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Review

Trends in the Research and Development of Soil Nitrogen Mineralization in Forests from 2004 to 2024

by
Xiumin Zhang
1,
Huayong Zhang
1,2,*,
Zhongyu Wang
1,
Yonglan Tian
1 and
Zhao Liu
2
1
Research Center for Engineering Ecology and Nonlinear Science, North China Electric Power University, Beijing 102206, China
2
Theoretical Ecology and Engineering Ecology Research Group, School of Life Sciences, Shandong University, Qingdao 266237, China
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(18), 7882; https://doi.org/10.3390/su16187882
Submission received: 18 July 2024 / Revised: 6 September 2024 / Accepted: 8 September 2024 / Published: 10 September 2024
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Abstract

:
Nitrogen (N) is a vital mineral nutrient for plant growth and occupies a pivotal position in biogeochemical systems. Soil nitrogen mineralization (SNM) in forests represents a significant limiting factor in terrestrial ecosystem productivity in the context of global climate change. To understand the research status and development trends of SNM in forests, 3576 articles spanning 2004 to 2024 from the Web of Science (WOS) database were analyzed using CiteSpace software. The results indicated that (1) the mean number of articles published in the recent ten-year period is 193, marking an approximate 17.8% increase compared to the preceding ten-year period (2004–2013), highlighting the continuous development of SNM research; (2) among the sampled articles, Soil Biology and Biochemistry, Forest Ecology and Management, Plant and Soil, and Biogeochemistry emerged as leading international journals that played a key role in shaping the development of the field and laid a solid foundation for future research efforts; (3) the USA and China emerged as the most productive countries in this field, with the Chinese Academy of Sciences standing out as a prominent institution at the forefront of this research domain; and (4) recent research is focusing on understanding the interactions between microbial communities and the environment during SNM. In summary, this study offers valuable insights into the research status and development trends of SNM in forests. It underscores the importance of ongoing interdisciplinary collaboration and innovation to further enhance our understanding of key ecological processes. Future research on SNM in forests is encouraged to delve deeper into its associations with forest productivity, carbon cycling, microbial functions, and global change. Additionally, exploring sustainable land management and process optimization is recommended to promote the healthy and sustainable development of forest ecosystems.

1. Introduction

As a key element in the biogeochemical system, nitrogen (N) affects the biogeochemical cycling of forest ecosystems in the context of global change [1,2,3,4,5]. The process of N mineralization transforms soil organic N into inorganic N that is accessible to plants, thereby influencing vegetation productivity and often serving as a major limiting factor for productivity in numerous forest ecosystems [6,7,8,9]. Soil nitrogen mineralization (SNM) plays a pivotal role in N cycling within forest ecosystems, exerting a direct influence on forest productivity, soil fertility, and ecosystem stability [10,11]. Therefore, it is imperative to stay abreast of the latest research findings on SNM in forests to comprehend the implications of environmental change and effectively manage forest ecosystems.
In recent years, research on SNM in forests has intensified in response to global climate change and the degradation of forest ecosystems [12,13,14]. This trend highlights the core role of SNM in the physiological, biological, and biogeochemical cycle of forest ecosystems and its importance in the structure and function of the plant–soil–microorganism system, which has become the focus of extensive research by numerous scholars [15,16,17]. Notably, research on the mechanism and dynamics of SNM has achieved a significant milestone. Studies have shown that increased plant diversity significantly enhances the activity and diversity of soil microorganisms, ultimately promoting the N mineralization rate [16]. Furthermore, investigations into specific soil fungal functional groups, particularly those closely associated with different mycorrhiza types, have provided valuable insights into the distinct contributions of these microorganisms to SNM in various forest ecosystems [17]. Despite these advancements, several pressing scientific issues remain. Firstly, there is a scarcity of quantitative data on the spatial and temporal changes in SNM rates across different forest types and regions, which is crucial for developing an accurate model for predicting N cycling in forest ecosystems on a global scale. Secondly, the structure and function of the soil microbial community and its interaction with SNM are complex, necessitating further study to elucidate the underlying mechanisms. Lastly, the impact of global climate change on the process of SNM in forests, along with the potential for human intervention to regulate soil N cycling as well as maintain and enhance the service function of forest ecosystems, pose significant scientific challenges that require further investigation.
Despite the considerable attention dedicated to studying SNM in forests and the abundant empirical and qualitative literature synthesized by experts, there remain limitations, including regional variations, functional traits, and database coverage [12,18,19,20]. It is noteworthy that there is a paucity of literature utilizing bibliometric analysis to explore the current state and research trends specific to SNM in forests. The bibliometric analysis method employs mathematical and statistical techniques to examine the external characteristics of literature, with the objective of describing, evaluating, and predicting the dynamic evolution of scientific knowledge [21] in the form of graphs. Additionally, it demonstrates various complex relationships among knowledge units, such as network, structure, interaction, intersection, and evolution [2,22,23,24]. Therefore, a comprehensive bibliometric analysis is crucial to provide an overarching perspective on the study of SNM in forests.
In this study, bibliometric techniques were utilized to identify, collate, and analyze publications on SNM in forests from 2004 to 2024 in the Web of Science (WOS), providing a systematic and objective overview of the scientific research development of SNM in forest soils. The objectives of this study are as follows: (1) to create a comprehensive understanding of SNM in forests regarding the trends of articles and citations, research subject categories, representative journals, and keywords; (2) to identify the representative countries, institutions, and authors in this research area; (3) to reveal the evolution of the knowledge structure and networks of SNM in forests through highly cited articles and frequent and bursting keywords; and (4) to explore the research hotspots and emerging trends of SNM in forest ecosystems.

