. Based on this, the main reason for the difference in keywords between domestic and foreign fields is that foreign basic disciplines pay more attention to this research, while domestic applied disciplines pay more attention to this research. The difference in keywords also reflects, to some extent, the different disciplines of research in this field at home and abroad.

This analysis result is consistent with the original intention of CNKI and WOS research in this direction. Although the core goal of CNKI and WOS research in this direction is positioned as sustainable development, it still reflects different attention tendencies toward ecological protection in WOS research and economic development in CNKI research. Both CNKI and WOS research has shown a strong concentration of core keywords, indicating that the research is well focused on the core objectives. The number of keywords in WOS research is significantly higher than that in CNKI research, reflecting the expansion of WOS research into a wider range of research fields. Compared to WOS researchers, the keyword structure in China is relatively simple, and there is still a significant gap in expanding research fields. Overall, (1) The disciplines studied in this direction are generally consistent with both CNKI and WOS, but more extensive expansion of biological solutions abroad and greater emphasis on place value enhancement in China; (2) The research hotspots in this direction are similar with both CNKI and WOS, with the core goal being the restoration of abandoned land from the perspective of sustainable development; (3) Compared with the research of CNKI, the research of WOS has a broader scope in this field and a more complex focus.

Based on the above analysis results, the researchers found that the research hotspots of ecological restoration and brownfield reuse reveal the current focus issues, but the hotspots themselves lack systematicity and theoretical depth. By further analyzing the main research theories and methods of ecological restoration and brownfield reuse in representative studies, we can gain a deeper understanding of the essence and solutions to these hot issues (Figure 11).

3.5.1. Related Theories of Ecological Restoration and Urban Brownfield Reuse

Ecological restoration and urban brownfield reuse are important ways to address environmental degradation and land resource scarcity in the process of urbanization. Both emphasize the coordinated development of ecology, economy, and society. Ecological restoration provides an environmental foundation for brownfield reuse, while brownfield reuse provides practical scenarios for ecological restoration. The relevant theories involved in this field include ecology, sustainable development, land economics, urban renewal, etc., emphasizing systematic, dynamic, and adaptive management, aiming to achieve a balance between ecological protection and urban development.

(1)

Sustainable Development Theory: Promoting Coordinated Development of Society, Economy, and Ecology

The reuse of urban brownfields requires a balance between economic, social, and environmental sustainability [77]. On the economic level, developing brownfields can create new growth points, such as transforming abandoned factories into creative parks to drive surrounding development. At the societal level, transforming into a park can improve the living environment and promote social interaction. At the environmental level, emphasis is placed on restoration and protection, using environmentally friendly materials and facilities to reduce negative impacts on the environment.

(2)

Landscape Ecology Theory: Constructing Organic Landscape Systems

The theory of landscape ecology can guide landscape planning and ecological design for the reuse of urban brownfields. Taking brownfields to park conversion as an example, based on the patch-corridor-matrix theory [78], green spaces, water bodies, and other landscape patches are connected through ecological corridors such as pedestrian walkways and water systems to construct an organic landscape ecosystem. Emphasize the integration with the surrounding urban architectural environment and integrate brownfield landscapes into the overall urban landscape.

(3)

Social Participation Theory: Promoting Diversified Development

Enhance social identity in brownfield renewal through community participation and cultural storytelling. For example, in Japan, brownfield regeneration emphasizes public participation to enhance project sustainability. In practical applications, there may be conflicts of interest among stakeholders, such as inconsistent goals between the government, developers, and residents, which can hinder the progress of planning. The theory of social participation can enhance public awareness and participation in ecological issues through educational and promotional mechanisms. It is also possible to encourage residents to supervise and provide feedback on ecological restoration projects through public supervision mechanisms.

(4)

Ecological Engineering Theory: Enhancing System Self-Regulation Capability

Ecological engineering theory is the design of artificial systems to restore and optimize damaged ecosystems by mimicking the structure and function of natural ecosystems, enhancing their self-regulation capabilities and ecosystem services. This theory emphasizes the optimization of material circulation and energy flow and combines landscape ecology principles to construct ecological corridors and patches, enhancing the connectivity and stability of ecosystems. At the same time, for abandoned industrial land spaces with cultural heritage value, it is also necessary to emphasize the inheritance of the site’s spirit and memory, the innovative application of industrial elements, and the protection of intangible cultural heritage, etc., to ensure the long-term effectiveness and sustainability of the project.

