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Review

Progress and Trends in Research on Soil Nitrogen Leaching: A Bibliometric Analysis from 2003 to 2023

by
Getong Liu
1,2,
Jiajun Sun
1,*,
Chenfeng Liu
1,
Huading Shi
1,
Yang Fei
1,
Chen Wang
1,
Guilong Zhang
3 and
Hongjie Wang
2,4
1
Technical Centre for Soil, Agriculture and Rural Ecology and Environment, Ministry of Ecology and Environment, Beijing 100012, China
2
Hebei Key Laboratory of Close-to-Nature Restoration Technology of Wetlands, School of Eco-Environment, Hebei University, Baoding 071002, China
3
Agro-Environmental Protection Institute, Ministry of Agriculture and Rural Affairs, Tianjin 300000, China
4
Engineering Research Center of Ecological Safety and Conservation in Beijing-Tianjin-Hebei (Xiong’an New Area) of MOE, Baoding 071002, China
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(1), 339; https://doi.org/10.3390/su17010339
Submission received: 7 November 2024 / Revised: 25 December 2024 / Accepted: 2 January 2025 / Published: 5 January 2025
(This article belongs to the Section Sustainable Agriculture)
Browse Figures
Versions Notes

Abstract

:
An in-depth discussion on the research progress and trends in soil nitrogen leaching is essential for the development of agricultural sustainability. However, not enough attention has been paid to the progress and future trends of soil nitrogen leaching research. Using software such as VOSviewer and CiteSpace, bibliometric analyses of a total of 2767 documents in the Web of Science Core Collection were conducted; the documents considered were published over the last 20 years (2003–2023). The results are as follows: (1) The research output on soil nitrogen leaching has been increasing steadily, showing a single-discipline dominance in agronomy, but a trend of multidisciplinary cross-research has gradually begun to emerge in recent years. (2) There has been close cooperation between authors, countries, and institutions; the main cooperation includes research on modelling and management, and research related to nitrogen management practices and soil types has a high international profile. (3) The research components at the heart of soil nitrogen leaching are nitrogen cycling in soils, soil properties, water environments and crops, greenhouse gas formation and emissions, and agronomic management practices and the research hotspot has gradually changed to applied research. (4) Increasing the considerations of management measures, deepening the related research on soil microorganisms, and constructing a complete evaluation system constitute the main future research directions. This study can provide valuable references for the sustainable development of agriculture.

1. Introduction

Nitrogen is an important nutrient for plant growth, and it can directly affect crop growth and development [1]. However, in order to increase crop yields in some areas, excessive nitrogen fertilizers have been injected into the soil for a long period of time; consequently, the nitrogen content of the soil is higher than the carrying capacity of the environment [2]. This, together with the frequent occurrence of human activities, such as inappropriate irrigation and the return of feces to the field, has resulted in nitrogen leaching from the soil [3]. Soil nitrogen leaching can cause agricultural and environmental problems such as groundwater pollution [4], soil nutrient loss [5], and greenhouse gas emissions [6], resulting in the loss of agricultural resources. Therefore, the development of accurate quantification and scientific control methods for soil nitrogen leaching and the in-depth discussion of research trends and progress of soil nitrogen leaching are conducive to the conservation of agricultural resources and the reduction of environmental pollution, which is of great significance to the sustainable development of agriculture.
Over a long period of time, scholars have conducted a lot of research in the field of soil nitrogen leaching, and gradually developed and used research methods such as sampling and monitoring [7], isotope tracing [8], and model simulation [9]. Early studies of nitrogen leaching mainly utilized tools such as the lysimeter [10], ceramic suction cups [11] for monitoring and quantifying. The monitoring methods mentioned above can be used to quantify nitrogen leaching under specific soil conditions over a short period of time, and they are easily limited by factors such as soil environment and field management patterns which makes it difficult for them to be applied to large-scale nitrogen leaching assessment [12]. Therefore, model assessment techniques based on computer technology are gradually becoming important methods for quantifying soil nitrogen leaching [13]. Reports on the use of modelling to assess nitrogen patterns began as early as the 1960s [14]. Over the past 50 years, there has been relatively good development of agricultural system models, and a variety of models, such as Denitrification–Decomposition (DNDC) [15] and an open soil–plant–atmosphere system model (DAISY) [16], have been validated to simulate quantitative nitrogen leaching while evaluating soil nitrogen transport under different farmland environments; these models have clarified the key factors affecting soil nitrogen leaching and provided important data references for the selection of nitrogen leaching management measures [17].
Scientific management practices are important for reducing soil nitrogen leaching, and nitrogen reduction [18], rational irrigation [19], and cover crops [20] are the more commonly used methods. These measures reduce nitrogen leaching mainly by reducing the nitrogen content in the soil and improving the nitrogen retention capacity of the soil. In recent years, the effects of the application of efficiency fertilizers [21] and the use of soil microorganisms to promote nitrogen cycling [22] on soil nitrogen leaching have also been increasingly reported. These measures reduce the leaching of nitrogen from the soil while contributing to the improvement of soil quality and crop growth [23]. In addition, the effectiveness of management practices to reduce nitrogen leaching is susceptible to limitations in terms of soil type, crop type, and local climatic conditions [24]; therefore, an increasing number of studies have used models to simulate differences in crop yields, irrigation application, and nitrogen leaching under different management practices, thereby helping people to select and evaluate the best management practices (BMPs) [25,26]. Originally proposed in the United States, BMPs are centered on nutrient management and integrate technical, economic, and institutional aspects to maximize the control of soil nitrogen leaching [27]. In recent years, BMPs have been effectively applied in an increasing number of countries, and management methods are more applicable to the management needs of different regions and complex soil conditions [28,29]. The rapid development of BMPs is beneficial to the scientific control of soil nitrogen leaching, thus promoting the development of sustainable agriculture.
In this context, an in-depth discussion on the research related to soil nitrogen leaching can further the understanding of the current status of the research in this field; such a discussion can generate new ideas to further enhance the scientific management of nitrogen leaching, develop sustainable agriculture, and reduce the risk of agricultural surface pollution. Bibliometric analyses, which include analyses of research content and research trends, as well as the issuing countries, authors, and institutions, are useful for assessing the quantity and quality of existing research outputs and for providing an objective picture of the focus and future directions of research in this field of study [30]. Currently, studies have been conducted to investigate the research hotspots and thematic evolution of soil nitrogen leaching through bibliometrics, but few studies have combined the characteristics of publications, collaborations, and research contents in the field of soil nitrogen leaching research in the last 20 years, with the goal of advancing the sustainable development of agriculture, to provide insights on the future research contents and development trends in the field of soil nitrogen leaching.
In this study, using the Web of Science Core Collection as a data source, we systematically determined the number of publications, basic characteristics, publishers, countries, institutions, highly cited papers, and research hotspots and trends related to the topic of soil nitrogen leaching over the last 20 years (2003–2023). In this study, we analyzed (1) the development trends and basic characteristics of the field of soil nitrogen leaching; (2) the cooperation and publication contributions among authors, countries, and institutions; (3) the topical elements of research in the field of soil nitrogen leaching; and (4) the future research content and trends in the field of soil nitrogen leaching. This study can provide a reference for the understanding of the scientific control of soil nitrogen leaching and the sustainable development of agriculture, and it can provide a scientific basis for constructing a complete evaluation system of soil nitrogen leaching.

