1. Introduction
Nitrogen is a pivotal element for plant growth and the total amount of resident nitrogen is estimated at 133–140 Pg for the upper 100 cm soil [1]. The Primary component of SN is in organic form [2,3]. However, plants mainly absorb inorganic nitrogen and can only absorb a small amount of low molecular weight of organic nitrogen under extreme conditions [4]. Soil nitrogen availability profoundly affects global crop growth and development [5]. Nitrogen (N) plays a vital role in terrestrial and aquatic ecosystems [6], N limiting plant growth is a widespread problem in terrestrial ecosystems [7] and is also a major factor limiting plant growth in acidic soils. Nitrogen is an important part of soil nutrients, and its spatial-temporal differentiation is an important reference index to reflect soil quality and evaluate land sustainability [8]. It is also a major factor affecting ecosystem productivity and ecosystem stability and is important in the study of the global carbon and nitrogen cycle and their impact on climate change [9,10].
Soil, an environmental factor in karst ecosystem, is the key link and link of controlling rocky desertification. The evolution of rocky desertification is accompanied by the change of soil quality. In order to achieve the United Nations Sustainable Development Goals (SDG), the EU Green Agreement and the EU biodiversity Strategy 2030, namely, to protect, restore and promote the sustainable use of terrestrial ecosystems, sustainable forest management, combat desertification, halt and reverse land degradation and halt the loss of biodiversity, to provide ES, agricultural production and support circular economy development, we need to understand the ES supported by soil quality, ecological products and vegetation diversity in degraded ecosystems.
The ecological environment of karst areas is fragile, and it faces a severe problem in its soil quality. In such a specific environment of rocky desertification with poor soil, high-exposure bedrock and low vegetation coverage, it will take around 10,000 years to form a 2.47 cm-thick soil layer [11]. Nutrient-carrying capacity is subject to shallow soil, which hinders growth of the plant [12]. In order to improve the restoration efficiency of ecological environments in harnessing rocky desertification, managing ecological environment must be addressed [13]. The inherent problem of the ecological environment is that the variation in land utilization has led to the transformation of ecosystemic structure [14]. The improvement of soil quality is most obvious in the early stage of secondary succession, and the restoration of natural vegetation can solve the problem of land degradation [15]. However, SN is an important indicator for evaluating changes in soil quality. Therefore, it is of great significance to study the spatial-temporal distribution of N in rocky desertification soils. Rocky desertification N is both the most important and the most difficult nutrient for plants to manage, and N availability often limits seasonal grassland productivity [16], other agricultural and forestry ecosystems are also constrained. According to research reports: at the present stage, tree, shrub and grass plants in karst areas present N deficiency and P sufficiency [17], and the growth of shrub in karst ecosystems is restricted by N deficiency [18]. Due to the diversity of various soil-forming factors, the morphological characteristics, components and properties of soil will have spatial distribution variability, correlation and temporal changes at various scales. However, ecosystems increasingly rely on self-regulating processes [19]. Since SN supports ES, it should be an integral part of ecological product quality supply, soil quality and vegetation diversity regulation. Therefore, we must elucidate the distribution of SN in rocky desertification ecosystems.
The predecessors have carried out a lot of research on the distribution of soil nitrogen in rocky desertification, but the current situation of SN heterogeneity in space and time has not been studied from the perspective of bibliometrics. Bibliometrics refer to the research methods employed in library and information science, which use quantitative analysis and statistics to describe patterns of article distribution within a given topic, field, institution and country. Bibliometrics is a useful tool to map the literature around a research field, which has been used in many global studies of specific fields recently [20,21]. For instance, the researchers evaluated the knowledge landscape of heavy metal phytoremediation from 1989 to 2018 by constructing a series of scientific maps to explore the research hotspots and trends of heavy metals [22]. To better understand the current state of research in the study of spatial-temporal distribution characteristics of soil nitrogen, a review of scientometric methods was used, which helps to show the popular research topics and their evolution in this field. This paper reviews the spatio-temporal heterogeneity of SN in karst rocky desertification area during the past 30 years, the spatio-temporal heterogeneity of soil nitrogen, its influencing factors and its service support to ecosystem were summarized, the key scientific and technological problems that need to be solved in the existing research are presented and provide a scientific basis for the study of SN in karst rocky desertification area, promotes the improvement of ES for rocky desertification control and maintains the integrity and sustainability of ecosystem [23].