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Land Economics Theory: Optimizing Land Resource Allocation

Studying the allocation, utilization, value, and economic relationships of land resources aims to optimize the distribution and use of land resources by analyzing issues, such as land supply and demand, land rent and land price formation mechanisms, land-use efficiency, and property rights systems, ultimately achieving a balance between economic, social, and ecological benefits. This theory focuses on the economic characteristics of land as a scarce resource, exploring how to improve land-use efficiency through market mechanisms and policy interventions and promote sustainable land use and socioeconomic development [79].

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Urban Renewal Theory: Guiding the Reshaping of Urban Functions

Urban brownfield is an important object of urban renewal. The theory of urban renewal aims to transform deteriorating urban areas and enhance urban functionality and quality. When reusing brownfield, it is necessary to replan the land functions, renovate or rebuild buildings, and integrate them into urban development. For example, the old port brownfield has been transformed into a commercial complex, with supporting transportation and landscape facilities, and transformed into a multifunctional urban area.

3.5.2. The Main Methods of Ecological Restoration and Urban Brownfield Reuse

The main methods of ecological restoration include vegetation restoration, soil remediation, water restoration, and ecological engineering technologies [80]. By planting local plants, treating polluted soil, improving water quality, and constructing ecological facilities, the damaged ecosystem functions can be restored. In the process of ecological restoration, eight principles need to be followed, including “supporting ecological recovery process”, “seeking the highest level of recovery attainable”, “employing a continuum of restorative activities”, etc. [81]. At the same time, brownfield reuse optimizes urban spatial layout by transforming abandoned or polluted land into residential, commercial, or green spaces through pollution assessment and control, land function transformation, green infrastructure construction, and community participation and policy support.

(1)

Physical Repair Technology: With the Help of Engineering Methods and Technical Equipment, the Short-Term Effect Is Significant

Land consolidation technology: including methods such as leveling land and building terraced fields. In mountainous areas where land degradation is caused by soil erosion, building terraced fields can reduce the runoff velocity of gentle slopes and minimize soil erosion. For example, in the Loess Plateau region, the construction of terraced fields effectively controls soil erosion, allowing previously barren land to be reused for agricultural production or vegetation restoration.

Soil physical improvement techniques: including deep plowing operations and pit-filling methods. Through deep plowing operations, the surface soil is mixed with deep soil to break soil compaction and increase soil permeability and permeability, but deep plowing may damage soil structure. The method of filling pits with sand can also be used, which involves digging pits in the field according to certain specifications, excavating the saline–alkali soil in the pit, and filling it with sand, but the cost is relatively high.

(2)

Chemical Remediation Technology: Dispensing Specific Chemical Reagents, Fast, Efficient, and Highly Targeted

Soil chemical improvement technology: using chemical agents to improve soil. Lime can be applied to acidic soils to increase soil pH. In some red soil areas such as Hunan Province in the southern region due to the strong acidity of the soil, applying lime can neutralize the soil acidity, increase the effectiveness of nutrients such as calcium in the soil, and promote the growth of crops and vegetation [82].

Chemical remediation technology for water bodies: For example, in eutrophic water bodies, chemical agents are used to remove phosphorus. Some lakes have high phosphorus content due to domestic sewage and agricultural non-point source pollution. By adding chemical agents such as aluminum and iron salts to the water, phosphorus is deposited, thereby reducing the phosphorus content in the water and controlling excessive algal growth.

(3)

Bioremediation Technology: Utilizing Biological Metabolism with Strong Specificity and Environmental Friendliness

Plant restoration technology: including plant extraction, plant stabilization, and plant volatilization. Plant extraction is the process of using plants to absorb heavy metals and other pollutants, such as centipede grass, to remediate arsenic-contaminated soil. Plant stability is achieved by fixing pollutants through root systems to prevent their migration, such as planting plants around tailings ponds to fix heavy metals. Plant volatilization is the process of converting pollutants into gaseous form for release, such as plant volatilization of mercury [83].

Microbial remediation technology: utilizing microorganisms to decompose organic pollutants. In sewage treatment, activated sludge method utilizes microorganisms, such as bacteria and fungi, to decompose organic matter, such as carbohydrates and proteins, in sewage into carbon dioxide, water, and simple inorganic substances, achieving purification. When repairing polluted soil, adding specific microbial agents can decompose organic pollutants, such as petroleum hydrocarbons.