2. Materials and Methods

2.1. Data Collection

The Web of Science (WOS) database contains authoritative academic journals of the highest level in a wide range of fields such as natural sciences, engineering, and technology from all over the world, and WOS is the most commonly used database in the academic world, with a high degree of influence in the international arena. The publications in this study were obtained from the WOS Core Collection using the “Advanced search” function and subject (TS)-based citation indexing. The rules of Boolean logic operations were applied [31]: the operator “AND” was used to combine two terms, and the operator “OR” was used to include synonyms or near-synonyms of terms. Since soil nitrogen leaching is mainly dominated by nitrate nitrogen [32], “nitrogen leaching” and “nitrate leaching” are considered as close synonyms in this study; it was searched by “TS = (Soil) AND TS = (“Nitrogen Leaching” OR “Nitrate Leaching”)”. In the search, “nitrogen leaching” and “nitrate leaching” were enclosed in double quotes to indicate an exact search term. The search was conducted on 30 September 2024, spanning the period 2003–2023, with “Article” and “Review Article” selected as article types; the language of the articles was English (Figure 1). The bibliometric data were downloaded in text file format containing details such as title, abstract, authors, keywords, country, institution, number of citations, etc. The above data were then filtered by eliminating data from two or more identical publications to a single set, resulting in valid bibliometric data for 2767 publications.

2.2. Knowledge Extraction and Analysis

VOSviewer (Version 1.6.20), CiteSpace (Version 6.3.R1), Scimago Graphica (Version Beta 1.0.45), and Origin 2024 were used for data analysis and visualization in this study [33]. Meanwhile, to ensure the scientific nature of the study and to make the bibliometrics sufficiently informative, the minimum threshold of keywords for VOSviewer was set to 30, and a total of 140 keywords were obtained; the minimum threshold of countries was set to 5, and a total of 56 countries were obtained; the minimum threshold of institutions was set to 25, and a total of 25 institutions were obtained; and the minimum threshold of authors was set to 5, and a total of 98 authors were obtained. In addition, to avoid limitations caused by synonyms and different expressions of author names, a new thesaurus file was created to merge synonymous keywords and author names in the bibliometric data before analysis using VOSviewer, such as “modelling” and “model” or “Di, h j” and “Di, Hong Jie”.
Information regarding the number of citations to journals, research areas, and publication titles was directly available from the bibliometric data (Figure 1). The annual trend of publications in the field of soil nitrogen leaching worldwide was mapped using Origin2024 to analyze the overall volume of publications (Figure 1). Collaboration analyses between different authors, countries, and institutions were carried out using VOSviewer (Version No. 1.6.20), and author collaboration grids were plotted. The grid parameters of countries and institutions were imported into Scimago Graphica (Version Beta 1.0.45) for visualization (Figure 1). Keyword co-occurrence mapping was performed using VOSviewer (version number 1.6.20) in order to perform a keyword frequency analysis (Figure 1). Finally, the keyword data were imported into CiteSpace (version 6.3.R1), and a keyword burst map was generated using the “Burstness” function that comes with CiteSpace (Figure 1).