2. Literature Review
2.1. Data Collection Process
Typically, bibliometrics is based on analysis of one or more of four databases: Web of Knowledge, Scopus, Google Scholar and PubMed [24]. Scopus database was used to search the literature related to SND in rocky desertification, because it contains the largest number of indexed journals in the research database, it is considered the largest research database [25]. Scopus has the advantage of loading sources 70% greater than WoS [26]. As the largest database of peer-reviewed literature abstracts and citations, it covers 15,000 scientific, technical and medical journals [27].
We have used the Scopus database search expressions in titles, abstracts and keywords to obtain global-level research output. By using Boolean operators “OR” and “AND”, search queries can be reached to assess the total number of publications related to SND at the global level; the retrieval type is as follows: (TITLE, ABS, KEY (“Karst*” OR “rocky desertification*”) and TITLE, ABS, KEY (“nitrogen”) and TITLE, ABS, KEY (“distribution” OR ”variation” OR ”change”). During the database search collection process, the subject areas relevant to the subject in the Scopus database were screened. It includes 11 subject categories including Agricultural and Biological Sciences, Biochemistry, Genetics and Molecular Biology, Environmental Science, Earth and Planetary Sciences. Document types include Article, Conference Paper, Review, Book Chapter, Erratum. Literature language selection includes English, Chinese, German, Czech, French, Estonian, Polish, Russian. The time range is the maximum time range that the database can retrieve literature from with a deadline of 4 March 2022. The flow chart of screening literature on N distribution in rocky desertification soil is shown in Figure 1.
2.2. Data Analysis and Methods
Data collected from the Scopus database were further analyzed by bibliometrics. It should be noted that the references listed at the end of the text are all cited in this study and the rest of the literature is not included. Co-occurrence analysis and research hotspot analysis were performed using VOSviewer software. In addition, we used Microsoft Excel 2020 and Origin 9.0 to analyze and plot the data. In co-occurrence and cluster analysis networks, nodes represent specific key terms, such as authors or keywords, where the larger the node size, the higher the frequency. The bigger the circle, the more important the node is. The thicker the relationship line, the closer the cooperative relationship. In addition, based on the VOSviever software, the closely related keywords are divided into one category to form a clustering network.
2.3. Literature Distribution Analysis
2.3.1. Literature Types, Numbers and Countries Distribution
A total of 344 articles were obtained from the Scopus database. According to the type of literature, the literature is divided into Article, Conference Paper, Review and Erratum (Figure 2a). The “Article” genre ranked first with 97.4%, this is followed by the “Conference Paper” type at 1.7%. It is worth noting that the proportion of “Article” type is higher than other fields, karst nitrogen deposition [28], nitrogen isotope [29], soil nutrients [30] and soil microbes [31,32]. This shows that relevant researchers in the field of rocky desertification SND have summarized and drawn conclusions based on the current research status, which has promoted the development and progress of this field. This also means that there may be more studies on SND in rocky desertification in the future.
There are 11 main research areas. (Figure 2b). Agricultural and Biological Sciences, Environmental Science and Earth and Planetary Sciences are three major fields of SND in rocky desertification. This is because nitrogen provides an indispensable role in supporting functions and processes in dynamic natural ecosystems [33,34]. It has attracted extensive attention in the fields of Agricultural and Biological Sciences and Environmental Science. In addition, the research direction of rocky desertification SN is changing by the direction of ES, including environmental science, biodiversity, soil quality and ecological product quality. This indicates that SN plays an important role in ES.
The year-to-year changes in the number of articles in each year are summarized as shown in Figure 2c. The number of publications on SND in rocky desertification is increasing gradually, with the number of published articles increasing from 1 in 1990 to 43 in 2021. Because the literature statistics of 2022 are not complete, the analysis will not be made from 2022. By 2021, 338 documents had been published. The initial development stage of SND in rocky desertification was relatively slow, and it needed time to be explored. It is worth mentioning that there was a significant increase between 2009 to 2012 compared with previous years. A total of 118 papers were published in the past three years, accounting for 34.30% of the total documents. These results indicate that more and more researchers are devoted to the study of SND in rocky desertification.