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Site Investigation and Evaluation Techniques: Collecting Diverse Data and Conducting Risk Assessments

Environmental investigation techniques: including the investigation of environmental factors such as soil, groundwater, and atmosphere. Sampling and analysis of brownfield soil to determine the types and contents of pollutants, such as heavy metals and petroleum hydrocarbons. Utilize groundwater monitoring wells to monitor water level and quality and determine whether pollutants are spreading. Conduct atmospheric monitoring on brownfields contaminated with volatile organic compounds to understand the volatilization of pollutants [84].

Risk assessment technique: conduct risk assessment based on site investigation results. Using a health risk assessment model, evaluate the potential risks of pollutants in brownfields to human health, such as carcinogenic and non-carcinogenic risks. At the same time, conduct an ecological risk assessment to analyze the impact of pollutants on the ecosystems within and around the brownfield, including the degree of harm to animals, plants, and microorganisms.

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Pollution Control Technology: Cross-Cutting Composite Technology for Soil and Water Treatment

Soil pollution control technology: soil gas-phase extraction technology in physical remediation, which introduces air into soil contaminated with volatile organic pollutants, extracts, and processes volatile pollutants. Chemical oxidation technology in chemical remediation utilizes strong oxidants to decompose organic pollutants. The biological ventilation technology of bioremediation injects air to promote the decomposition of organic pollutants by microorganisms.

Groundwater pollution control technology: mainly includes extraction treatment technology, that is, the contaminated groundwater is extracted from the ground, treated by physical, chemical, or biological methods, and then reinjected or discharged. There are also in situ remediation techniques, such as permeable reactive wall technology, which sets up reactive walls in the groundwater flow path. The reactive materials inside the walls react chemically with pollutants to remove them [85].

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Architectural Renovation and Design Technology: Integration of Planning and Design to Create Distinctive Landscapes

Building renovation and reconstruction technology: old buildings on brownfields are renovated or rebuilt according to reuse goals. When renovating buildings, techniques such as reinforcement, exterior façade renovation, and internal space redistribution are used, such as transforming old factory buildings into art studios [58]. Building reconstruction involves demolishing unsuitable buildings and constructing them according to new plans, such as building new residential communities using modern and green building technologies.

Landscape design and construction technology: landscape design is an important part of brownfield reuse. Using landscape construction techniques to create artificial wetlands, utilize wetland plants to purify water quality and beautify the environment. Construct a rainwater garden, collect and purify rainwater, and use it as a landscape node. Plant local plants to build communities, providing habitats for organisms and creating distinctive regional landscapes.

3.6. Main Strategies for Ecological Restoration and Urban Brownfield Reuse in Different Regions

By analyzing the publication time of literature based on keyword timeline spectra, research on English literature can be roughly divided into three stages (Figure 12 and Figure 13). Prior to 2008, studies were generally conducted on different types of brownfields and pollution sources, such as land contaminated with heavy metals or crude oil, attempting to use ecological restoration methods to restore the damaged ecological environment [53]. Since 2008, research on the treatment of various pollution sources has continued, and some studies have begun to focus on the benefits of humans in this process or the survival of multiple ecological communities or flora and fauna. Keywords such as “bioavailability”, “community”, “fish”, and “birds” have become hot topics, and the focus has gradually shifted from the site itself to the biological communities that use the site [86]. Since 2018, the focus of attention has been more on humanity itself. The emergence of keywords such as “human health”, “indigenous people”, and “environmental justice” indicates that brownfield reuse is no longer a simple ecological issue, and more scholars are paying attention to the social issues involved in this process [87]. The timeline spectrum indicates that the hot research in this direction abroad has gradually shifted from focusing on the site to focusing on the users themselves. At the same time, ecological governance methods for various pollution sources and multi-level biological community restoration from macro to micro levels have always been hot research topics in different disciplines.