2. Methods

2.1. Data Sources

WOS has progressively evolved into the world’s largest comprehensive and multidisciplinary academic retrieval platform, serving as the primary reliable source of citation data [25]. It encompasses the Science Citation Index Expanded (SCI), the Art & Humanities Citation Index (A&HCI), the Social Science Citation Index (SSCI), and numerous other indices [2]. A targeted search within the SCI database yielded pertinent publications focusing on the themes of “Soil nitrogen mineralization” and “Forest”. The document types were refined to “article” and “review”, as these indicated the majority of studies with comprehensive research results. The language was set to “English”, and the research data were confined to the period spanning from 2004 to 2024. After merging and deduplicating the data, a total of 3576 publications were collected on 1 January 2024. All records were analyzed using VOSviewer (version 1.6.20) and CiteSpace 6.1.6 (64 bit).
Initially, a literature search was conducted to gather relevant data, which were subsequently analyzed to ensure the accuracy and replicability of the research. This was achieved by applying consistent search criteria and filtering principles. Microsoft Excel 2019 was utilized for the analysis and export of documents exhibiting the highest citations or productivity, based on factors such as authors, countries/regions, publications, journals, and institutions. Tables and graphical data were extracted from published articles using Origin (2021) software. Selected experiments underwent detailed analysis, and data were extracted for further examination.

2.2. Data Analysis

The full record, along with its citation, was extracted from WOS into a plain text file, with each entry separated by a unique label. Utilizing the annual number of publications, a time series network was constructed. CiteSpace (version 6.1.6) ingested a comprehensive set of bibliographic records and subsequently modeled the fundamental intellectual landscape of the given research field [2,21,26]. This software supports various bibliometric research methods, including collaboration network analysis, geospatial visualizations, co-citation studies of both authors and institutions, and co-word analysis [26]. Duplicate publications were identified and removed from the dataset. Essential information, such as authors, institutions, countries, keywords, cited references, and their relationship matrices, was extracted. Subsequently, Bibexcel was employed to further elaborate on the network mappings. The research data encompassed a time span from 2004 to 2024, with a time slice of 1 year. Depending on the research objectives, a range of analytical techniques were selected, including collaboration network analysis of authors, countries, and institutions, as well as co-citation analysis of institutions, journals, and references cited. Co-word analyses were also selected separately. The “Pruning the merged network” function was applied to the Pathfinder algorithm. The log likelihood ratio (LLR) was adopted for the clustering analysis of co-words, while the remaining parameters were set to the system default values. Additionally, VOSviewer was used to obtain tables of the most frequently cited journals, authors, institutions, and countries.
In this study, all statistical data were converted into tabular form, and the mean number of citations per item was calculated for each table. The degree of academic influence was comprehensively evaluated using the total number of publications (TPs), the total number of citations (TCs), and the average citations as judging indicators. To analyze the nature of the cooperative relationship, the centrality index provided by CiteSpace was selected as the most appropriate indicator.

3. Results and Discussion

3.1. Overall Status of Nitrogen Mineralization in Forest Soils

The number of published articles serves as a reflection of the development strength and course of the research field. Furthermore, it enables the division of the research field into distinct stages and allows for dynamic analysis of the change in the number of documents by plotting the curve of published articles over time [27]. The research on SNM in forests demonstrated a consistent upward trajectory in recent years, indicating a growing interest in this research field (Figure 1). From 2004 to 2014, the field progressed through a gradual development phase, with annual publications fluctuating between 140 and 190, averaging 166 per year. However, since 2016, while annual publication counts continued to experience minor fluctuations, the overall trend indicated an increasing number of articles, signifying continual advancement in research. From 2016 to 2023, the average number of publications per year was 194, representing a mean annual growth rate of 3.2%. It is noteworthy that in 2022, the number of publications peaked at 225, suggesting that this research was in a “growth phase” and possessed considerable potential for further expansion. In recent years, research on SNM under different forest types and forest management within the context of global change has garnered increasing attention [19,28,29]. Currently, investigations are focused on the effects of various factors on soil N cycling, microbial community composition, and enzyme activity [29,30].