3. Results and Discussion

3.1. Trends in Issuance and Basic Characteristics of Publications

3.1.1. Analysis of Output Trends

Based on the search results from the WOS Core Collection, the trends of 2767 publications over time were analyzed (Figure 2). To some extent, the level of annual publications may reflect the growth and change in research in the field [34].
As can be seen in Figure 2, the number of publications in the field of soil nitrogen leaching has exceeded 80 per year over the past 20 years, and the overall trend has been upward, indicating that there was already a certain research foundation in this field in 2003 and, at the same time, that it is still in the process of continuous development and progress. The research of almost 20 years can be divided into three phases, based on the publications issued each year. The first phase, from 2003 to 2008, focused on the effects of management measures, such as cover crops [35] and the use of nitrogen application [36] to control soil nitrogen leaching. The annual volume of publications in this phase fluctuates and does not grow significantly, indicating a slower development in this phase. The second phase, from 2008 to 2020, is a period of rapid growth in research output in the field, with an average increase of 9.1 articles per year and a peak of 197 articles in 2020. This is probably because as the world’s economies have continued to grow, the emphasis on sustainable agricultural development has gradually increased [37]; meanwhile, agricultural modelling has been further developed, and as the use of multiple process models of the terrestrial nitrogen cycle and metabolic models of agricultural crops has matured [38,39], there has been an increasing interest in nitrogen use efficiency. The third phase is after 2020, during which the number of annual publications stabilized at over 170. The number of publications issued in 2023 may be reduced by factors such as publication delays and lagging updates [40], but the overall change is not significant. This suggests that the research related to soil nitrogen leaching is relatively mature after 2020.
Overall, research outputs on soil nitrogen leaching have increased steadily over the last 20 years, indicating a gradual increase in international attention to sustainable agricultural development and the management of pollution from agricultural non-point source; however, there is less change in the research outputs as the research matures after 2020. Given this trend, it is expected that the number of articles on soil nitrogen leaching will continue to stabilize without major fluctuations in the coming years.

3.1.2. Analysis of the Basic Characteristics of Publications

Based on the results of the WOS Core Collection, Table 1 lists the top 10 published journals in the field of soil nitrogen leaching in terms of the number of publications. Agricultural Ecosystems Environment and Agricultural Water Management, which are closely related to agriculture, are the two journals with the highest number of publications, accounting for 6.4% and 5.0% of the total number of publications, respectively, which reflects the dominant position of agriculture in the field of soil nitrogen leaching. Meanwhile, the top 10 journals in terms of number of publications are all related to agriculture and environmental sciences ecology (Table 1), which indicates a concentration of journal types for soil nitrogen leaching line pipe studies. In addition, the top 10 journals in terms of number of publications have a high impact factor, reflecting the high quality of research in the field.
Based on the results of the WOS Core Collection, the annual number of publications was analyzed for research areas with more than 100 total publications (Figure 3). Studies have shown that soil nitrogen leaching is dominated by a single discipline, but the trend of multidisciplinary cross-research has begun to emerge gradually in recent years. A total of seven research areas had more than 100 publications, with agriculture and environmental sciences ecology consistently being the two areas with the highest number of publications; this is consistent with the findings of the journals in which they are published. The number of publications in the field of agriculture has remained in first place; before 2018, the number was basically maintained in more than 60% of the total number of publications (excluding individual years, Figure 3) and occupied a clearly dominant position, which indicates that the research in the field of soil nitrogen leaching represents the domination of a single discipline. Starting in 2018, the share of publications in the field of agriculture gradually declined and remained below 60%, while plant science showed a significant improvement over the more stable development of 2010–2017, and the number of publications in the field of science technology other topics also gradually increased from 0 before 2005 (Figure 3). These findings indicate that research on agriculture in the field of soil nitrogen leaching has decreased in recent years, and a trend of multidisciplinary cross-research has gradually begun to emerge. Multidisciplinary cross-disciplinary research is conducive to innovation and comprehensive development in the field of soil nitrogen leaching [41].

3.2. Countries of Publications

3.2.1. Analysis of the Number of Publications by Countries

The number of publications according to country indicates the level of development in the relevant field, reflects the current state of research in the field, and provides a reference for international cooperation and exchange [42]. Table 2 shows the top 10 countries in the field of nitrogen leaching during the period 2003–2023, with a total of 2485 publications from 10 countries, accounting for 89.8% of the total number of publications in the world. Five countries had more than 150 publications, namely the USA, China, Germany, New Zealand, and Denmark, which accounted for 22.7%, 19.4%, 10.4%, 7.5%, and 5.5% of the total, respectively (Table 2). Figure 4 shows the trend in publications for these five countries over the last 20 years, and the data suggest that the USA consistently has a high volume of publications and is at the forefront of the research. The USA, Germany, New Zealand, and Denmark have shown a small increase in the number of publications in the last 20 years, but the overall change has not been significant, indicating the relative maturity of the research in these countries. China is the fastest-growing country in terms of publications and showed a particular surge after 2016, gradually overtaking the United States as the country with the largest number of publications, which accounted for 37.4% of the total in 2023 (Figure 4). These results indicate that China is developing rapidly in the field of soil nitrogen leaching, probably because the Chinese government strengthened the monitoring and control of nitrogen pollution in water bodies during the 13th Five-Year Plan period [43], which has greatly promoted the research on soil nitrogen leaching. The Carbon Peaking and Carbon Neutrality Goals in 2020 has increased societal concern about soil nitrogen leaching and agricultural non-point source pollution [44]. In addition, China has introduced a series of policies to develop smart agriculture [45], which has promoted agricultural modelling research and applications [46] and has contributed to the rapid progress of research in the field of soil nitrogen leaching in China.
The number of citations can be a proxy for the international impact of a publication [47]. Publications from European countries such as France and England have the highest average number of citations, probably because European countries have enacted a number of policies since 1991, such as the EU Nitrates Directive, to reduce nitrogen emissions [48]; at the same time, European countries regularly monitor and evaluate the effectiveness of nitrogen emission reduction in each member state [49]. Therefore, the research related to the evaluation of nitrogen leaching in European countries started early, and the research is relatively mature with strong international influence.