Regional distribution studies were also conducted on the literature (Figure 2d). It is worth noting that there are seven documents whose research countries are not counted, so the distribution of the research areas of 334 documents is analyzed. China, Germany, Norway and the United States are the five countries with the most research, with China’s research accounting for 59.3%. This is related to the fact that South China Karst is the largest contiguous karst area in the world, therefore, the research on karst ES is highly typical, representative and exemplary in the world.
2.3.2. Analysis of Author Cooperation
Figure 3 shows a visualized map of density associated with published studies on SN from rocky desertification-item density. The total number of occurrences of author keywords was analyzed, and the minimum instances of a keyword being used was set to seven times. Out of a total of 1016 keywords, 10 reached this threshold. The number of co-occurrence links for each of the 10 keywords were counted. The center keyword is located in the darkest region (yellow to blue). The same is true for Figure 4, which shows the results of author keyword co-occurrence analysis in the form of a network visualization. As shown in Figure 4, the authors were explicitly divided into three groups based on colors. Here, He, X., Zhang, W. and Xiao, D. appear in the same research group. Wang K. is in the middle of the network and has connections to the other two groups of authors, this shows that Wang, K. maintains a strong academic collaboration with leading researchers in the field. At the same time, Wang, K. has the largest node. He works closely with all authors, which means that Wang K. is considered one of the top scholars in the field.
2.3.3. Keyword Co-Occurrence Analysis
Keyword analysis is crucial because keywords carry the most important and core information and represent the content of the article, playing an important role in the information retrieval process [35]. Analysis of keywords helps to understand research hotspots in this field. Through the analysis of literature keywords by VOSviewer software, the minimum occurrence of keywords were set to 5. Among 5592 keywords in total, 44 reached this threshold, thus obtaining Figure 5 and Figure 6. Figure 5 shows the keyword density mapping. The color range is from cool (blue) to warm (yellow), indicating that the higher the co-occurrence frequency of keywords, the higher the literature research intensity. ‘Karst’ and ‘nitrogen’ are the most used keywords, while ‘picea abies’, ‘Norway’, ‘Spruce’ and ‘karst ecosystem’ are more common. This indicates that soil nitrogen has received extensive attention in the karst picea abies (norway spruce) ecosystem in the past. Figure 6 shows the keyword clustering graph. The first group (green) focuses on the study of soil nutrients, soil organic matter, plant diversity and vegetation root systems in karst areas; the second group (yellow) focuses on the study of tree species in karst areas or southwest China, and the third group (blue) the focus is on land use in karst ecosystems; the fourth group (dark yellow) focuses on nitrogen, Picea abies and vegetation in Norway spruce soil from a biological perspective. It is expected that China will continue to be an active country in this area of research, as it is the only country name to appear in the list of countries with the highest frequency of high incidence.
3. Main Progress and Achievements
3.1. Heterogeneity of Spatial-Temporal Distribution of Soil Nitrogen Forms
3.1.1. Temporal Heterogeneity
Among plant nutrients, nitrogen has the greatest demand and is recognized as the major limiting element in terrestrial ecosystems on a global scale [36,37,38]. Many studies have reported various developments in SN storage including an increase in rocky desertification [39,40], reduction [41] and unchanging [42]. The variation of SN storage in rocky desertification is the result of the migration and transformation of different forms of SN in the atmosphere, soil and plants. However, the distribution of SN was different in different stages of rocky desertification succession and vegetation restoration strategies.
Yan Ping [43] showed that SN decreased with the increase in rocky desertification grade, the research results are inconsistent with the trend of a decrease followed by an increase in the study of Sheng Maoyin [44]. Soil nitrogen increases with the positive succession of vegetation. Seasonal changes in soil moisture and temperature are also an important aspect of temporal changes in SN. Compared with the rate of ammoniation and nitrification, the rate of soil net nitrogen mineralization showed obvious seasonal variation, which indicated a trend of being high in summer and low in winter [45]. Over the past century, large parts of the karst region have been severely degraded by intensive human interference. Most of the degraded land in the region has been ecologically restored through management or natural vegetation restoration with the support of the “Green for Food” project [46]. Practically, long-term management and natural restoration of vegetation are also the process of soil quality improvement, in which SN plays an irreplaceable role and its effectiveness plays an important role in determining the structure and function of the ecosystem [47].
