Analysis of the Development Trend in Agriculture Non-point Source Pollution Based on CiteSpace

. This paper uses CiteSpace to visualize and analyze the literature related to agricultural non-point source pollution in China Knowledge Network (CNKI) from 1993~2022. The results showed that the research on agricultural non-point source pollution has gone through three stages: the beginning (1993~2006), booming development (2007~2011), and stable development (2011-2022). The research on agricultural non-point source pollution focuses mainly on agricultural non-point source pollution, pollution control, and model simulation. Agricultural non-point source pollution has always played a leading role in the development of the research field since the research until now, and model simulation and spatial distribution have been continuously cited from the emergence of clustering until now, and are widely available in the research field.


Introduction
The global situation of agricultural non-point source(ANPS) pollution is very serious, and ANPS pollution has become the main source of eutrophication in water bodies [1] . According to statistics, the global surface water affected by non-point source pollution occupies about 30%~50% [2] , about 12% of the 12×10 8 hm 2 of arable land with varying degrees of degradation is caused by ANPS pollution [3] , and in China, ANPS nitrogen and phosphorus pollution loads account for 81% and 93% of the total nitrogen and phosphorus pollution loads in water bodies, respectively [4] , and gradually becoming a new focus of research as well as attack by scholars from various countries.
Since the research on ANPS pollution began in the late 1990s, there have been numerous research results on it in China [5] . With the accumulation of results, it is of great theoretical and practical significance to study the development of ANPS pollution. Bibliometrics plays an important role in elucidating the current status and trends of scientific research and predicting possible future hotspots [6] .Therefore, this study used CiteSpace to visualize 385 ANPS pollution-related literatures from 1993~2022 in the China Knowledge Network (CNKI) database to analyze the current status of it in China as a whole and clarify the research hotspots and development trends in this field, in order to provide a reference basis for future agroecological research and agricultural environmental protection policy formulation.

Methods
CiteSpace is a visualization software for documentation statistics developed by Chaomei Chen in 2004 [7] , and first introduced to China in 2007, it is able to visualize the unquantifiable scientific literature knowledge through data analysis in the form of scientific knowledge mapping [8] .In this study, we used "ANPS pollution" as the keyword to conduct multivariate cluster analysis, combine with Origin statistics such as the number of articles, centrality, citation frequency and keywords to conduct ranking analysis. Finally present and describe the research status, research hotspots and development trend of Chinese ANPS pollution research literature by means of knowledge mapping, visualization analysis and statistical tables.

Data sources
The data for the study were obtained from the CNKI, The 627 retrieved documents were screened, and 385 academic journal articles on "agricultural nonpoint source pollution" were compiled to form the literature sample for this study. The data were saved as plain text files in "Refworks", which were used as data samples for the analysis of the paper.  Keyword is a high level summary of the research topic which represent the core ideas of the article [9] . Fig 2 shows the co-occurrence mapping of high-frequency keywords in the field of ANPS pollution. We can know that the keywords with high frequency are ANPS pollution, ANPS, pollution control and estimation model, etc. The largest key node is also the ANPS pollution, The current research on ANPS pollution in China includes both the accounting of pollution and the prevention and control of pollution, and gradually began to use model simulation to estimate the amount of pollution, so that the research and management of agricultural non-point source pollution is more time and effort-saving [10] . To further understand the importance of keywords and the relationship between them, Combining the research content and research status, this study selects keywords with frequency ≥ 11 for statistical analysis (Table 1), it can be seen that there are more keywords in the research topic, among which the frequency of ANPS pollution is the highest (173 times), and the highest centrality is also the ANPS pollution with a centrality of 1.02. Followed ANPS, pollution control, model simulation, spatial analysis and key source area also show close relationship with other keywords.  Timeline mapping can visually characterize the evolution of research topics. The thickness of node circles indicates the citation frequency of the literature, and the difference in grayscale of cluster word color characterizes the time difference of the first appearance of co-citation connection of the cluster. From the timeline plots (Fig 4), the duration of the ANPS pollution research themes varies, The cluster #0 has lasted for 20 years and has played a leading role in the development, with a significant number of citations, and is still an active cluster. Citation of cluster #4 has been published recently, the spatial distribution is a hot topic of research in this field in recent years. Model simulation and spatial distribution have been continuously cited since the emergence of clustering and have been cited a lot vertically, indicating that this cluster is highly adaptable, widely available in the research field.

Conclusions
This paper uses bibliometric methods to visualize and analyze the literature related to ANPS pollution, and explores the current status, research hotspots and development trends of ANPS pollution, with the following main findings: (1) The literature on ANPS pollution in China showed a yearly increase from 1993 and then gradually stabilized. 1993~2006 as the initial stage of ANPS pollution research, 2007~2011 as the booming stage of ANPS pollution research, and 2011~2022 as the stable development stage of ANPS pollution research.
(2) The current hotspots of ANPS pollution in China are mainly agricultural non-point source pollution, non-point source pollution, pollution control and model simulation. The research on ANPS pollution includes both pollution accounting and pollution prevention and control, and gradually begins to use model simulation for pollution volume estimation.
(3) The duration of ANPS pollution research topics in China varies. ANPS pollution emerging since the late 1990s, lasting up to 20 a and playing a commanding role in the development of the research field. Model simulation and spatial distribution have been continuously cited as future research directions since the emergence of clustering.
ANPS pollution as a theoretical research hotspot, the current research on agricultural non-point source pollution in China has a relatively mature theoretical basis, but is still in the development stage, mainly because China's research on agricultural non-point source pollution started late, the research method is still mainly reference and reference to foreign research results [11] .In order to quickly catch up with international development frontiers, China should innovatively research agricultural non-point source pollution models that are applicable to its own characteristics, study the integration of multi-disciplinary communication and focus on agricultural non-point source pollution management.
This study is based on the Chinese literature under CNKI database, and explores the development context and research status of ANPS pollution in China. In order to comprehensively explore the research status and development trend of ANPS pollution, different foreign language databases will be further added for analysis in the future.