3.2.2. Analysis of Cooperation of Countries

By analyzing the cooperation of the countries with publications (Figure 5), this study demonstrated that the international cooperation in the field of nitrogen leaching evaluation is close and forms two major global nodes, represented by China and the USA. The USA and China, as the two countries with the largest number of publications and the largest number of international cooperation publications, work most closely with each other. The USA and China collaborated on 83 publications, accounting for 13.2% and 15.5% of the total number of publications from the two countries, respectively. The collaboration between China and the USA was dominated by the use of agricultural models, and as early as 2003, China and the USA jointly developed a transfer function model and tested its effectiveness in evaluating nitrogen leaching [50]. Since then, models such as DNDC [15], a model of water flow and solute transport (HYDRUS) [51], and Soil Water Heat Carbon and Nitrogen Simulator (WHCNS) [52] have been the main research areas of cooperation between China and the USA. At the same time, China and the United States have cooperated with each other and conducted frequent studies on best management practices (BMPs) [53,54]. BMPs are effective in reducing nitrogen emissions from agricultural land, as has been demonstrated in several locations in the USA [55].
In addition, China and the USA have relatively close cooperation with countries such as Australia and Canada (Figure 5). China and Australia have collaborated on 33 publications, with studies covering the retaining of straw on a soil surface [56], water and nitrogen management in desert areas [57], soil environmental pollution in coastal watersheds [58], and the use of multiple agricultural models [51,59]. The USA and Australia collaborated on 28 publications, with the main research covering nitrogen fertilizer management [60], fungal regulation of nitrogen loss [61], microbial communities [62], and modelling analysis [63]. Twenty-six publications resulted from collaborations between China and Canada, with studies covering climate change [64], nitrogen fertilizer [65], management strategies [66], and the use of agricultural models [67,68]. In summary, the main research components of international cooperation include nitrogen management practices, the use of agricultural models, etc., and in-depth research will facilitate international knowledge sharing.

3.3. Authors of Publications

3.3.1. Analysis of the Number of Publications by Authors

Statistical analyses of published authors provide information about representative research scholars and core research strengths in the field [69]. Table 3 lists the top 10 authors with publications in the field of soil nitrogen leaching from 2003 to 2023, with Hongjie Di, Jørgen E Olesen, and Keith C Cameron in the top 3. The earliest publication of Brendon Malcolm and Grant R Edwards was in 2014, which the latest publication year of the 10 authors, suggesting that they have been more active in their research over the past 10 years. In addition, four authors, Hongjie Di, Jørgen E Olesen, Keith C Cameron, and Klaus Butterbach-Bahl, have an average number of citations for their publications of more than 50, indicating that they have balanced both a high quantity and a high quality of research with a high international impact.

3.3.2. Analysis of Cooperation of Authors

Price’s law [70] can be used to determine core author publication thresholds based on the number of publications by the most prolific authors in the field, as follows:
N = 0.749 n max
where nmax is the number of publications by the most prolific authors in the field, and N is the publication threshold for core authors. From Table 3, nmax = 48, then N ≈ 5; so, the authors with five or more publications can be defined as core authors. The clustering of core authors was shown using VOSview, with a high number of clusters in the current field of soil nitrogen leaching; there were 14 in total (Figure 6). Among them, Cluster1, Cluster2, and Cluster3 have more than 10 authors, with more publications and better representation.
Cluster1 contains 14 authors, including Klaus Butterbach-Bahl, Minghua Zhou, and Bo Zhu, who collaborated with each other and focused on nitrate leaching and greenhouse gas emissions from field soils [71,72], with a focus on management practices in maize–wheat cropping systems [73,74].
Cluster2 includes 11 authors, including Kelin Hu, Baoguo Li, and Hao Liang, who have paid particular attention to agricultural modelling, assessing the effects of different nitrogen management practices on soil nitrogen leaching using different agricultural models, such as WHCNS [52,53], and focusing on exploring management practices in greenhouse vegetable production systems [75,76].
Cluster3 includes 10 authors, including Elly Møller Hansen, Iris Vogeler, and Mark Shepherd, who focused on the effects of management practices, such as catch crops [77], delayed cover crops [78], and altered nitrogen fertilization [79], on soil nitrogen leaching and assessed the quantitative effects of different models [80].
The other clusters, with less than 10 authors, are relatively small, but their studies are equally well characterised. The authors of Cluster4 focused their research on soil profiles [81,82] and have conducted studies related to organic carbon and soil microorganisms [83]. The authors of Cluster7 focused on nitrogen leaching in plant–soil systems, assessing the effect of growing different wheatgrasses or plants on soil nitrogen leaching [84,85]. The authors of Cluster10 focused on soil nitrogen leaching from cornfields under the influence of biogas and digesters [11,86]. In summary, the collaborative research between the authors is comprehensive and varied, with the focus of the collaboration on the use of agricultural models and the selection of nitrogen management practices.

3.4. Institutions of Publications

3.4.1. Analysis of the Number of Publications by Institutions

The publication status of research institutions can clarify the research strength of different institutions in related disciplines and reflect the current research status in the field [87]. Table 4 shows the top 10 institutions in terms of the number of publications from 2003 to 2023, with the Chinese acad sci, China agr univ, and Aarhus univ being the 3 institutions with the highest number of publications, reaching 164, 92, and 92 publications, respectively. Among the top 10 institutions in terms of the number of publications, 4 are from China, and 3 are from the United States, indicating that institutions in China and the United States have made a lot of achievements in the field of soil nitrogen leaching. In addition, China agr univ, Chinese acad sci, and Inra ranked among the top three institutions with an average number of citations of more than 50, indicating that the above three organisations have a high international influence in the field of soil nitrogen leaching.