3.2. Co-Occurrence Analysis of Journals

This study conducted a statistical analysis of 3576 research articles listed in the WOS database, encompassing a comprehensive evaluation of journals covering soil science, ecology, botany, environmental science, and allied fields. The analysis, which spanned the period from 2004 to 2024, aimed to identify the journals with the highest number of publications. Generally, the total number of citations (TCs) of a paper reflects its impact. Since the influence of a journal may vary depending on the research field, the average number of citations per paper in a journal (TCs/TPs) serves as a relatively good indicator of the relative importance of the journal in a particular field. All the journals in a subject are sorted in descending order according to the impact factor of the journals in a given year and then divided into four categories (each comprising 25%), namely Q1, Q2, Q3, and Q4 [31]. This categorization, along with the impact factor of these journals, provides insights into the standing of a journal among its peers and its role and position in scientific communication.
The journals encompassed in this analysis have published cutting-edge research across numerous domains. For instance, Soil Biology and Biochemistry concentrates on advancements in soil biology and biochemistry; Forest Ecology and Management covers all facets of forest ecology and management; Plant and Soil publishes research on the interaction between plants and soil and its impact on the environment, and so forth. As evident from Table 1, Soil Biology and Biochemistry emerged as the leading journal with 398 articles, trailed by Forest Ecology and Management and Plant and Soil, which contributed 194 articles each. In general, the results of the TC/TP values highlighted the significance of Ecology, Oecologia, and Global Change Biology. These were highly influential mainstream journals that exhibited a correlation to N mineralization and other fields. Overall, the top 10 high-impact journals encompassed a broad spectrum of publications in soil science, ecology, botany, and environmental science, indicating that this research area is a vibrant and dynamic field of study with substantial contributions from these esteemed journals. This analysis provides valuable insights into the research output and trends within these important scientific domains.
By analyzing the literature on SNM in forests with high citation frequencies in the WOS database, a literature data table showcasing the top 10 most cited articles was compiled (Table 2). The prevailing research topics within the selected fields of study were centered on soil microorganisms and ecosystem function, carbon, N, and phosphorus cycles and dynamics, soil respiration, and N dynamics, as well as the impact of global change on soil ecosystems. These studies were crucial for gaining insights into soil ecosystem functioning and the ramifications of global change. It is noteworthy that these highly cited references originated from a wide array of countries, suggesting that SNM in forests research has garnered significant international attention. Specifically, the top three most cited studies were centered on the critical role of soil microorganisms in plant diversity and productivity, N cycling, and ecosystem function [32,33,34]. Additionally, considerable research was conducted on the cycling and dynamics of carbon, N, and phosphorus in soils and ecosystems, encompassing their mineralization, decomposition, and cycling processes [35,36,37], which played a pivotal role in understanding the N cycling mechanism of an ecosystem and the dynamic changes in soil N content and density [38]. Furthermore, the effects of global change (e.g., climate change, dry–wet cycles, etc.) on soil ecosystems have been extensively investigated in numerous studies [39,40]. These insights were of utmost importance for evaluating the resilience and vulnerability of forest soil ecosystems in the context of the increasing prevalence of global perturbations.

3.3. Co-Occurrence Analysis of Authors

By analyzing the network of co-authors, it is possible to determine which scientists collaborate closely in this field and to explore the influence of collaboration on their academic research [42]. The utilization of co-citation analysis provides valuable insights into the most influential academic groups operating within this research field [43]. The collaborative network (Figure 2) contained a total of 228 nodes, with the size of each node being directly proportional to the frequency of publications by the respective author. Therefore, the larger the node, the more contributions an author made. Furthermore, the presence of 186 connecting lines within this network indicated the existence of collaborative relationships between the authors. The thickness of the lines represented the strength of the collaborative efforts between the authors. The calculated cooperative network density of 0.0072 indicated a substantial level of collaboration within this research community, thereby highlighting the existence of cohesive scientific groups.
Out of the total 3576 papers and 228 authors considered, Table S1 underscores a select group of authors who have made notable contributions. This elite cohort comprises Mueller, C.; Zhang, J.B.; Cai, Z.C.; Chang, S.X.; and Kuzyakov, Y., each of whom has published more than 17 articles. Moreover, Groffman, P.M. and Bai, E. emerged as the most influential researchers in this field, evident from their highest average citations. The high-yield analysis presented in Figure 2 visually depicts the profound influence of these high-frequency authors within the academic community. Their dense and interconnected presence within the network demonstrates that increased collaboration among researchers not only propels the advancement of scientific knowledge but also ensures the sustainability of this vital area of research. Additionally, it is noteworthy that authors such as Mueller, C.; Zhang, J.B.; and Cai, Z.C., despite publishing their first papers later than Chang, S.X., have achieved remarkable outputs. Their subsequent prolificacy indicates a high level of engagement and rapid progress in the field. This observation underscores the significance of continuous engagement and dedication to research, irrespective of the initial entry into the field.