Chinese literature research is much later compared to foreign countries, and there are fewer keyword clusters formed. Its research direction is clearly influenced by both foreign research hotspots and domestic policy orientations, and its keyword clustering has the same or similar meaning as most foreign countries. Before 2011, China’s research in this area had been developing toward landscape and functionality from the beginning. Apart from “ecological restoration”, other hot keywords mainly include “landscape creation”, “place spirit”, “parkification”, etc. From 2011 to 2017, domestic research mainly focused on the restoration of a single type of brownfield [54,88]. “Mine landscape” as a major type of brownfield has sparked a research boom in China, and other keywords also reflect research types such as “industrial brownfield” and “waterfront brownfield.” Since 2018, influenced by foreign research and national policy requirements, domestic research has begun to focus on the relationship between urban planning and brownfield reuse, with keywords such as “sponge city”, “urban double repair”, and “rural revitalization” gaining popularity [59].

Based on the above analysis results, the researchers further summarized eight strategies for ecological restoration and brownfield reuse, intended to provide systematic and actionable solutions for theoretical research and practical applications in this field. These strategies not only cover the core issues of ecological restoration and brownfield reuse but also comprehensively consider the multidimensional needs of ecology, economy, society, etc., in order to achieve sustainable development and harmonious coexistence between humans and nature (Figure 14).

3.6.1. Main Strategies for Ecological Restoration

(1)

Combining Engineering Construction and Strengthening Management Methods to Reduce Soil Erosion

For areas with severe soil erosion, engineering measures such as terraced field construction and ditch and dam construction are often adopted to slow down water flow velocity, increasing soil and water infiltration and retention. Combine biological measures such as planting slope protection plants and constructing ecological revetments to enhance the self-healing ability of ecosystems. Select locally suitable plants for vegetation restoration to enhance soil stability and water retention capacity [57,89]. Protect the vegetation within the enclosed area, prohibiting indiscriminate logging and overgrazing in order to maintain the balance of the ecosystem. Reasonably adjust the land-use structure based on terrain, climate, and soil conditions, such as returning farmland to forests and grasslands, to reduce soil erosion. Optimize agricultural planting structure, develop ecological agriculture and green agriculture, and improve agricultural production efficiency.

(2)

Using Chemical Degradation and Biological Agriculture Methods to Treat Water Pollution

Firstly, for heavily polluted water bodies, technologies such as chemical precipitation, biological flocculation, and plant remediation are used to purify pollutants in the water [90,91]. For water bodies in urban brownfields, landscape design can also be combined to create ecological embankments, wetland parks, etc., to enhance the ecological and landscape value of water bodies. For water bodies in rural areas: more attention needs to be paid to protecting water sources, preventing agricultural non-point source pollution, adopting measures such as ecological agriculture and ecological breeding, and maintaining the cleanliness and health of water bodies. Wetland ecosystems depend on specific hydrological conditions, such as changes in water level and water flow velocity. Many wetlands have suffered hydrological damage due to reclamation, water conservancy projects, and other reasons. Dismantle unreasonable dams and reclamation facilities and restore the natural water system of wetlands.

(3)

Adopting Grassland Rejuvenation Techniques and Afforestation to Restore Ecological Communities

In arid and semi-arid regions: grassland rejuvenation techniques, such as fence enclosure, grass cutting, and grass loosening, can be used to improve soil permeability and promote grassland restoration. At the same time, combined with artificial grassland planting technology, suitable grass varieties are selected for sowing to improve grassland coverage and grass yield. In wet areas: pay more attention to the restoration of forest vegetation, adopt measures such as afforestation and forest nurturing, and improve the stability and biodiversity of forest ecosystems [92,93]. One researcher focuses on the vegetation in a disused steel plant’s brownfield area in Chongqing, China, with Broussonetia papyrifera, Pteris vittata, and Debregeasia orientalis, etc., proving that phytoremediation is a sustainable method for purifying urban soil [94].

(4)

Enhancing Biodiversity Through Animal Introduction and Construction of Ecological Seawalls

Wetland systems have abundant biodiversity, but it is declining due to factors such as pollution and habitat destruction. Planting aquatic plants such as reeds and Acorus calamus in small wetlands around cities can absorb nutrients from water, purify water quality, and provide food and habitat for fish, birds, and other species. The aquatic community in wetlands can be restored by releasing local fish fry and other methods. Coastal areas are facing issues such as rising sea levels and coastal erosion. For example, a marsh-fronted seawall at Juniper Cove, Massachusetts, is proving a hybrid nature-based coastal protection solution because it attenuates wave energy, reduces erosion, and provides ecosystem services [95].

