1. Introduction
Agricultural land is the material basis and source of wealth for human survival and development, as well as an important site for the migration, transformation, and deposition of heavy metals. Over the past decades, China’s rapid urbanization and industrialization have generated significant emissions of heavy metal pollutants, and the accumulated heavy metals are transformed into organic complexes through organisms in the soil, which are persistent, non-degradable and bioaccumulative, posing a serious threat to the health of urban and rural habitats and the ecological and environmental security of agricultural land.(Dai et al., 2018; Zang et al., 2017) It poses a serious threat to the environmental health of urban and rural people and the ecological safety of agricultural land. Due to the rigidity of the demand for food and agricultural products caused by the huge population in China, agricultural production has to rely on intensive farming and high rates of fertilizer and pesticide application to ensure yield, thus causing a decline in the quality of agricultural land and a further increase of risk of heavy metal contamination in the soil.(Shang, Xu, Zhang, & Huang, 2018; G. Wang et al., 2017) About 10% of China’s arable soils are contaminated with heavy metals to varying degrees, according to research findings, and the exceedance rate of heavy metals in arable soils in the main grain-producing regions of China is 21.49%, which is highly concentrated in urban areas, especially in the peripheral urban agricultural areas where pollution is relatively low.(Chabukdhara & Nema, 2013; Shang et al., 2018; Q. Yang et al., 2018) The rate of exceedance was 21.49% and was highly concentrated in urban areas, especially in agricultural areas in the periphery of cities with relatively low pollution.
While arable soil in the urban periphery play an important role in providing a daily supply of agricultural products for residents and in the health of urban and rural ecosystems, the level of heavy metal contamination in arable soil is directly related to food safety and the well-being of agroecosystems.(Fujita et al., 2014; Lu & Yu, 2018) Therefore, the potential heavy metal contamination in peri-urban agricultural land has become an important challenge in the rapid urbanization process in China, and its changes in content, sources, and spatial and temporal distribution patterns should be of great concern.
At present, studies on heavy metals in arable soil mainly focus on content characteristics, spatial distribution, pollution level, source analysis, and health risks.(Dong et al., 2019; Ismail Umer, Younis Fatah, Ramadhan Abdo, Abdulaziz Karim, & Nabil Abdulrahman, 2021; Mapani et al., 2010) The spatial pattern of soil heavy metals is largely caused by present and past human activities, such as industrial agriculture, due to their significant cumulative nature.(Y. Wang, Duan, & Wang, 2020) . Therefore, the application of multi-period heavy metal content measurement, spatial and temporal variation analysis of pollution, and pollution source analysis can more accurately reveal sources and spatial and temporal distribution patterns of heavy metal pollution in soils, and can effectively curb the deteriorating trend of soil pollution(H. Y. Chen, Teng, Wang, Song, & Zuo, 2013) It can effectively curb the deterioration of soil pollution. The study of spatial and temporal variability of soil heavy metal pollution can be carried out by comparing spatial interpolation maps of different periods based on geostatistical methods through GIS characterization.(X. Li et al., 2012; Y. Yang, Wu, & Christakos, 2015) Heavy metal spatial variability can be studied by GIS characterization and geostatistical method comparing spatial interpolation maps in different periods. Soil heavy metal pollution source analysis methods include the Emission inventory method, Chemical massbalance (CMB) in receptor model, Hybrid approach, Positive Matrix Factorization (PMF), UN-MIX model, etc.(Lang, Li, Wang, & Peng, 2015; Y. Liu, Liu, Yousaf, Zhang, & Zhou, 2020; Wu, Li, Teng, Chen, & Wang, 2020) Among them, PMF was first used for source of atmospheric airborne particulate matter analysis in the 1990s, and in recent years it has also been widely used for soil analysis.(Tan et al., 2016) It is a new and effective method for source analysis of heavy metals, which is easy to operate and effective in identifying the sources and assigning their contribution to each heavy metal.(Chai et al., 2021) It is a simple and effective method to analyze source heavy metals.
Chengdu is a central city in western China and a typical representative of the fast-growing cities in China in recent years. At the same time, as an important food and agriculture production base in China, it also carries the important responsibility of food security in China. However, there are still ecological risks of soil heavy metals in this region. Numerous scholars have conducted studies to assess the pollution of cultivated soils in Chengdu.(Pang, Li, Zhang, Wang, & Yu, 2011; S, H, Q, Y, & H, 2013) .However, current studies on soil contamination in the region are mainly limited to specific administrative regions, specific periods, or specific soil use types, while there are relatively few systematic studies on the levels, sources, spatial and temporal patterns of hazardous metals on agricultural land in different urban circles in the context of rapid urbanization. The Chinese government has proposed a strategy to build the Chengdu-Chongqing City Cluster to guide the growth of west China and to boost the integration and interplay with the ‘Belt and Road’ and Yangtze River Economic Belt construction strategies. The research can give useful direction to the development of relevant policies. In addition, the study of heavy metal contamination in arable soils in Chengdu will not only improve the understanding of arable soils in the Yangtze River upstream area, but also provide a basis for arable soil conservation and management in the rapidly urbanizing areas of China.
The primary goals from the study were to (1) analyze the spatial and temporal patterns of soil heavy metal contamination in farmlands of typical rapidly developing cities in China; and (2) analyze causes and effects between soil pollution sources and soil pollution and environmental variables in agricultural fields of different circles of rapidly developing cities. We hypothesize that ”rapid urban development and human activities are the key drivers of increasing soil heavy metal pollution over time and space” in the study area. Management decisions require a process-based analysis of pollution sources and capture key factors in terms of their spatial and temporal dimensions.