3.1.2. The Changing Characteristics of Space
Studies have shown that understanding spatial patterns of SN availability, mineralization and controlling factors is critical for assessing ecosystem function, particularly in the sustainability of ES in ecologically degraded areas that vegetation restoration projects aim to improve [47]. Thomas E. [48] also shows that: karst habitats, after introducing Micronesia cryptosporidium plants, decrease soil phosphorus content in five years. Six years later, the SN content increases, the plant and its surrounding soil show unique interaction, some of these features include the increase in carbon (C) and N fixation and increasing the spatial heterogeneity of soil chemistry and improvement to the ES. The distribution and influence of SN were different at different spatial scales. For instance, when N inputs are large and continuous, anthropogenic N addition in temperate ecosystems has been shown to affect a wide range of ecosystem characteristics and processes, altering net primary production and nutrient cycles, interacting with elevated C dioxide [49]; in tropical and subtropical ecosystems, even small changes in biogeochemical cycles can have global significance; in the Qinghai-Tibet Plateau, the relative effects of climate change and human activities can cause significant spatial and temporal differences in NPP changes [50,51]. Xiao Xuefeng [52] showed that N deposition accelerated soil acidification, increased the remaining amount of litter and affected the decomposition of soil organic matter. In fact, karst soil nutrients are mainly replenished by litter.
Researchers found that the vertical distribution of SN has clear surface aggregation, and this phenomenon is consistent in the distribution of rocky desertification and non-rocky desertification areas. Li Xiaoliang [53] and other studies found that the contents of total nitrogen (TN) and total organic carbon in the soil are in the topsoil layer, core soil layer, and subsoil layer in order from largest to smallest amounts, respectively, and with the deepening of rocky desertification, the topsoil erosion phenomenon is shown. Zhang Dongqing [54] found that in the soil profile, the contents of organic matter and various forms of nitrogen were mainly distributed in the soil surface, and gradually decreased with the deepening of soil layer. The proportion of various forms of organic nitrogen in total N also decreased, but the proportion of acid non-hydrolytic nitrogen in total N gradually increased. Zheng Hua [55] studied the impact of land use on soil nutrients in karst peak forest valley and highlighted that the total N content in paddy soil was significantly higher than that in forest land, and both were significantly higher than that in dry land, and the soil C/N ratio was influenced by the composition of organic matter and fertilizer application. Li Xinai [56] showed that soil total N content in paddy field was significantly higher than that in forest land, and that in forest land was significantly higher than that in dry land. The content of soil microbial biomass N was almost the same in paddy field and forest land, but significantly lower in dryland than in paddy field and forest land.
The inconsistencies of these results suggest that SN dynamics during vegetation restoration are complex and may be influenced by multiple factors. Many studies have assessed the effects of vegetation restoration, tillage systems, and geographic scales (elevation, aspect, position, regional, regional, and field) on SN variables and their transformations, but the resulting patterns are highly spatio-temporal heterogeneous. Mediating this variability includes both biological and abiotic factors.
3.2. RSN Provides Service Support to Ecosystem
In the 1980s, Ehrlich P, a famous ecologist, first proposed the term “ecosystem services” to promote the concept and emphasize the importance of ES. In terms of soil nitrogen, this concept can help improve the quality of ecological products, maintain soil quality, maintain plant diversity [57,58,59,60] and improve ecosystem functions such as C and N storage, mineralization and absorption [61,62].
3.2.1. Soil Quality Service Support
Nitrogen is one of the key nutrients that indicate soil quality and an important component for plant development [63]. Soil quality is one of three components of environmental quality, along with water and air quality [64]. Soil quality is often more broadly defined as: the ability of soil to maintain biological productivity, maintain environmental quality and promote plant and animal health within ecosystem and land-use boundaries [65]. Notably among scientists, the concept of ES is often used in connection with the concept of soil functions and shifting the research approach from sustainability of crop production to the provision of ES [66]. Translating soil indicators into impacts on ES and social benefits is particularly useful for supporting land users and policymakers to build the case for scaling up sustainable land management [67].
The supporting services that are provided by soil quality to the ecosystem include soil formation process, including supporting water and nutrient cycling, biological population regulation and maintenance of soil structure and habitat [68]. Soil nitrogen plays a pivotal role in all of the above. Karst is an integral part of the fragile ecological region in the world, and the degradation of the karst rocky desertification ecosystem is also a research focus determined by the United Nations Sustainable Development Goals. Due to the shortage of nitrogen resources in the karst areas of Southwest China, N leaching from the system is critical.
