Land degradation in Central Asia: a review of papers from the Scopus database published in English for the period of 2000-2020

. Land degradation has a big influence for Central Asia. The territory of Central Asia is rarely discovered and it can be seen by the number of published papers in English. There is a high potential of applying remote sensing (RS) and geographical information systems (GIS) for the mapping and monitoring of degraded lands. In the present research we tried to collect all Scopus-based publications in English for 20 years using land degradation as a keyword. We analyzed the most popular journals, top authors, top-cited papers, top countries, and the role of remote sensing (RS) and geographic information systems (GIS) in the land degradation issues of Central Asia. RS and GIS technologies are good tools, but in Central Asia researchers have to spread them widely. Data-sharing systems and lack of international cooperation could bring more joint projects and publications on land degradation issues.


Introduction
Land is a resource providing first of all a food for human beings [1].Humanity should treat land resources as non-renewable sources.Due to the ecological, environmental, and social aspects, we are facing the worldwide expansion of land degradation issue [2].Land degradation became one of the most important challenges of the world and brought a lot of economic losses to countries.Land degradation refers to the reduction of soil efficiency due to the different factors [3,4].United Nations Convention to Combat Desertification (UNCCD) and Sustainable Development Goals (SDG) have the initiative to reduce land degradation processed worldwide [5].Land degradation has a negative impact on the ecosystem and food security, therefore this topic was crucial in 20th century and became more crucial in the 21st century [6].The UNCCD and SDG work closely to solve problems related to land degradation neutrality and land monitoring.UNCCD reports that nearly 250 million people felt the direct influence of land degradation.Globally the territory of the degraded lands in arid regions equals to 54 million km2.Around 34% of this territory located in Asia, 24% in Africa, 24% in America, 15% in Australia and 3% in Europe [7].Tilahun et al., [8] did a report to achieve SDG 15.3, and they stated that the investment in sustainable land management could bring a positive effect to achieve other SDGs for Asian countries.Delang [9] did a review on the direct and indirect consequences of soil degradation in China.Baumhardt et al., [10] focused on North American Soil Degradation issues.Main focus of their research was sustainable land management strategies for the soil conservation.
Land degradation is one of the most harmful natural hazards for Central Asian countries [2,11].Negative effects of land degradation we can observe on crop and livestock production.There are several major types of land degradation available in Central Asia and they are soil salinity, soil erosion, desertification.The major triggering factors of land degradation in Central Asia are unstable agriculture and farming, increasing the territory of crops, overgrazing of rangelands, and poor irrigation systems [12].Lal et al., [13] in their chapter discuss the effect of land use practices to land degradation in Central Asia.Robinson [14] reviews land degradation issue in Central Asian countries.The chapter describes degradation issues of the region and how scientific and public policies have been changed after the independence of countries.Mirzabaev et al., [12] in their research refer to the economic consequences and triggering factors of land degradation in Central Asia.
There are several statistical reviews or bibliometric research was done in the field of land degradation.Almost all of them focused to a global scale and taking into account just Web of Science database [15][16][17].It is known that most of the Central Asian researchers publish their papers in Russian and there are not so many English-based papers on land degradation.The main objective of our study is to reveal the most popular journals, top authors, top-cited papers, top countries, and the role of remote sensing (RS) and geographic information systems (GIS) in the land degradation issues of Central Asia.

Methodology
Based on the objective of the research, three subject areas selected from the Scopus database for the period of 2000-2020 for the Central Asia region.Name of subject areas: 1. Agricultural and Biological Sciences -4205 papers 2. Earth and Planetary Sciences -4998 papers 3. Environmental Science -4020 papers.All publications were analyzed and reviewed using land degradation as the keyword.Total of 70 Central Asia based publications sorted out for the further analysis on land degradation issue.Then, a database was categorized including the year of publication, journal names, authors' names, countries, the type of publication, the number of citations per paper, the number of citations per journal, the percentage of publications by the topic cluster name.At the end we have analyzed the role of RS and GIS technologies on land degradation issue of Central Asia. Figure 1 shows the flow of the selected methodology for the research.

Journals on land degradation
Selection of the appropriate journal is essential during the publication process.Scopus based 70 papers on land degradation issue published in 56 different journals.About 34% from the total number of papers published in first 10 journals given in Table 1

Top authors on land degradation
Auuthors play an important role in a development of the particular field of research in any region.During the last two decades total of 295 different authors worked to publish 70 papers for the land degradation issue in Central Asia.Nearly 6% of the papers published by the single authors, while the rest part published by two, three or group of authors.Four authors Kurochkina, L.Y., Suleimenov, M., Khamraev, S.R., Rahimon, R.M. published their papers as a single author.In the review we took the first 11 top authors who published at least 3 and more papers which is shown in Figure 3. From the list we can see that almost half of the top authors are not from Central Asian countries, but they have jointly published papers in a high ranked journal.Authors Lamers, J.P.A., Khamzina, A., Toderich, K. are leading the list of top authors with a number of papers 6, 5, 5 respectively.

