The Spatial Distribution, Contamination Status and Contributing Factors of Heavy Metals in Cropland Soils of Twelve Cities in Shandong Province, China  
[PERSON]  
1 College of Geography and Environmental Science, Shandong Normal University, Jinan 250300, China; [EMAIL_ADDRESS] (W.X. ); [EMAIL_ADDRESS] (A.J.)110051@sdnu. characteristics of toxicity, abundance and persistence [4; 9]. Thus, heavy metal contamination in soils has become a serious problem in China, and even in the world [10]. Heavy metals can be bio-accumulated into vegetation and absorbed by the contactee, and then transferred into human bodies directly or indirectly [11]. Therefore, metal contamination in agricultural soils has threatened human health and should be given more attention [11]. In recent years, cropland soils in many areas of China have been suffering from the contamination of chromium (Cr), copper (Cu), lead (Pb), zinc (Zn), nickel (Ni) and cadmium (Cd) [5; 12; 13]. For example, Cd and Pb were excessively enriched in cropland soils near the Pearl River Delta, South China [12]. Previous reports showed that distribution and accumulation of heavy metals in agricultural soils of Shunyi, Beijing, Shanghai, Huizhou and many other cities significantly differed [12; 13; 14; 15; 16]. However, there are few studies concentrated on the spatial distribution patterns of heavy metals in the agricultural soils of different cities in the same province systematically, which can present the accumulating difference and similarity and contamination status of heavy metals in the areas, and can also provide data references for relevant research to explore the macro trends of metal accumulation among the provinces, China and even the world. Then, municipal management and control measures can be implemented based on the pollution status. Due to different climates, humidity, etc., the world contains many different soil types, which can hold different contents of heavy metals [17]. For example, yellow brown soil and cinnamon soil show different contents and absorption of Cu, Pb and Cd [18]. Thus, soil types may be an important factor affecting the spatial features of heavy metals, but are rarely studied up to now. Moreover, previous studies indicated that anthropogenic activities most resulted in the metal contamination in cropland soils [12; 13]. For example, industrial and agronomic practices, industrial future, coal burning exhausts and domestic waste reportedly led to various degrees of heavy metal accumulation to the local cropland soils in Northeast China [19], [PERSON] et al. (2019) suggested that human activities contribute 65.8%-86.0% to the metal accumulation in surrounding farmland soils [20]. It is unclear whether human activities can lead to large areas of agricultural soil pollution or not. Economic production dominates the effects of human activities on metal contamination in soils [12; 20; 21]; thus, the exploration on economic producing and metal contents may indicate their correlations. Shandong Province is situated in the eastern coast of China. Its total area is 155,800 square kilometers, and the total soil area is 121,100 square kilometers. It has a population of over 100.47 million and governs 16 cities (Figure 1). The topography of Shandong Province is complex and diverse; its eastern area belongs to the Shandong peninsula, the western and northern areas are parts of the North China Plain, and the southern area has various mountains [22]. The Yellow River, Haihe River, Huaihe River and many tributaries are the dominant rivers that flow through Shandong Province from the east to the west and finally discharge into coastal zones. In addition, brown soils, cinnamon soils and fluvo-aquic soils are three dominant soil types which account for 14.09%, 14.66% and 38.53% of the soil areas in Shandong Province, respectively [23; 24]. The cultivated rate and agricultural added value in Shandong Province ranked first in China, among which, corn and wheat are the main crops.