3.2.2. Service Support for Ecological Product Quality
Nitrogen availability may have a positive effect on the C source-sink balance. Carbon supply improves leaf and whole-tree photosynthesis capacity, promotes leaf area and fruit enlargement, and increases soluble sugars in fruit due to C supply or improved starch degradation [69]; under the condition of high N application, the firmness of fruits at harvest or storage decreases [70], and the effect of N fertilizer application on fruit quality needs to be further explored. In the ecologically fragile areas of karst rocky desertification, the development of ecological industries provides a wide variety of ecological products (pasture, medicinal materials, economic forests, etc.). Studies have shown that N addition can effectively improve the nutritional quality of herbage [71]. It is suggested that the rational application of N fertilizer can not only improve the N supply capacity, but also enhance the supply capacity of ecological products and improve the ES value of the karst rocky desertification ecosystem.
3.2.3. Plant Diversity Service Support
Many countries believe that human behavior is destroying the earth’s ecosystems, eliminating genes and species at an alarming rate, and biological characteristics of this observation has led to such a problem, namely how the biodiversity loss will change the function of ecosystems and how societies will provide goods and services needed for prosperity [72]. Species diversity is a component of biological diversity, including animal, plant and microbial diversity. Some studies show that herbivore assemblage combined with SN quantity accounts for 41% of the variation in plant α-diversity, while herbivore assemblage combined with SN heterogeneity explained 15% of the variation in plant β-diversity [73]. Both plant diversity and functional composition were important controlling factors for soil and plant N pools. Plant communities with more diverse mixtures may use limited resources more efficiently. Nitrogen accumulation is the main driver of species composition change across different ecosystem types by driving competitive interactions that lead to composition changes or disadvantage the conditions of certain species. Plant diversity has an important impact on ecosystem functions, such as productivity and C and N storage in agroforestry, grassland and forest.
4. Key Issues to Be Addressed
4.1. Aiming at the Problem of Unclear SN Sources in Karst Areas and Considering the Particularity of Karst Environment, a Comprehensive N Source Assessment Mechanism Should Be Established
Soil nitrogen is mainly produced from the biological fixation of molecular N in the atmosphere, N carried by rainwater and irrigation water and the application of organic fertilizer and chemical fertilizer. In the process of rock desertification control, a series of ecological restoration projects have been implemented, such as mixing agricultural and forestry land and grassland into fruit forests. However, the application of chemical fertilizers and pesticides is not conducive to the evaluation of SN in the non-intensive management mode of rock desertification area, while ensuring plant yield and avoiding pests and diseases. Considering the particularity of karst environment, it is suggested to use stable isotope SIAR mixture model as a reliable “fingerprint” tool, which can be successfully used to estimate the contribution of various carbon and N sources in complex ecosystems [74]. The prediction accuracy of the model can also be evaluated based on the cross-validation procedure, and the soil monitoring data of synthetic aperture radar (SAR) earthworks can be evaluated and predicted based on the model [75]. A combination of multiple techniques (model building) was used to establish a comprehensive evaluation mechanism for the N sources in karst soils.
4.2. In View of the Unclear Rules of N Migration in Karst Areas, the Process and Rules of SN Migration Were Explored, and the Whereabouts and Leaching Risk of SN after Migration Were Evaluated
With the development of soil solute transport theory, the study of N transport and transformation has been developed. Simulation models are useful tools for understanding the interaction between N transformation and transport in the field [76]. The DRAINMOD and CREAMS models simulate and study the transport and transformation of N in soil, respectively, and their research on SN pollution through their models has attracted extensive attention internationally [77,78]. The transport and transformation of N in soil has become a popular topic in environmental and soil science [79,80]. Soil nitrogen is important in ecosystem material cycling and nutrient flow. The dual structure of surface and subsurface in karst region determines the complexity of soil nitrogen migration, and establishes a comprehensive mechanism to evaluate the destination and leaching risk of soil nitrogen after migration.