3.4.2. Analysis of Cooperation of Institutions

The study screened out organisations with 25 or more publications and analyzed the collaboration of these organisations (Figure 7), of which there were 25 in total. The study demonstrated a very close institutional collaboration in the field of soil nitrogen leaching, with the Chinese acad sci having the greatest strength of cooperation. The two institutions that collaborated most with the Chinese acad sci were Univ Chinese acad sci and China agr univ. There were 58 publications by the Chinese acad sci in collaboration with the Chinese acad agr sci, and the main studies dealt with the effects of nitrification inhibitors [88], nitrate accumulation [89], and organic fertilizer substitution [90] on soil nitrogen leaching; research on smart agriculture and agricultural modelling [91,92]; and research on agricultural management practices [93]. The Chinese acad sci and China agr univ collaborated on 18 publications, with research focusing on the application of nitrogen management practices in China [94,95] and the development of optimised agricultural models [57,96]. These studies show that agricultural modelling and nitrogen management practices are key elements of the cooperation of institutions. In addition, institutions in different countries are also cooperating more closely, including the Chinese acad sci and Karlsruhe inst technol, and Lincoln univ and Agresearch (Figure 7), which is important for international knowledge sharing.

3.5. Analysis of Highly Cited Articles

The number of citations to a publication reflects the impact of the publication and is representative of the research frontiers for the coming years [30]. Table 5 shows the five most cited articles in the field of soil nitrogen leaching from 2003 to 2023; three of these have more than 1000 citations. The most cited publication was “Reducing environmental risk by improving N management in intensive Chinese agricultural systems”, published by Xiaotang Ju in Proceedings of The National Academy of Sciences of the United States of America in 2009, with 2075 citations. Studies have demonstrated that rational fertilizer reduction can reduce the risk of environmental nitrogen pollution while maintaining crop yields [94]. “Water pulses and biogeochemical cycles in arid and semiarid ecosystems” by Amy T Austin, published in Oecologia in 2004, has been cited 1067 times; it provided a detailed overview of the effects of water inputs on soil microbes and soil nitrogen cycling in arid and semi-arid regions [97]. “Long-term effects of organic amendments on soil fertility. A review” by Francesco Montemurro, published in Agronomy for Sustainable Development in 2010, has been cited 1032 times; it reviewed long-term experiments on the effects of organic amendments used for organic matter replenishment and to avoid the application of high levels of chemical fertilizers [98]. The fourth and fifth most highly cited articles were from Germany and New Zealand, respectively. The former reviewed the effects of soils with different moisture levels on soil carbon and nitrogen cycling [99], and the latter reviewed the processes involved in each of the nitrogen loss pathways and showed that best management practices can reduce the environmental impact of nitrogen leaching [100]. In summary, the highly cited articles are published in diverse countries and journals, and the research is mainly related to soil types and management practices. The research related to soil types and management practices has received more attention and has had a significant international impact.
Table 5. Top 5 most cited publications.
Table 5. Top 5 most cited publications.
TitleCitationsCorresponding AuthorCountrySourceYear
Reducing environmental risk by improving N management in intensive Chinese agricultural systems [94]2075Ju XTChinaProceedings of The National Academy of Sciences of the United States of America2009
Water pulses and biogeochemical cycles in arid and semiarid ecosystems [97]1067A. T. AustinArgentinaOecologia2004
Long-term effects of organic amendments on soil fertility. A review [98] 1032Montemurro FItalyAgronomy for Sustainable Development2010
Reappraisal of drying and wetting effects on C and N mineralization and fluxes in soils [99]896Borken WernerGermanyGlobal Change Biology2009
Nitrogen losses from the soil/plant system: a review [100]886Cameron K. C.New ZealandAnnals of Applied Biology2013