3.6.2. The Main Strategies for Urban Brownfield Reuse

(1)

Pollution Control and Landscape Reshaping

Before brownfield management, a detailed pollution assessment is required, including soil and groundwater pollution levels and ecological risk analysis. Using physical, chemical, and biological remediation techniques to treat polluted soil, the restored land can be redeveloped and utilized according to urban planning, such as building residential buildings [96]. The High Line Park in the United States transforms abandoned railways into urban linear green spaces, combining landscape design with community activities, becoming a model for brownfield regeneration. The Ruhr Industrial Zone in Germany has transformed abandoned mining areas into ecological parks and cultural tourism complexes through ecological restoration and industrial heritage protection. Taking the typical industrial abandoned land in Changchun City, China, as an example, the regeneration planning and design strategies for industrial abandoned land are proposed, which include preserving the original structures of the industrial abandoned land, renovating the original structures of the industrial abandoned land and developing underground space [55].

(2)

Mine Backfilling and Land Reclamation

For some mining land, mining operations will result in the formation of large areas of mines, tailings piles, etc., which will damage the original topography and ecological environment. Measures such as backfilling the mine pit and reshaping the terrain are taken. In the restoration of brownfields in some open-pit coal mines, the stripped topsoil is backfilled into the mine pit, and then the terrain is shaped to create slopes and valleys suitable for vegetation growth. At the same time, covering the surface with soil amendments and planting vegetation adapted to the local environment gradually restores the ecosystem. Taking the mining sites in the urban area of Xuzhou City as an example, based on the principles of landscape ecology and restoration ecology, the researchers constructed an ecological restoration theoretical system suitable for the characteristics of coal cities in the plain areas of China [56]. Another case, such as the Hegang Mine Geopark, which was once an open-pit mine, has been meticulously designed and developed into a national-level mine park. This not only realizes the utilization of land in the coal mining subsidence area and enhances the level of intensive land use, but also plays a significant role in improving the urban ecological environment.

(3)

Public Service and Commercial Space Renovation

Old industrial factories and sites in the city have been abandoned due to industrial restructuring. With the expansion of the city, the location of many old factories has now become a valuable resource in the city center [97]. For example, several creative industry parks in Shanghai were originally renovation projects of old factories, located in bustling areas such as Jing’an District and Huangpu District. Secondly, from a cost perspective, the renovation cost of old factory buildings is relatively low, far less than the cost of new construction. For example, the renovation cost of large-span factory buildings is usually controlled within 10–20% of the total cost. Furthermore, from a property perspective, the tall space, multi-story structure, and large-scale building clusters of the old factory provide flexibility for the functional repositioning of the project. For example, the 798 Art District in Beijing was transformed from old electronic industrial factories, such as the former state-owned 798 Factory, retaining the architectural style of the original factory buildings and transforming them into art spaces.

(4)

Tourism Development and Leisure Space Design

Some brownfields have unique landscapes and tourism value, which can be developed for tourism and leisure [98]. For example, the Ruhr area in Germany is a model of brownfield management in Europe, adopting a “divide and conquer” strategy: firstly, for land that is not too polluted but relatively easy to manage, after thorough removal, the land use can be changed. For example, a large shopping center has been built on the site of the Oberhausen factory in the Ruhr industrial zone, attracting a large number of people. Secondly, for land that is not severely polluted but difficult to control, its original facilities should be basically maintained and used for industrial heritage tourism. Duisburg Landscape Park did not demolish the giant steel structure but was transformed into a historical museum introducing steelmaking and blast furnace technology. Thirdly, for areas with severe pollution, comprehensive governance should be carried out to transform them into public spaces. The Emsher Canal system was designed as a water park, and abandoned land was transformed into a leisure and entertainment venue for nearby residents.

4. The Main Challenges and Future Research Prospects of Ecological Restoration and Urban Brownfield Reuse

4.1. The Main Challenges Currently Facing Ecological Restoration and Urban Brownfield Reuse

The main challenges currently facing ecological restoration and urban brownfield reuse include multiple aspects, such as technology, law, funding, management, and society. To overcome these challenges, it is necessary for the government, enterprises, and all sectors of society to work together to strengthen technology research and development, improve laws and regulations, broaden funding sources, optimize management mechanisms, and increase public participation to enhance its comprehensive benefits (Figure 15).