3.4. Collaboration Network Analysis of Institutions and Countries

The analysis of the mechanism cooperation network generated a mechanism cooperation map (Figure 3), which revealed 180 nodes and 312 connecting lines, indicating a network density of 0.0194. It signifies that there were robust collaborative relationships among all institutions engaged in the field of SNM in forests. The map further illustrates that numerous institutions collaborate, fostering a mutually beneficial environment that advances the development of this research field. The extensive research on SNM in forests, reflected in the SCI documents dedicated to this topic, encompasses a global scope, involving 180 diverse countries and regions. To gain deeper insights, we designated institutions that have published over 20 studies on SNM in forests as high-frequency publishing institutions. The scientific and technological statistical analysis of the research results of high-frequency publishing institutions (with SCI as the index) clearly demonstrates the development process and research results of SNM in forests in the institution (Table 3). The Chinese Academy of Sciences topped the list with 403 publications (largest node in Figure 3), reflecting its vast contributions and profound exploration in this field. Boston University led in TC/TP values (74.95), indicating that its publications have a high influence or citation rate. Most institutions started publishing around 2004, and the University of Chinese Academy of Sciences is one of the institutions that started publishing later (Table 3). A closer examination revealed a relatively focused research area (0.08), which highlights the necessity for broader international collaboration and exchange. Enhancing such collaborations will undoubtedly accelerate global research progress in SNM in forests.
The discipline knowledge map of cooperative countries depicts the intricate interconnections within the research landscape centered on N mineralization in forest soils (Figure 4). The research analyzed encompassed 81 nations globally, with the majority of studies being conducted in Europe, America, Asia, and Oceania, highlighting the global reach and substantial impact of this field of inquiry. A statistical analysis of the co-occurrence network of countries with high publication frequencies over the past two decades provides valuable insights into the most productive nations in this research domain. Notably, the United States led with 1072 publications, followed by China with 803, Germany with 356, Canada with 280, Australia with 191, Sweden with 175, etc. (Table 4). The United States occupies a dominant position in terms of the number and centrality of publications, while Sweden boasts the highest TC/TP values, indicating that its publications may wield considerable influence. The frequency of collaborations and exchanges among these nations underscores the pivotal role of international cooperation in enhancing the quality of research and academic influences.
Table 5 provides a comprehensive enumeration of the papers published by the top 10 disciplines in the field of SNM research in forests. The analysis revealed a notable concentration of publications in disciplines such as soil science, environmental science, ecology, forestry, plant science, and agriculture. It is noteworthy that the earliest publications in these disciplines can be traced back to 2004 or even earlier, indicating a long-standing and intricate interdisciplinary engagement in this research field. Therefore, the study of SNM in forests has long been a topic of interest across various scientific domains, fostering a rich tapestry of research contributions and advancements.

3.5. Research Hotspots

The title of a study serves as a concise encapsulation of its core content, enabling scientific researchers to efficiently search for relevant literature by identifying subject-specific keywords and narrowing their professional focus [44]. The titles of the 3576 articles were analyzed to identify the keywords that exhibited high frequency and centrality, thereby pinpointing current research hotspots. In the CiteSpace interface, the node to “Keyword” and the threshold to Top N = 25 were set, generating the keyword co-occurrence network (Figure 5). The extensive distribution, multiple interconnected branches, and overlapping patterns of the network indicated the field’s substantial and concentrated research contents. Table S2 provides a comprehensive overview of the frequency, mean centrality value, and year of origin for the top 20 keywords. When combined with the visual representation in Figure 5, keywords pertaining to soil nitrogen transformation mechanisms and dynamics, including “mineralization”, “forest”, “soil”, “organic matter”, and “microbial biomass”, emerged as prominent due to their consistently high frequency and centrality within the network.
Through cluster analysis, the keywords were categorized into seven distinct research hotspots, namely nitrogen deposition, climate change, dissolved organic carbon, microbial community, microbial biomass, boreal forest, and soil organic matter (Figure S1 and Figure 6). The largest cluster, Cluster #0, focused on nitrogen deposition, microbial biomass, N mineralization, and so forth. It was evident that excessive nitrogen deposition had a detrimental impact on ecosystem integrity, leading to a decline in biodiversity and compromising the stability and crucial service functions of the ecosystem [14,45,46,47,48,49]. Given that a robust ecosystem served as the foundation for sustainability, effective management of nitrogen deposition was of paramount importance for the preservation of ecological health and the advancement towards sustainable development, leading to its imperative status in the research field of SNM. Cluster #1 included 33 projects that addressed various aspects of climate change, such as soil respiration, N mineralization, litter decomposition, fertilization, forest soils, etc. [35,37,39,50,51,52,53]. It was widely acknowledged that climate change exerted a significant impact on the process of SNM [18]. Therefore, it was essential to monitor and investigate the N mineralization process in forests to enable the implementation of timely and appropriate management strategies that would safeguard the health of forest ecosystems and foster their sustainable development. Cluster #2 and Cluster #6 represented soil matrix modules for SNM research. Specifically, Cluster #2 highlighted organic carbon, temperature sensitivity, substrate availability, microbial biomass, stable isotopes, soil warming, and organic carbon turnover [54,55,56,57,58]. Cluster #6 encompassed soil organic matter, soil density fractionation, Fagus Sylvatica, Pinus Sylvestris, nitrogen saturation, protease, and so on [5,35,56]. This module emphasized the key role of soil organic matter in SNM research. Both soil organic matter and SNM served as the basis for maintaining soil fertility and productivity. By maintaining and improving the content of soil organic matter, the process of N mineralization can be promoted, thereby increasing the availability of N in the soil and providing forest plants with an abundant source of nutrients. This, in turn, contributes significantly to maintaining the vitality and stability of the forest ecosystem. Cluster #3 was comprised of microbial community, enzyme activity, nitrate, PLFA, bacterial genus group, soil disturbance, etc. [38,46,59,60]. Cluster #4 was characterized by microbial biomass, humic substance, carbon stocks, labile organic matter, soil disturbance, etc. [14,47,61]. Finally, Cluster #5 explored 20 keywords related to boreal forest, nitrogen transformations, phenolic compounds, birch litter, soil carbon, and related topics [49,62,63,64], which may represent another research hotspot.
To clearly depict the time sequence and correlation of keywords, we conducted an analysis of the keyword time zone diagram using the co-occurrence spectrum (Figure 6). Table S2 and Figure 6 demonstrate that key terms such as “nitrogen mineralization”, “carbon”, “decomposition”, “nitrification”, and “kinetics” have been prevalent since 2004 or earlier, thereby underscoring their enduring significance within this research domain. Over time, the emergence of new keywords diminished, exhibiting a decline in both frequency and centrality, resulting in a fragmented and diversified research landscape. Recently, the research focus has shifted towards new areas, including “bacteria”, “soil organic carbon”, “pH”, “mechanism”, “diversity”, “carbon use efficiency”, and “community”.

