Heavy metal pollution levels and ecological risk assessment across various in soil of different land use types in a typical red soil regions

Guangze County is situated in the northern section of Wuyi Mountains, which is a typical red soil distribution area within the hilly and mountainous regions of the south of the Yangtze River. This region is endowed with an abundance of natural resources, including mountains, rivers, forests, fields, lakes and grasslands. Additionally, it serves as a major grain-producing county at the provincial level in Fujian Province. Agricultural activities have impacted the soil quality of agricultural lands to varying degrees. To investigate the current status of heavy metal pollution in soils across different land use types in Guangze County, this study collected a total of 75 soil samples from various locations within the county: dry land (SC), paddy fields (SD), fir forest (SM), secondary broad-leaved forest (KY), and bamboo forests (ZL). The concentrations of eight heavy metals, including cadmium (Cd), lead (Pb), zinc (Zn), copper (Cu), chromium (Cr), nickel (Ni), mercury (Hg), and arsenic (As), were analyzed. Pollution risks were assessed using several methods: single-factor index, Nemero comprehensive index, ground accumulation index, and potential ecological hazard index evaluation methods. The results indicated that the average concentrations of heavy metals Cd, Pb, Hg, and Zn across different land use types in Guangze County exceeded the background values of these elements in Fujian Province. Notably, high levels were observed for Cd and Pb, with 9.33% and 25.33% exceeding the screening values set by soil pollution risk control standards for agricultural land (2018), respectively. The single factor index (Pi) of Pb in the soil was higher than that of other heavy metals, and the ground accumulation index (Igeo) of Pb and Cd in the soil was relatively high, indicating an average pollution level ranging from none to moderate. The content of Cd in the study area’s soil exhibited a significantly positive correlation with soil pH (p<0.01), whereas the Pb content showed a significantly positive correlation with altitude (p<0.01). The Cd content in the soils of SC and SD was significantly higher than that of other non-agricultural land types at the same latitude (p<0.05), with 27.00% and 20.00% of sampling points exceeding the screening values set by soil pollution risk control standards for agricultural land (2018). The potential ecological risk (RI) of heavy metals in the soil was classified as moderate risk level, which was significantly higher than that observed in KY, SM, and ZL. No significant difference was found in Pb content between agricultural and non-agricultural soils (p>0.05), nor in the Pi or Igeo. However, there were different proportions of slight pollution-related potential ecological risks. Therefore, agricultural utilization is likely the primary source of Cd pollution in the soil of Guangze County, and targeted agricultural improvement measures, such as increasing soil pH and applying organic fertilizers, can be implemented for prevention and control. Meanwhile, the pollution sources of Pb in the soil may involve a combination of natural and agricultural origins.