3.6. Research Keywords

3.6.1. Keyword Co-Occurrence Analysis

The co-occurrence analysis of keywords gives an idea of the frequency and focus of the research related to the different keywords in the field [101]. A total of 140 keywords with a frequency of occurrence of more than 30 were selected for co-occurrence analysis (Figure 8). It was found that apart from the search words “nitrate leaching”, “nitrogen leaching”, and “soil”, the keywords that appeared more frequently were “management”, “losses”, “model”, “carbon”, and so on, which demonstrated the focus of research in these areas. The keywords can be roughly divided into four clusters (Figure 8).
Cluster 1 includes 49 keywords, such as “nitrogen”, “nitrate”, and “carbon”. The research focus of this cluster includes the cyclic transformation of nitrogen in the soil as well as soil properties. The keywords “nitrate”, “nitrification”, and “decomposition” in this clustering can reflect the mechanism of soil nitrogen leaching to some extent. Excess nitrogen fertilizers enter the soil under the action of enzymes and microorganisms, through the process of mineralization and decomposition; the vast majority of them exist in the form of inorganic nitrogen, and the change in the form of nitrogen affects the process of nitrogen cycling processes such as nitrification and denitrification, resulting in the accumulation of soil nitrogen and thus exacerbating the loss of nitrogen [102]. The nitrogen leaching form in the soil is dominated by nitrate nitrogen, which has low soil adsorption, but also includes nitrite nitrogen, ammonium nitrogen, and other types of water-soluble nitrogen; in the process of moving with the water, water-soluble nitrogen occurs in the nitrogen leaching losses [32]. In addition, soil nitrogen leaching in different ecosystems and land use types is often accompanied by the leaching of other nutrients, such as carbon and phosphorus [103], reducing nutrient efficiency [104].
Cluster 2 includes 36 keywords, such as “yield”, “water”, and “model”. The research in this cluster focuses on the water environment and crops. Soil nitrogen leaching has a large impact on the water environment, and agricultural non-point source pollution caused by nitrogen leaching is one of the major causes of groundwater pollution [105]. At the same time, nitrogen leaching can lead to the eutrophication of water, further aggravating underground drinking water pollution [5]. Some studies have shown that long-term consumption of water with excessive NO3− content can cause hypertension and cardiac system diseases [106]. Secondly, a series of keywords related to crop growth, such as “maize”, “yield”, and “plant”, illustrate the close relationship between soil nitrogen leaching and crop growth, plant–soil interactions are an important part of research in the field of soil nitrogen (Figure 8), which is consistent with previous research [107]. Crop growth-related processes, such as water evaporation and humicification, have a significant impact on water balance and nutrient dynamics in the soil, which in turn affects nitrogen leaching [55]; thus, soil nitrogen leaching assessment is usually studied in conjunction with crop growth. In addition, “model” has a high frequency of occurrence in this cluster, probably because modelling is widely used as an important tool for assessing nitrogen leaching, both in risk assessment of the water environment and in crop growth assessment [108,109].
Cluster 3 includes 29 keywords, such as “fertilization”, “nitrous oxide”, and “losses”. The research in this cluster focuses on the formation and emission of greenhouse gases. These studies show that the study of soil nitrogen leaching involves the whole nitrogen cycle and that nitrogen leaching is often accompanied by other nitrogen contamination, such as nitrous oxide and ammonia volatilization [110,111]. Therefore, issues such as nitrogen transformation and greenhouse gas emissions are also important considerations in the control of soil nitrogen leaching. In addition, management practices targeting nitrogen leaching, such as manure application and the retaining of straw on the soil surface, may also contribute to increased emissions of greenhouse gases such as nitrous oxide [112]. An in-depth study of greenhouse gas emissions during nitrogen leaching and the development of appropriate nitrogen management practices can help reduce nitrogen losses and emissions and promote sustainable agricultural development.
In cluster 4, “management” is the largest node; it is the core of the network and maintains a close relationship with all the other clusters. Keywords such as “cover crops”, “catch crops”, and “use efficiency” are also the focus of this cluster. The research in this cluster focuses on the effects of different agronomic management practices on nitrogen leaching or nitrogen use efficiency. The importance of management practices has already been mentioned in 2.2–2.5, and the effectiveness of management practices is susceptible to constraints imposed by different soil environments and local economic and policy conditions; thus, research into best management practices under different conditions can help to safeguard crop yields and improve economic efficiency while reducing nitrogen leaching [113,114].

3.6.2. Keyword Burst Analysis

A keyword burst is analyzed to show the evolution of research hotspots and trends in the field over different time periods [115].
The bibliometric data were analyzed for keyword bursts for the period from 2003 to 2023 (Figure 9). The main keywords that started to burst before 2005 were “denitrification”, “nitrogen deposition”, “decomposition”, and so on, indicating that research in this period focused on the cycling and transformation of nitrogen in the soil, with an emphasis on the formation mechanism of nitrogen leaching. After 2005, keywords such as “nitrogen dynamics”, “groundwater”, and “cropping systems” burst. Soil dynamics are essential to minimise the impact of nitrogen leaching on groundwater and to implement appropriate soil management practices [116]. The research during this period focused on simulation and analysis of the effects of soil nitrogen leaching on groundwater and cropping systems through nitrogen dynamics. In recent years, keywords such as “nitrogen use efficiency”, “greenhouse gas emissions”, “responses”, and “maize yield” have been bursting. These emerging keywords could continue to burst until 2023, indicating that the hotspot of research in recent years will continue into the future and will involve the adoption of rational countermeasures to improve soil nitrogen use efficiency, reduce nitrogen leaching and greenhouse gas emissions, and, at the same time, safeguard the yields of crops such as maize. It is worth mentioning that the ‘microbial community’ continues to burst until 2023, indicating that more and more studies in the field of soil nitrogen leaching have been combined with microbiology and other disciplines in recent years, which is consistent with previous findings [117] and contributes to a more comprehensive understanding of the biogeochemical processes and expands the methods and scope of the study of soil nitrogen leaching.

3.7. Prospects for Future Research

In conjunction with the studies in 3.1–3.6, the field of soil nitrogen leaching has continued to develop over the last 20 years. While research has long focused on nitrogen leaching management practices and model evaluation, greenhouse gases, crop yields, and soil microbiology have also received increasing attention. Therefore, based on the bibliometric analysis, three important soil nitrogen leaching field development components were expanded with the goal of promoting agricultural sustainability.
(1)
Increase considerations of management measures and further refinement of agricultural models. With the increasing focus on greenhouse gases, crop yields, soil microbes, and other components, the impact of management practices on other environmental characteristics and crop characteristics should be fully considered along with the reduction of soil nitrogen leaching. By refining existing or developing new agricultural models that incorporate more environmental and crop characteristics into the simulation, thus facilitating the selection of best management practices. This will improve the science and availability of management practices and promote more systematic and comprehensive development of sustainable agriculture.
(2)
Deepening the research content of soil microorganisms in the field of soil nitrogen leaching. It has been shown that microbial-driven elemental cycling has a relatively intuitive impact on the soil environment, nitrogen leaching and crop yield [23]. Therefore, in-depth study of soil microbial community structure, functional genes in the field of soil nitrogen leaching has great prospects for development. Using correlation analysis and modeling analysis to combine soil microorganisms with crop yield and nitrogen leaching, exploring the mechanisms by which soil microorganisms affect crop yield and nitrogen leaching will be beneficial in enhancing the promotion of sustainable agriculture at the intersection of other disciplines.
(3)
Construct a complete nitrogen leaching evaluation system. As research on soil nitrogen leaching matures, a complete system is needed to evaluate it in terms of quantification and management. Integrated use of hierarchical analysis, peer review and other methods to evaluate the accuracy of quantification of soil nitrogen leaching, the effectiveness of management practices and the reasonableness of model parameters. The construction of a complete soil nitrogen leaching evaluation system will help to promote the integrated development of sustainable agriculture in monitoring, early warning and decision-making.