4.1.1. Technical Dilemma: Lack of Targeted Ecosystem Reconstruction Technology

Polluted brownfield soil often suffers from nutrient imbalance, excessive levels of heavy metals or organic pollutants, leading to the destruction of the original soil microbial community structure. At present, there is a lack of technology that can quickly and effectively restore the diversity and function of soil microbial communities, making it difficult to accurately construct microbial communities with specific ecological functions based on different types and degrees of pollution. The integrity of ecosystems cannot be separated from the participation of animals, but there is relatively little research on the construction techniques of animal habitats in brownfield ecosystem reconstruction. Lack of habitat design and construction techniques for different types of animals makes it difficult to provide suitable food sources, habitats, and breeding environments for animals, making it difficult to attract them back and affecting the biodiversity and ecological functions of the ecosystem.

4.1.2. Legal Dilemma: Insufficient Policy Support and Imperfect Laws and Regulations

At present, the government’s special fiscal appropriations are relatively insufficient, and there is a lack of diversified financing policy support. In the process of reusing brownfield, for example, in different regions and states of the United States, there are many policy restrictions on changes in land properties and land transfer methods. Due to historical reasons, some brownfields have complex land ownership and difficulties in land-use conversion and lack targeted land policies to solve such problems. In terms of laws and regulations, there are difficulties in identifying the responsible parties for brownfield pollution, especially for some historical brownfields that have undergone multiple corporate restructuring and property rights changes, making it difficult to clarify the attribution of pollution responsibility [99]. At the same time, in the process of ecological restoration, the division of responsibilities among the government, enterprises, and society is not clear enough, leading to a tendency to shift blame to each other in the restoration work.

4.1.3. Financial Difficulties: Difficulty in Raising Funds and Lack of Sustained Financial Support

Ecological restoration and brownfield reuse require significant investment in site investigation, risk assessment, planning, and design in the early stages. Brownfield redevelopment in the European Union is usually funded by local sources or private investment. The EU also has a complex framework of funding to support regional, cross-border, and multicity redevelopment projects. The primary funding mechanisms are the Cohesion Fund and the European Regional Development Fund (ERDF), etc. [100]. But unfortunately, compared with the European Union, enterprises or related entities in most countries need to advance funds by themselves before generating any benefits. For entities with weaker financial strength, the burden is heavy, which may lead to difficulties in the early preparation stage of the project. Even if the project is repaired and redeveloped, continuous financial investment is still required for subsequent operation and maintenance. In addition, it takes a long time from land consolidation and restoration to transfer, and the speed of fund recovery is slow.

4.1.4. Management Dilemma: Complex Land Ownership and Uneven Distribution of Benefits

In terms of land ownership, the land ownership of urban brownfields is often complex, involving multiple property rights units and stakeholders. In the process of land acquisition, compensation, and redevelopment, it is easy to cause conflicts of interest and contradictions. In terms of profit distribution, different stakeholders have different demands for benefits in brownfield reuse projects [101]. For example, the government hopes to enhance the city’s image and land value through the project, enterprises pursue economic benefits, and residents pay attention to environmental improvement and quality of life. In the process of land use change, if the distribution of benefits is unreasonable, it can easily lead to project obstacles. It is necessary to coordinate multiple departments and stakeholders to handle approval procedures. Moreover, the standards and requirements for land use change vary among different departments, further increasing the difficulty of land ownership adjustment.

4.1.5. Social Dilemma: Insufficient Public Awareness and Poor Channels for Participation

Many members of the public have insufficient understanding of the concept of brownfield, lack awareness of the importance and significance of ecological restoration and urban brownfield reuse, and even have misunderstandings and resistance toward the project, which poses obstacles to its progress. In the planning, design, and implementation process of the project, the channels for public participation are not smooth enough, and there is a lack of effective communication and feedback mechanisms, which makes it difficult for public opinions and suggestions to be fully valued and adopted. The governance and reconstruction of most brownfields abroad are promoted by the central government, implemented by local governments, and supported and encouraged by the participation of non-governmental organizations in the governance and development of brownfields. In China, the transformation of brownfields lacks incentive mechanisms for non-governmental organizations, and the participation of non-governmental organizations is low, making it difficult to form social synergy.