4.3. The Aim Is to Solve the Problem of Unclear SN Form Occurrence in Karst Rocky Desertification Ecosystem
Nitrogen performs a major role in regulating plant nutrient uptake, root development, photosynthetic physiology, yield and quality formation and N transporter genes [81]. At present, many studies have been conducted on SN in rocky desertification, such as soil quality [82,83], soil nutrients [84,85], soil microorganisms [86,87] and plant communities [88]. Most studies discuss the response and influence of TN, ON or IN on geographical factors. However, the status, form and occurrence of SN in different types of ecosystem (forest, agroforestry, grassland and farmland) with different levels of rocky desertification has not been systematically described. It is urgent to clarify the regularity of N form and occurrence in rocky desertification soil.
4.4. According to the Unclear Spatio-Temporal Heterogeneity of SN since the Control of Karst Rocky Desertification, We Selected the Time scale of Drought and Rainy Season and the Typical Spatial Scale to Carry out the Research on SN Support to ES
Under the influence of rainfall and human activities, temporal and spatial variation of SN exists at both large and small scales [89]. The ecological reconstruction in karst rocky desertification areas is restricted by the environmental characteristics of thin soil layers, poor water and soil conservation ability, nutrient deficiency and large seasonal difference [90]. For areas with diverse types of ecosystems and significant spatial heterogeneity of ecological environment, researchers can make ecological partitions in the study area and then construct different ecological vulnerability evaluation systems according to local conditions [91]. At present, the spatial or temporal variation of SN in karst areas is mostly discussed only, but the temporal and spatial variation of SN is seldom combined to discuss the long-term series, and the spatial and temporal variation of SN in karst rocky desertification is more systematically discussed. The spatio-temporal heterogeneity of SN in karst rocky desertification was studied by selecting a long-time scale and representative regions in the long-term self-succession of the ecosystem to provide a scientific basis for ecological environment management and sustainable development in karst rocky desertification area.
4.5. In View of the Lack of N Management in Karst Rocky Desertification Soil, How to Develop N Requirements and Management Technologies for Different Karst Ecosystems, and Form an Evaluation System for Sustainable N Management
The relationship between N budget, balance and surplus is complex. Therefore, in the process of establishing N balance, according to different purposes, the details of input and output items should be considered differently. The allowable residual amount of nitrate N in soil after harvest of dryland crops is an index for evaluating N management [92]. China’s operation mode is dominated by small farmers, which is a complex operation unit without convenient restrictions, which is not conducive to nitrogen cycling and flow management of small farmers. For karst areas, due to poor soil resulting in slow growth and poor growth of regional crops, it is more necessary to develop and apply a N management index system by coordinating the ratio between each nutrient, with the aim of evaluating and achieving optimal management of N to achieve productivity, environmental protection and improved soil fertility in karstic stone desert areas and to achieve the orderly realization of the ecosystem material cycle, the sustainable use and succession of the environment.
4.6. For the Impact of N on Product Quality in Karst Soil, Further Research Is Needed on How to Comprehensively Evaluate N on Ecological Product Quality through the Key Link of N Cycle
Generally, the level of nitrogen application affects the firmness of the fruit when it is received or stored. When N supply increases, soluble sugar in the fruit increases with improved carbon supply or starch degradation. Different nitrogen forms also have significant effects on fruit quality. Yang Yang [93] obtained the influence of nitrogen forms on jufeng grape quality, indicating that urea and appropriate NO3-N ratio are conducive to the improvement of jufeng grape quality. In the process of comprehensive control of karst rocky desertification, ecological industries were derived while ecological restoration was carried out, and the product quality was closely related to nitrogen dosage or practicability. Especially in the current stage of rock desertification control, assessing the value of nitrogen to ecological products and ES plays a positive role.
4.7. The Overall Problem of SN Cycling in Karst Rocky Desertification Is That There Is No Complete and Applicable Research Model to Strengthen SN Cycling in Rocky Desertification Areas
Nitorgen is a key limiting resource in many terrestrial ecosystems, and its cycling affects almost all aspects of ecosystem function [94]. Ecosystems have the functions of energy flow, material circulation and information transfer. The energy flow of ecosystems drives the circulation of various materials between biological communities and the inorganic environment. Chen Shuting [95] discussed the spatial-temporal evolution pattern and driving mechanism of vegetation NPP on the Tibetan Plateau by using a barycentric model and correlation analysis, which is conducive to revealing the correspondence of vegetation ecosystem to globalization. Common ecosystem functional material cycles include the nitrogen cycle. Nitrogen is a very stable gaseous monomer, and nitrogen fixation refers to the process of fixing nitrogen into other available compounds by natural or artificial means. After the nitrogen is fixed, the nitrogen produced by the reduction in denitrifying bacteria by the action of soil microorganisms is returned to the atmosphere to start a new cycle or is absorbed and used by the roots of plants. There is no complete study of soil nitrogen cycling integrity based on the atmosphere-plant-soil system in karst environments.