3.6. Emerging Trends

To identify and track frontiers, mutated keywords were extracted, pinpointing the topics that garnered particular attention from the scientific community during a specific period [65]. As an effective analytical tool, burst detection was utilized to identify keywords that have garnered significant attention within the relevant scientific circles from 2004 to 2024 [66]. Figure 7 presents an emergent network map of the top 25 keywords, arranged based on their emergence intensity, providing insights into the trending topics through an analysis of their annual frequency. The results revealed that keywords such as “microbial community”, “priming effect”, “organic matter”, “nitrification”, “vegetation”, and “temperature sensitivity” exhibited high emergence intensity, indicating their prominence as primary research topics.
The research process of SNM in forests can be categorized into three distinct stages. The initial stage, spanning from 2004 to 2010, marked the nascent understanding of the scientific community regarding the significance of SNM in forest ecosystems [19]. This process was recognized as an indispensable component of nutrient cycling and plant productivity, with key topics encompassing vegetation, soil N, coniferous forest, nutrients, organic N, tropical forest, and others [1,36,67]. Notably, carbon sequestration, a pivotal aspect of sustainable ecosystem development, emerged as a prominent research area [36]. During the second stage of the research process, from 2010 to 2017, scientists extensively deliberated on the effects of environmental factors on SNM in forests, including soil temperature, humidity, pH, and nutrient availability [12,13]. Additionally, the impacts of plant species diversity and community composition on the rate of SNM garnered considerable interest [16]. The research field broadened further to include dissolved organic carbon, microbial community, heterotrophic nitrification, and temperature sensitivity [12,20]. At this stage, the microbial community and its role in soil N turnover became the focal point of research, with comprehensive examinations of the effects of various biological and abiotic factors on SNM and N content [68,69,70,71]. The third stage, defined as the period between 2017 and 2024, shifted the research focus to the far-reaching impact of global environmental change, particularly climate change and intensified nitrogen deposition, on SNM in forests [18]. There was a notable surge in research activity related to bacteria, mechanisms, diversity, carbon utilization efficiency, and community composition during this stage [16,20]. The study not only provided a detailed examination of the impact of diverse factors on soil N cycling, microbial community composition, and enzyme activity [14,15,45,64] but also offered a comprehensive analysis of the spatial and temporal variations in SNM rates across different forest types and regions, providing valuable insights for global forest N cycling management [52,57,72].
The research on SNM is currently advancing at an unprecedented rate, with the mechanism of SNM involving the decomposition of organic nitrogen by soil microorganisms, the release of inorganic nitrogen, and plant absorption [1]. As science and technology progress, complex mathematical models and microbial functional gene analysis have become essential tools for elucidating the internal mechanism of SNM [69,70,72,73]. These studies have offered a novel perspective on the impact of global climate change on forest ecosystems and have provided a scientific foundation for forest management, contributing to the healthy, stable, and sustainable development of forest ecosystems. Specifically, using the model to predict SNM allows for comprehensive consideration of the influences of various environmental factors (such as climate, soil conditions, vegetation types, etc.) on the N mineralization process, thereby enabling more accurate predictions of N availability and forest productivity. Concurrently, microbial functional gene research has elucidated the species and activities of microorganisms involved in N mineralization in soils [11,74], further disclosing the micro-mechanism of N mineralization and providing robust support for the formulation of more precise forest management strategies. These studies are of great significance for understanding the N-cycling process of forest ecosystems and provide important scientific support for coping with global climate change, maintaining biodiversity, and ensuring human welfare. Therefore, by continuously promoting research on SNM and its related fields, greater contributions to the health, stability, and sustainable development of global forest ecosystems can be achieved.