Based on the systematic analysis of the existing challenges, strategies, and theoretical methods of ecological restoration and urban brownfield reuse mentioned above, there are many interdisciplinary theories and diverse methods and strategies. Combining the evolution of technology and methods in this field with the needs of socioeconomic development, the key areas for future research on ecological restoration and urban brownfield reuse are proposed as follows (Figure 16).

4.2. The Main Strategy of Urban Brownfield Reuse

4.2.1. Multi-Domain Integration: Research on System Coupling and Resilience Enhancement

The research on ecological restoration and urban brownfield reuse involves many fields, with climate change adaptation and watershed regional system governance being the focus of attention. Collaborative research on climate change adaptation requires the integration of ecosystem service assessment models and climate scenario simulation tools to quantify the synergistic effects of ecological restoration on carbon sequestration, heat island mitigation, and rainwater storage. By constructing a “restoration technology climate resilience” coupled database, restoration projects can be transformed from “passive defense” to “active adaptation” [102]. The study of the relationship between independent restoration units and watersheds/regions requires a systematic thinking approach to coordinate the multi-factor feedback mechanism of “mountain-water-forest-field-lake-grass”. Based on hydrological connectivity analysis and digital twin technology, reveal the cascading effects of ecological restoration projects on surface runoff, surface water, and groundwater conversion in the watershed. Constructing a paradigm of “source-corridor-strategic point-network” at the regional scale, coordinate the synergistic effect of local restoration and regional ecological security pattern through the spatial coupling of ecological red lines and urban development boundaries [103]. Current research often focuses on static ecological goals or local restoration units, making it difficult to address the complex challenges of frequent extreme events and cross-scale ecological interactions caused by climate change. In the future, there is an urgent need to build a research framework that integrates multiple fields. Through dynamic adaptive restoration, cross-scale collaborative planning, and systematic ecological governance, the contradiction between ecological restoration, urban brownfield reuse goals, and urban development needs can be alleviated.

4.2.2. Digitalization-Driven: Research on Technological Innovation and Combined Applications

In the research of digital technology applications, the collaborative application of satellite remote sensing, unmanned aerial vehicles, and AI technology should be deepened. With the help of hyperspectral remote sensing technology, detailed surface information can be provided to dynamically monitor the diffusion path of brownfield pollution, the evolution of soil physicochemical properties, and the process of vegetation restoration [104]. By using drones equipped with multispectral sensors, it is possible to monitor land cover types and vegetation types, accurately identify the spatial pattern of biological habitats, and improve the efficiency of ecological restoration [105]. By combining AI technology and big data analysis, a dynamic feedback mechanism and ecological risk model for ecological restoration projects can be established. Currently, these technologies still face many challenges in their application, such as severe data silos, difficulty in effectively integrating data from different sources, and the inability to fully realize the value of data. The model has insufficient generalization ability and poor adaptability in different scenarios. In the future, it is necessary to strengthen the integrated monitoring network of “sky-ground-air-water”, establish a more complete data-sharing mechanism, reduce data transmission delay, achieve real-time processing and analysis of data, and promote the transformation of ecological restoration from traditional “experience-driven” to “data-driven intelligent”. At the same time, actively explore the combination and optimization solutions of different technologies, find the most suitable technology combination for different ecological restoration scenarios, and improve the efficiency of restoration and reuse.

4.2.3. Multidisciplinary Intersection: Theoretical Mutual Learning and Technology Integration Research

The coordinated promotion of ecological restoration and urban brownfield reuse is essentially a coupled process of “natural system restoration” and “urban functional regeneration”, involving deep interdisciplinary fields such as ecology, environmental science, geography, urban-rural planning, computer science, sociology, and economics. Environmental science and ecology are the cornerstone of ecological restoration, playing a leading role in pollution risk assessment, research and development of soil water restoration technologies, and analysis of ecosystem succession laws. Geography can rely on technologies such as remote sensing and drones to build a monitoring network that integrates “sky-ground”, providing spatial decision support for multi-scale restoration strategies. Urban and rural planning studies integrate systematic governance and ecological restoration through collaborative design of sponge facilities, biological corridors, and other technologies in the renewal and transformation of urban stock spaces. Computer and information technology can empower data fusion analysis, establish ecological restoration decision support systems, and construct quantitative evaluation models. Sociology and economics focus on stakeholder research, balancing the long-term public welfare attributes of ecological restoration with the short-term economic demands of urban development [106]. The current research still faces problems such as the solidification of disciplinary barriers and the fragmentation of technical tools, which constrain multi-objective collaborative optimization. In the future, there is an urgent need to build an interdisciplinary knowledge sharing platform that can solve the disconnect between ecological restoration and urban regeneration through theoretical mutual learning, methodological collaboration, and technological integration [107].