5. Conclusions
In this paper, 344 articles collected from the Scopus database were analyzed and systematically reviewed. The main conclusions are as follows: (1) The literature type of the study was mainly journal papers, with 97.40% of the document types categorized as an “Article”; (2) agriculture and biological science, environmental science and earth and planetary science are the three major fields of SND during rock desertification; (3) the spatio-temporal heterogeneity of soil nitrogen in karst rocky desertification increased, and the volume of published articles in the past three years accounted for 34.30% of the total, which indicates that soil nitrogen in karst rocky desertification is attracting more attention and discussion; (4) China, Germany, Norway and the United States are the four countries with the most research on this topic; (5) the keyword atlas of the literature shows that the words with the highest frequency are related to RSN, and the ultimate purpose of RSN research is to improve the service ability of rocky desertification control ecosystem and achieve sustainable development.
This literature review summarizes several key scientific issues, and these seven issues are worthy of further study in the future: (1) the mechanism of SN sources in rocky desertification; (2) the process and law of soil nitrogen migration and the risk assessment of soil nitrogen leaching after migration; (3) solving the problem of unclear SN form in karst rocky desertification ecosystem; (4) soil nitrogen supporting ES; (5) the study of SN management of rocky desertification; (6) the effects of nitrogen on the quality of ecological products; (7) SN cycling in karst rocky desertification. These questions require further study. This research assists the development of N management strategies to mitigate nitrogen contamination in rocky desert-vulnerable ecosystem soils and assists in providing sustainable development of soils that may be achieved with such management measures.
P.W., writing—original draft preparation, software and formal analysis; K.X., methodology, writing—review, and editing and funding acquisition; L.Z., conceptualization, methodology, validation, and supervision. All authors have read and agreed to the published version of the manuscript.
Not applicable.
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Date are contained within the article.
We appreciate the anonymous reviewers for their invaluable comments and suggestions on this manuscript.
The authors declare no conflict of interest.
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Enlarge this image.Figure 2. Proportion of literature types on nitrogen distribution in rocky desertification soils (a). Distribution of research directions (b). Number of literature in the period 1990–2022 (c). Distribution of research areas of the literature (d).
Enlarge this image.Figure 3. Density visualization map-item densities were correlated with published studies on the distribution of nitrogen in stony desert soils. The co-occurrence of the authors’ keywords was analyzed with the minimum number of occurrences of a keyword was set to 5. Out of the 1016 keywords, 62 keywords met the threshold value. For each of these 62 keywords, the number of co-occurring links was calculated. The keyword with the maximum number of links was selected.
Enlarge this image.Figure 4. Web visualization maps related to published studies on nitrogen distribution in rocky desert soils. The co-occurrence of authors’ keywords was analyzed, with the minimum number of occurrences of a keyword set to 7. Of the 1016 keywords, there were 10 keywords that met the threshold value. For each of these 10 keywords, the number of co-occurring links was calculated. The keyword with the maximum number of links was selected. Keywords with the most intra-cluster co-occurrence relationships were placed in the same cluster (in this case, there were 7 clusters).
Enlarge this image.Figure 5. Network visualization map in association with published research that utilized the AHP method. Analysis of co-occurrence of author keywords, with the minimum number of occurrences of a keyword set to 5. Of the 1055 keywords: 102 keywords meet the threshold. For each of the 102 keywords, the number of co-occurrence links was calculated. The keywords with the largest number of links are selected. Keywords which have the most intra cluster co-occurrence relations are arranged in the same cluster (in this case, there were 6 clusters).
Enlarge this image.Figure 6. Network visualization map in association with published research that utilized AHP method. Analysis of co-occurrence of author keywords, with the minimum number of occurrences of a keyword set to 5. Of the 1055 keywords: 44 keywords meet the threshold. For each of the 44 keywords, the number of co-occurrence links was calculated. The keywords with the largest number of links are selected. Keywords which have the most intra cluster co-occurrence relations are arranged in the same cluster (in this case, there were 6 clusters).
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