3.7. Future Research Directions and Policy Recommendations

Based on the above literature review, we propose the following directions for future research on SNM:
(1)
A comprehensive modeling approach to SNM: With the increasing availability of high-resolution data on soil properties, plant diversity, and environmental factors, there is an opportunity to develop more complex models that integrate the various processes and factors affecting SNM in forest ecosystems. These models can assist researchers in gaining a deeper understanding of the intricate interrelationships between the various components of the system, ultimately enabling more precise projections of future developments.
(2)
The microbial ecology of SNM: Advances in microbial ecology and sequencing technology have allowed researchers to describe soil microbial communities in unprecedented detail. Future research should focus on identifying the specific microbial groups and functional groups responsible for SNM in different forest types and environments. This information can be utilized to develop targeted management strategies aimed at promoting healthy soil microbial communities and N cycling.
(3)
Interactions between SNM and other ecosystem processes: SNM is closely linked to other ecosystem processes, such as carbon cycling, water availability, and plant growth. Future research should explore the interactions between SNM and these other processes to comprehensively understand how they interact and promote overall ecosystem function.
(4)
Combining the practice of desertification control and land management: Exploring the optimization of the SNM process through land management measures (such as afforestation, returning farmland to forest, etc.), improving soil fertility and productivity, and promoting the healthy and sustainable development of forest ecosystems are attractive research directions. In general, future research on SNM in forests should give more attention to its relationship with forest productivity, carbon cycling, microbial function, and global change, and actively explore sustainable land management measures to optimize the SNM process and achieve healthy and sustainable development of forest ecosystems.
In light of the challenge of SNM in forests, it is recommended that government departments adopt a scientific approach to the management and rational utilization of forest resources. The following policy recommendations are therefore proposed. Firstly, it is imperative to reinforce the protection and restoration of forests, with particular emphasis on broad-leaved forests, mixed forests, and other forest types that play a pivotal role in SNM. The expansion of forest areas and the restoration of farmland to forest ecosystems will enhance the efficiency of carbon storage and N cycling. Secondly, a scientific forest management plan should be formulated in order to ensure the sustainable utilization of resources. It is important to safeguard soil and vegetation during logging operations to prevent the disruption of the N mineralization process. Soil N monitoring should be strengthened to provide a scientific basis for management. Thirdly, in areas adjacent to agricultural and forest land, the promotion of eco-friendly agricultural technology, a reduction in the use of chemical fertilizers, and the mitigation of the negative impact on SNM are recommended. The utilization rate of soil N can be enhanced through the implementation of scientific fertilization and a reasonable rotation strategy, thereby reducing the nutrient pressure on forest ecology. Fourthly, it is recommended that the government implement improvements to relevant policies and regulations, and clarify the responsibilities associated with forest protection and management. Concurrently, it is encouraged that all sectors of society participate in forest protection and restoration. The sustainable development of forest ecosystems can be promoted through the implementation of policy guidance and financial support.

4. Conclusions

4.1. Limitations

While this research provides considerable insight into research hotspots and trends in the field, it also has some limitations. Firstly, our analysis was limited to literature from the WOS database, without consideration of other databases. This approach may introduce bias into our findings, and further improvements are necessary to ensure the accuracy of our results. Secondly, although we identified key research hotspots and their evolution, further investigation is required to gain a deeper understanding of each of these hotspots, including the methods used, theoretical frameworks, and key findings of individual studies. Finally, it is important to note that, despite its widespread use in bibliometric studies, the CiteSpace software has its own limitations. The process of analyzing maps and data is inherently subjective, which could potentially lead to variations in the results of the analysis. However, the findings presented in this paper are based on objective data and are considered to be stable and reliable.

4.2. General Conclusions

This paper employed a bibliometric approach to select and review 3576 articles from the WOS database, with a focus on research on SNM in forests over the past two decades. The findings demonstrated a consistent increase in research activity, evidenced by a notable expansion in publications since 2004, accompanied by a growing number of scientists engaged in this field. A thorough examination of the research hotspots highlighted the mechanisms and dynamics underlying soil nitrogen transformation, including N mineralization, nitrification, influencing factors, N cycling, forest productivity, environmental impacts, and control measures. In recent years, there has been a pronounced emphasis on elucidating the effects of microbial community characteristics, microbial functional genes, land use patterns, and environmental variables on SNM. It is notable that the leading institutions and authors are primarily based in the USA and China, with the Chinese Academy of Sciences emerging as a leading research hub in this field. Soil Biology and Biochemistry, Forest Ecology and Management, Plant and Soil, and Biogeochemistry contributed significantly to shaping the field’s development and laying a robust foundation for future advancements. The field of SNM is inherently interdisciplinary, drawing upon and contributing to ten closely related disciplines (principally soil science, environmental science, ecology, forestry, plant science, and agriculture). In general, future research on SNM in forests should pay more attention to its relationship with forest productivity, carbon cycle, microbial function, and global change, and actively explore sustainable land management measures to optimize the N mineralization process in order to realize healthy and sustainable development of forest ecosystems.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su16187882/s1. Figure S1. Co-occurrence network map of keywords during 2004–2024. Notes: Colors indicate the cluster in which each keyword was related the most. The size of a node is proportional to the literature number of the subject category. Lines represent the co-occurrence link strength among terms. Table S1. Author published more than 10 papers in the field of soil nitrogen mineralization in forests. Table S2. The top twenty keywords of the most frequently appeared.