Top institutions on land degradation
Ranking of institutions mostly based on the quality of the papers published by the researchers of that institutions.Eighty-nine different institutions worked in cooperation to publish 70 papers on land degradation in Central Asia for the period of 2000-2020.Tashkent Institute of Irrigation and Agricultural Mechanization Engineers, University of Bonn, Kazakh National Agrarian University leading the list of top institutions in Figure 5.Each of these institutions published 7 papers in the cooperation with other institutions.In Figure 4 we showed institutions where 3 and more papers were published.Four institutions included to the top list from Uzbekistan and 2 from Kazakhstan.Fig. 4. List of top institutions on land degradation issue in Central Asia

Publication type on land degradation
There are different publication types where researchers can show their results.Papers published on land degradation issue in Central Asia for the given period presented in 5 different publication types shown in Figure 5. Fifty research papers published on land degradation issue.Next comes chapter type of publication with 9 papers from the total publications.All 9 chapters published during the second decade of analysis.Six review papers published during this period.All review papers also published during the second decade.

Top cited papers on land degradation
Number of citations shows the quality and novelty of the conducted research.Fifteen mostly cited papers on land degradation shown in the Figure 7.Total of 961 citations given to 70 publications on land degradation for the Central Asia for the given period.Six of the top cited papers published during the first decade and the rest 9 during the second.Almost 71% of citations given to 15 papers from the Figure 6.First 15 top cited papers are consisting of one chapter, 2 reviews and 12 research articles.

Publications by the topic cluster name on land degradation
There are different topic cluster names available for the subject areas given in Scopus database.Most of the papers published on land degradation issue in Central Asia belong to 4 different topic cluster names in Figure 8. Wetlands, Desertification, Plateau topic cluster name covers 37% of total publications, while Water, Water Resources, Water Management 35%, Remote Sensing, Image classification, Satellite Imagery 15% and Soil, Biochar, Soil Organic Carbon cluster covers 13% of total publications.and GIS technologies are so popular now in all disciplines and still Central Asian countries do not widely use them.Main reason for that could be improper data-sharing system and lack of international cooperation and consequently lack of joint projects.Proper management of all these issues could bring more international joint projects and new publications not only in land degradation but in other fields as well.

Fig. 2 .
Fig. 2. Number of papers on land degradation by the year of publication in Central Asia

Fig. 3 .
Fig. 3. List of top authors published on land degradation issue in Central Asia

Fig. 6 .Fig. 7 .
Fig. 6.List of top cited publications on land degradation in Central Asia

Table 1 .
. Key journals showing land degradation issues in Central Asia are Journal of Arid Environments, Land Degradation and Development, Arid Ecosystems, Ecological Indicators, Arid Ecosystems, Ecology, Environment and Conservation, Environmental Research Letters.List of the journals on land degradation in Central Asia

Role of RS and GIS technologies on land degradation RS
[3,[18][19][20][21][22][23] used in almost all fields of science since the last 2-3 decades[3,[18][19][20][21][22][23].The analysis of the land degradation publications for the Central Asia during the last decades shows us that almost 25% of publications utilized remote sensing and GIS technologies.Figure9states that just 2 papers published during the first decade on land degradation issue and the rest 15 papers during the second decade.All RS and GIS based papers published in 16 different journals with the high reputation.Almost 90 authors from different countries took a part to publish those papers.Nearly 31% of all citations are from papers published using RS and GIS and 30% of them are listed as a top cited paper on land degradation.Land degradation is a crucial topic for the Central Asia region.Based on the Scopus database we tried to analyze papers published on land degradation issue in English for the Central Asia during 2 decades.We analyzed number of papers published, number of top journals on land degradation issue.Lamers, J.P.A., Khamzina, A., Toderich, K. are top authors for the Central Asia.Total of 29 countries worked in a cooperation to publish all papers and Uzbekistan, Kazakhstan, Germany and Russian Federation are top countries on land degradation issue.Tashkent Institute of Irrigation and Agricultural Mechanization Engineers, University of Bonn, Kazakh National Agrarian University leading the list of top institutions.Almost 71% of total publications on land degradation were published as a research article.Land degradation publications got total of 961 citations and we listed top 15 publications and journals.Journal of Arid Environments, Environmental Research Letters and Land Degradation and Development journals are top journals in the field of land degradation for Central Asia.1.Wetlands, Desertification, Plateau, 2. Water, Water Resources, Water Management, 3. Remote Sensing, Image classification, Satellite Imagery and 4. Soil, Biochar, Soil Organic are mostly used topic cluster names for the land degradation papers.At the last we have analyzed the importance of RS and GIS technologies on land degradation issue in Central Asia.Almost 25% from the total publication applied these technologies.RS Fig. 9. Remote sensing and GIS based papers on land degradation in Central Asia 4 Conclusion