4.2.4. Multi Subject Co-Construction: Research on Economic Development and Social Governance

Ecological restoration and urban brownfield reuse require the participation of multiple stakeholders. The collaborative cooperation of multiple entities, such as government, enterprises, social organizations, and residents, plays an important role in promoting ecological restoration and social development. Clarifying the relationship between ecological restoration, residents’ livelihoods, and human well-being is a crucial aspect [108]. In ecologically fragile areas, although ecological restoration can enhance ecosystem functions and provide resource security for humans, there are also situations where it can lead to an increase in unemployment and a decrease in residents’ income. In the future, it is necessary to scientifically select advantageous species, optimize restoration configuration models, combine green enterprise development, explore poverty alleviation models, promote ecological compensation, improve residents’ production and life, and achieve a win-win situation between ecological restoration and socioeconomic development. The core goal of multi-party joint construction is to achieve win-win benefits for multiple parties [109]. For example, the researchers have proposed strengthening the network relationship between the government and environmental non-governmental organizations (ENGOs), improving the monitoring efficiency and network status of news media, establishing a construction consultation platform that effectively serves the public and end users, forming alliances among end users, and enhancing the stability of the relationship network between the public and non-governmental organizations [110].

5. Conclusions

This study systematically reviewed the research status, hotspots, and trends in the field of ecological restoration and brownfield reuse through bibliometric analysis. The main conclusions drawn are as follows:

The research on ecological restoration and brownfield reuse has shown a rapid growth trend in the past few decades, especially in the past decade, where research results have significantly increased. The research hotspots mainly focus on ecosystem services, pollution control technologies, land-use planning, public participation, and other aspects. Developed countries are at the forefront of research in this field, while research in developing countries has grown rapidly in recent years, but there is still room for improvement in overall influence. At present, there are still limitations in research on this topic. (1) Insufficient theoretical integration: Existing research is mostly focused on a single disciplinary field, lacking the integration and application of interdisciplinary theories. (2) Technical application limitations: Some ecological restoration technologies face high costs and long cycles in practical applications, and there is an urgent need for technological innovation and optimization. (3) Insufficient attention to socio-cultural factors: Existing research focuses more on ecological and economic dimensions, with relatively less attention paid to socio-cultural factors such as public participation and cultural inheritance. (4) Lack of long-term monitoring and evaluation: Most studies lack long-term monitoring and evaluation of the effectiveness of ecological restoration and brownfield reuse, making it difficult to fully reflect their sustainability. Meanwhile, we also need to clearly recognize that the limitation of this study in terms of quantitative research methods is that only CiteSpace was used for visual analysis. However, there is a lack of visual demonstration in terms of methods, theories, techniques, and challenges. But we have endeavored to conduct an analysis by selecting typical research cases. For instance, both America and China have successful cases of using native tree species to solve the problems of ecological restoration and brownfield reuse, such as the High Line Park in the United States. By preserving and replanting drought-resistant and pollution-resistant local herbs (such as Symphyotrichum and Andropogon) and trees (such as Gleditsia triacanthos), the soil structure of railway wasteland is restored, and biodiversity is enhanced. In the brownfield ecological restoration of the Shougang Industrial Zone in China, soil heavy metals were purified by selecting Salix matsudana, Sophora japonica, and Platycladus orientalis. By integrating the constructed wetland system, a collaborative governance model of “phytoremediation + constructed wetland” has been formed. And it is necessary to explore localized development models for ecological restoration and brownfield reuse based on the actual national conditions of each country.

In summary, scholars at home and abroad have gradually realized that research on ecological restoration and brownfield reuse should not only focus on the innovation and promotion of ecological technologies but also strengthen sociological attention to aspects such as “people” and “environmental justice”. Furthermore, we are aware that different countries require different strategies based on their socioeconomic and cultural conditions. Evidently, solutions that are effective in European and American countries may not be directly transferred to Asian countries.