Author Contributions

Conceptualization, X.Z. and H.Z.; methodology, X.Z. and H.Z.; software, X.Z.; writing—original draft preparation, X.Z.; writing—review and editing, Z.W. and Y.T.; visualization, X.Z.; project administration, Z.L.; funding acquisition, H.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Major Science and Technology Program for Water Pollution Control and Treatment (No. 2017ZX07101-002) and the Discipline Construction Program of Huayong Zhang, at the School of Life Sciences, Shandong University (61200082363001).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References

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Figure 1. The number of SNM in forests publications by year from 2004 to 2023.
Figure 1. The number of SNM in forests publications by year from 2004 to 2023.
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Figure 2. The map of authors collaborated from 2004 to 2024. Notes: Nodes represent authors. The size of the font is proportional to the number of papers produced by each author. The links between nodes represent the collaborative relationship between different authors.
Figure 2. The map of authors collaborated from 2004 to 2024. Notes: Nodes represent authors. The size of the font is proportional to the number of papers produced by each author. The links between nodes represent the collaborative relationship between different authors.
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Figure 3. Knowledge map of cooperative institutions from 2004 to 2024. Nodes represent institutions, with the size of a node being proportional to the number of papers produced by the institution. The links between nodes represent the collaborative relationship between different institutions. The color of the links corresponds to the year. The concentric circles indicate high centrality.
Figure 3. Knowledge map of cooperative institutions from 2004 to 2024. Nodes represent institutions, with the size of a node being proportional to the number of papers produced by the institution. The links between nodes represent the collaborative relationship between different institutions. The color of the links corresponds to the year. The concentric circles indicate high centrality.
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Figure 4. Knowledge map of cooperative countries from 2004 to 2024. Notes: The links represent the collaborative relationship between different countries.
Figure 4. Knowledge map of cooperative countries from 2004 to 2024. Notes: The links represent the collaborative relationship between different countries.
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Figure 5. Co-occurrence network of keywords from 2004 to 2024. Notes: Nodes represent different keywords. The size of a node is proportional to the quantity of literature related to the keyword. Lines between nodes represent the co-occurrence link strength among keywords.
Figure 5. Co-occurrence network of keywords from 2004 to 2024. Notes: Nodes represent different keywords. The size of a node is proportional to the quantity of literature related to the keyword. Lines between nodes represent the co-occurrence link strength among keywords.
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Figure 6. Time zone map of keywords from 2004 to 2024.
Figure 6. Time zone map of keywords from 2004 to 2024.
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Figure 7. Top 25 keywords with the strongest citation bursts (2004–2024). The beginning of a blue line represents when an article is published. The beginning of a red mark represents the beginning of a period of burst, and the end of the red mark is the end of the burst period.
Figure 7. Top 25 keywords with the strongest citation bursts (2004–2024). The beginning of a blue line represents when an article is published. The beginning of a red mark represents the beginning of a period of burst, and the end of the red mark is the end of the burst period.
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Table 1. List of the most highly published journals on SNM in forests.
Table 1. List of the most highly published journals on SNM in forests.
JournalsPublicationsTCs/TPsJCRIF
Soil Biology and Biochemistry39865.60Q19.8
Forest Ecology and Management19433.72Q23.7
Plant and Soil19434.50Q13.9
Biogeochemistry13854.41Q24.0
Geoderma12032.18Q16.1
Applied Soil Ecology11735.17Q14.8
Science of the Total Environment11522.46Q19.8
Global Change Biology10495.32Q111.6
Biology and Fertility of Soils8839.17Q16.5
Ecosystems8662.15Q13.7
Forests847.65Q12.9
Soil Science Society of America Journal6945.08Q22.9
Journal of Soils and Sediments6119.80Q13.6
Biogeosciences5170.60Q14.9
European Journal of Soil Biology4825.89Q24.2
Oecologia46102.67Q12.7
Canadian Journal of Forest Research4229.43Q22.2
Ecology41161.76Q14.8
Eurasian Soil Science406.08Q21.4
PLOS ONE3924.79Q13.7
Journal of Geophysical Research: Biogeosciences3628.22Q13.7
Scientific Reports3432.65Q14.6
European Journal of Soil Science3030.30Q14.2