4. Conclusions

In this study, a total of 2767 publications in the field of soil nitrogen leaching from 2003 to 2023 were systematically analyzed using bibliometric tools, such as VOSviewer and CiteSpace. The results of this study demonstrated a steady increase in research output on soil nitrogen leaching over the last 20 years, but there was a gradual maturation of the research after 2020, with less change in the research output. Soil nitrogen leaching has been dominated by a single discipline in agriculture, but the trend of multidisciplinary cross-research has gradually begun to emerge in recent years.
Four authors, Hongjie Di, Jørgen E Olesen, Keith C Cameron, and Klaus Butterbach-Bahl, have a high quantity and quality of research outputs and are the key authors on soil nitrogen leaching. The USA and China lead the world as the two countries with the largest number of publications and the largest number of international collaborative publications. In addition, China’s research output continues to improve, mainly through mutual cooperation between institutions such as the Chinese acad sci, Chinese acad agr sci, and China agr univ. Close cooperation between authors, countries, and institutions has contributed to the development of soil nitrogen leaching, with cooperation focusing on the use of agricultural models and nitrogen management practices. At the same time, research on soil types and nitrogen management practices has received more attention and has a high international profile.
The field of soil nitrogen leaching can be divided into a total of four clusters, with the core research areas being the cyclic transformation of nitrogen in soils and soil properties, water environments and crops, greenhouse gas formation and emissions, and agronomic management practices. At the same time, keywords such as “management”, “losses”, “model”, “carbon”, and “wheat” appear with high frequency, representing the research hotspots in this field. In addition, the research hotspot of soil nitrogen leaching has gradually changed to applied research. The research trend in recent years will be towards rational countermeasures to improve soil nitrogen use efficiency, reduce nitrogen leaching and greenhouse gas emissions, and, at the same time, safeguard crop yields.
Based on the bibliometric analysis, three important soil nitrogen leaching field development components were expanded with the goal of promoting sustainable agriculture. First, increase considerations of management practices and further refinement of agricultural models. Second, deepen the research content of soil microorganisms in the field of soil nitrogen leaching. Third, construct a complete nitrogen leaching evaluation system.
This study has some limitations: a purely quantitative analysis of the literature data may lead to a study that does not fully evaluate the scientific results of the articles, while the keywords may not fully reflect the subject matter of the publication, which may lead to a low correlation between some of the articles and soil nitrogen leaching. However, the current status of development, research hotspots, and future research trends in the field of soil nitrogen leaching were comprehensively discussed using bibliometric methods. This study provides a reference for the understanding of the scientific control of soil nitrogen leaching and the promotion of the sustainable development of agriculture, and it provides a scientific basis for the construction of a complete evaluation system of soil nitrogen leaching.

Author Contributions

G.L.: data curation, formal analysis, software, visualization, writing—original draft; J.S.: validation, data curation, supervision, writing—review and editing; C.L.: data curation, project administration, writing—original draft; H.S.: methodology, project administration, writing—original draft; Y.F.: data curation, writing— original draft; C.W.: visualization, writing—original draft; G.Z.: formal analysis, writing— original draft; H.W.: validation, writing—original draft. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the National Key Research and Development Program of China (2023YFD1701800), the National Key Research and Development Project of China (2023YFD1701805), and the Youth Science and Technology Innovation Fund Program of Technical Centre for Soil, Agriculture and Rural Ecology and Environment, Ministry of Ecology and Environment (QKC2024011).

Data Availability Statement

The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding authors.

Acknowledgments

The authors thank the financial support from Guilong Zhang, Chenfeng Liu and Yang Fei; and the reviewers for their valuable comments and suggestions on our manuscript.

