## 3 Results and Discussion
### The Distribution Pattern and Source Analysis of Heavy Metals in Different Soil Types  
Based on the second national soil census in China, brown soils, cinnamon soils and fluvo-aquic soils are the main soil types in Shandong Province [8]. The sampling area in this study involved 26 sites of cinnamon soils, 14 sites of fluvo-aquic soils and 15 sites of brown soils. Previous studies showed that heavy metals had different accumulating features in different soil types, as the soils had different values of organic matter, pH, structural parameter, soil fertility, etc. [5; 38; 39]. In this study, concentrations of Cr in brown soils and cinnamon soils were 152.06 mg\(\cdot\)kg\({}^{-1}\) and 149.38 mg\(\cdot\)kg\({}^{-1}\), respectively, which were significantly higher than Cr in fluvo-aquic soils (72.86 mg\(\cdot\)kg\({}^{-1}\); Figure 5 and Table 2). Moreover, the distribution pattern of Cr was the opposite of the pH in the soil types, which was significantly higher in fluvo-aquic soils (8.16) than in brown and cinnamon soils (6.59 & 6.98). Cr is easily migratory and sensitive to pH; thus, the significantly contradictory pattern suggested that the low pH was prone to the migration and bio-accumulation of Cr in the cropland [40; 41]. In addition, [PERSON] et al. [39] also suggested that yearly cultivation acidized the farmland, which was more vulnerable to metal accumulation. Furthermore, the higher contents of Cr in brown and cinnamon soils may also be attributed to the different soil fertilities--a soil's ability to supply and coordinate nutrients, moisture, air and heat for biological growth [42; 43].  
Figure 5: The distribution pattern of heavy metals in the three soil types. Bars sharing the same letter are not significant at \(\alpha=0.05\) (Duncan test).  
The concentrations of Cu, Ni, Cd and Pb in the soil types were ordered as: cinnamon soils > brown soils > fluvo-aquic soils. The differences were not significant, indicating that the three soil types had no prominent and differentiated impact on the accumulation of these metals (Figure 5). The Pearson correlation analysis indicated that metals of Cu, Ni and Zn were significantly related, and Pb and Cd were also significantly associated (\(p<0.01\); Table 3). [PERSON] et al. [PERSON] et al. (2019) showed that discharge of Cu, Zn and Cd were closely correlated with the industrial distribution. Cr, Ni and Pb were from parent sources in Daye City, China, and some reports indicated that Cd and Pb were mostly enriched from industrial effluent, such as coal burning exhausts in Northeast China (19,37). Thus, the spatial distribution of and contamination by heavy metals greatly differed among cropland areas of China, as did the metal correlations; this was confirmed in previous reports (12,45). Crustal materials and anthropic pollution were the main sources. The metal contamination in farmland in China is mainly affected by local background values of heavy metals and human activities (45). In this study, the contamination of Cu, Ni and Zn, and the accumulation of Pb and Cd in the cropland soils in Shandong Province were from different human activities and industrial producing. The contents of Cr were high and the correlation of Cr and other metals was not significant; thus it is suggested that sources of Cr may be various and complex. Thus, regulation and management on the discharge of industrial waste and on the agricultural practices need to be improved.