Notes: TCs/TPs—indicates the average number of citations per paper for a journal; JCR—Journal Citation Reports™: journals were divided into four categories (25% each), namely Q1, Q2, Q3, and Q4; and IF—impact factor of 2023.
Table 2. List of the most cited articles in the WOS database.
Table 2. List of the most cited articles in the WOS database.
TitleAuthorsJournalYearCitations
1The unseen majority: soil microbes as drivers of plant diversity and productivity in terrestrial ecosystemsVan Der Heijden, M.G.A.; Bardgett, R.D.; Van Straalen, N.M. [32]Ecology Letters20083045
2Nitrogen mineralization: Challenges of a changing paradigmSchimel, J.P.; Bennett, J. [33]Ecology20041725
3Microbial stress-response physiology and its implications for ecosystem functionSchimel, J.; Balser, T.C.; Wallenstein, M. [34]Ecology20071580
4Phenol oxidase, peroxidase and organic matter dynamics of soilSinsabaugh, R.L. [35]Soil Biology and Biochemistry2010911
5Reappraisal of drying and wetting effects on C and N mineralization and fluxes in soilsBorken, W.; Matzner, E. [39]Global Change Biology2009856
6Controls on nitrogen cycling in terrestrial ecosystems: A synthetic analysis of literature dataBooth, M.S.; Stark, J.M.; Rastetter, E. [40]Ecological Monographs2005809
7Calcium additions and microbial nitrogen cycle processes in a northern hardwood forestGroffman, P.M.; Fisk, M.C.; Driscoll, C.T.; Likens, G.E.; Fahey, T.J.; Eagar, C.; Pardo, L.H. [38]Ecosystems2006604
8Stoichiometric controls on carbon, nitrogen, and phosphorus dynamics in decomposing litterManzoni, S.; Trofymow, J.A.; Jackson, R.B.; Porporato, A. [36]Ecological Monographs2010555
9The global stoichiometry of litter nitrogen mineralizationManzoni, S.; Jackson, R.B.; Trofymow, J.A.; Porporato, A. [37]Science2008493
10A global model of carbon, nitrogen and phosphorus cycles for the terrestrial biosphereWang, Y.P.; Law, R.M.; Pak, B. [41]Biogeosciences2010469
Table 3. The top 10 institutions of the ranking of centrality in the research of SNM in forests from 2004 to 2024.
Table 3. The top 10 institutions of the ranking of centrality in the research of SNM in forests from 2004 to 2024.
InstitutionPublicationsCentralityFirst Published YearTCs/TPs
Chinese Acad Sci4030.54200536.94
Cornell Univ590.23200468.09
Univ Alberta720.18200432.56
Swedish Univ Agr Sci760.13200460.60
Boston Univ310.12200674.95
US Forest Serv780.11200440.59
Univ Chinese Acad Sci1390.08201334.98
Kyoto Univ520.08200425.32
INRA350.08200655.09
Univ Göttingen330.08200757.88
Notes: Chinese Acad Sci—Chinese Academy of Sciences; Cornell Univ—Cornell University; Univ Alberta—University of Alberta; Swedish Univ Agr Sci—Swedish University of Agricultural Sciences; Boston Univ—Boston University; US Forest Serv—United States Forest Service; Univ Chinese Acad Sci—University of Chinese Academy of Sciences; Kyoto Univ—Kyoto University; INRA—Institute National de la Recherche Agronomique; and Univ Göttingen—University of Göttingen.
Table 4. The top 10 countries of the number of papers published on SNM in forests from 2004 to 2024.
Table 4. The top 10 countries of the number of papers published on SNM in forests from 2004 to 2024.
CountryPublicationsCentralityFirst Published YearTCs/TPs
United States of America10720.31200460.06
China9870.1200428.95
Germany3560.22200449.69
Canada2800.07200438.04
Australia1910.18200446.97
Sweden1750.08200460.97
Spain1420.07200439.88
Japan1290.14200422.55
France1200.26200450.00
Brazil1140.01200431.26
Table 5. Statistical table of co-occurrence characteristics and frequency of subjects from 2004 to 2024.
Table 5. Statistical table of co-occurrence characteristics and frequency of subjects from 2004 to 2024.
Web of Science CategoriesRecord Count% of 3576
Soil science149241.72
Environmental sciences85323.85
Ecology78722.01
Forestry58716.42
Plant sciences42711.94
Agronomy3108.67
Multidisciplinary geosciences2667.44
Biodiversity conservation1574.39
Multidisciplinary sciences1123.13
Microbiology611.71
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Zhang, X.; Zhang, H.; Wang, Z.; Tian, Y.; Liu, Z. Trends in the Research and Development of Soil Nitrogen Mineralization in Forests from 2004 to 2024. Sustainability 2024, 16, 7882. https://doi.org/10.3390/su16187882

AMA Style

Zhang X, Zhang H, Wang Z, Tian Y, Liu Z. Trends in the Research and Development of Soil Nitrogen Mineralization in Forests from 2004 to 2024. Sustainability. 2024; 16(18):7882. https://doi.org/10.3390/su16187882

Chicago/Turabian Style

Zhang, Xiumin, Huayong Zhang, Zhongyu Wang, Yonglan Tian, and Zhao Liu. 2024. "Trends in the Research and Development of Soil Nitrogen Mineralization in Forests from 2004 to 2024" Sustainability 16, no. 18: 7882. https://doi.org/10.3390/su16187882

APA Style

Zhang, X., Zhang, H., Wang, Z., Tian, Y., & Liu, Z. (2024). Trends in the Research and Development of Soil Nitrogen Mineralization in Forests from 2004 to 2024. Sustainability, 16(18), 7882. https://doi.org/10.3390/su16187882

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