Conflicts of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Bibliometric search, screening, and analysis processes.
Figure 1. Bibliometric search, screening, and analysis processes.
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Figure 2. Annual publication volume in the field of soil nitrogen leaching.
Figure 2. Annual publication volume in the field of soil nitrogen leaching.
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Figure 3. Trends in publications in different research areas.
Figure 3. Trends in publications in different research areas.
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Figure 4. Annual publishing trends of top 5 countries.
Figure 4. Annual publishing trends of top 5 countries.
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Figure 5. Cooperation between different countries in the field of soil nitrogen leaching. Each node represents a country, and the size of the node is proportional to the amount of publishing carried out by the country. The color of the nodes represents the intensity of cooperation in the country, with red representing a high intensity of cooperation and yellow representing a low intensity of cooperation. The lines between the nodes indicate cooperation between countries, with thicker lines and darker colors indicating closer cooperation.
Figure 5. Cooperation between different countries in the field of soil nitrogen leaching. Each node represents a country, and the size of the node is proportional to the amount of publishing carried out by the country. The color of the nodes represents the intensity of cooperation in the country, with red representing a high intensity of cooperation and yellow representing a low intensity of cooperation. The lines between the nodes indicate cooperation between countries, with thicker lines and darker colors indicating closer cooperation.
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Figure 6. Collaborative clustering of core authors in soil nitrogen leaching studies. Each node represents an author; the size of the node is proportional to the author’s publication volume; the closely connected nodes are in the same circle, indicating the same cluster.
Figure 6. Collaborative clustering of core authors in soil nitrogen leaching studies. Each node represents an author; the size of the node is proportional to the author’s publication volume; the closely connected nodes are in the same circle, indicating the same cluster.
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Figure 7. Cooperation between different institutions in the field of soil nitrogen leaching. Each node represents an institution, and the size of the node is proportional to the amount of publishing carried out by the institution. The color of the nodes represents the intensity of cooperation in the institution, with red representing a high intensity of cooperation and yellow representing a low intensity of cooperation. The lines between the nodes indicate cooperation between institutions, with thicker lines and darker colors indicating closer cooperation.
Figure 7. Cooperation between different institutions in the field of soil nitrogen leaching. Each node represents an institution, and the size of the node is proportional to the amount of publishing carried out by the institution. The color of the nodes represents the intensity of cooperation in the institution, with red representing a high intensity of cooperation and yellow representing a low intensity of cooperation. The lines between the nodes indicate cooperation between institutions, with thicker lines and darker colors indicating closer cooperation.
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Figure 8. The map of the co-occurrence network of keywords; each node represents a keyword; the size of the node is proportional to the frequency of occurrence of the keyword; the closely connected nodes are in the same circle, indicating the same cluster.
Figure 8. The map of the co-occurrence network of keywords; each node represents a keyword; the size of the node is proportional to the frequency of occurrence of the keyword; the closely connected nodes are in the same circle, indicating the same cluster.
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Figure 9. Soil leaching nitrogen keyword burst map, with light blue representing years in which the keyword did not appear, blue representing years in which the keyword began to appear, and red representing years in which the keyword burst.
Figure 9. Soil leaching nitrogen keyword burst map, with light blue representing years in which the keyword did not appear, blue representing years in which the keyword began to appear, and red representing years in which the keyword burst.
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Table 1. Top 10 journals with the most publications.
Table 1. Top 10 journals with the most publications.
Source TitleNumber of PapersPercentage (%)5-Year Impact Factor
Agriculture Ecosystems Environment1766.46.4
Agricultural Water Management1375.06.3
Science of The Total Environment1043.88.6
Nutrient Cycling in Agroecosystems903.33.5
Journal of Environmental Quality812.92.7
European Journal of Agronomy642.35.1
Plant and Soil521.94.6
Soil Use and Management521.93.9
Agronomy-Basel491.83.7
Communications in Soil Science and Plant Analysis421.51.9
Table 2. Top 10 countries with the most publications.
Table 2. Top 10 countries with the most publications.
CountryPublicationsPercentage (%)CitationsAverage Number of Citations
USA62922.724,94939.7
China53719.421,34739.8
Germany28910.411,53539.9
New Zealand2077.5709834.3
Denmark1515.5666344.1
Australia1435.2621443.5
Canada1385.0324423.5
Spain1334.8628547.3
England1314.7626847.8
France1274.6667152.5
Table 3. Top 10 authors with the most publications.
Table 3. Top 10 authors with the most publications.
AuthorPublicationsEarliest Posting TimeCitationsAverage Number of Citations
Di, Hong Jie482004274357.1
Olesen, Jørgen E382003213256.1
Cameron, Keith C332004230869.9
Vogeler, Iris25200641816.7
Butterbach-Bahl, Klaus222005120854.9
Hansen, Elly Møller20200664132.1
Hu, Kelin19200869336.5
Malcolm, Brendon17201423413.8
Edwards, Grant R17201438222.5
Thomsen, Ingrid Kaag16200338824.3
Table 4. Top 10 institutions with the most publications.
Table 4. Top 10 institutions with the most publications.
InstitutionsCountryPublicationsCitationsAverage Number of Citations
Chinese acad sciChina164851051.9
China agr univChina92648670.5
Aarhus univDenmark92281930.6
Usda arsUSA84365443.5
Lincoln univUSA83389346.9
Univ FloridaUSA75237231.6
Swedish univ agr sciSweden60251341.9
Univ Chinese acad sciChina58218837.7
Chinese acad agr sciChina58183931.7
InraFrance55349563.5
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Liu, G.; Sun, J.; Liu, C.; Shi, H.; Fei, Y.; Wang, C.; Zhang, G.; Wang, H. Progress and Trends in Research on Soil Nitrogen Leaching: A Bibliometric Analysis from 2003 to 2023. Sustainability 2025, 17, 339. https://doi.org/10.3390/su17010339

AMA Style

Liu G, Sun J, Liu C, Shi H, Fei Y, Wang C, Zhang G, Wang H. Progress and Trends in Research on Soil Nitrogen Leaching: A Bibliometric Analysis from 2003 to 2023. Sustainability. 2025; 17(1):339. https://doi.org/10.3390/su17010339

Chicago/Turabian Style

Liu, Getong, Jiajun Sun, Chenfeng Liu, Huading Shi, Yang Fei, Chen Wang, Guilong Zhang, and Hongjie Wang. 2025. "Progress and Trends in Research on Soil Nitrogen Leaching: A Bibliometric Analysis from 2003 to 2023" Sustainability 17, no. 1: 339. https://doi.org/10.3390/su17010339

APA Style

Liu, G., Sun, J., Liu, C., Shi, H., Fei, Y., Wang, C., Zhang, G., & Wang, H. (2025). Progress and Trends in Research on Soil Nitrogen Leaching: A Bibliometric Analysis from 2003 to 2023. Sustainability, 17(1), 339. https://doi.org/10.3390/su17010339

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