Ecological risks of heavy metals in soil under different cultivation systems in Northwest China

The increasing population worldwide and high demand for food have changed the conventional open-field cultivation system (OFS) to plastic shed cultivation system (PSS), resulting in serious concern about soil contamination by heavy metals due to the intensive planting in PSS. However, there is limited research on heavy metals ecological risk after converting the cultivation system from OFS to PSS in Northwest China. Therefore, a total number of 341 soil samples were collected in Shaanxi province to measure heavy metals contents (As, Cd, Cr, Cu, Hg, Pb, and Zn) and assess the ecological risk. In this study, the PSS cultivation system exhibited more severe contamination than the OFS; whereas the heavy metals pollution in different regions was in the following order: southern Shaanxi (SS) > central Shaanxi (CS) > northern Shaanxi (NS). Although the mean values of the selected heavy metals were below the national limited standard, Cd and Pb posed considerable accumulation in both cultivation systems and the growing cultivation years increased the Cd, Hg and Zn contents in soils of PSS. The results of correlation analysis revealed a significant correlation between total phosphorus (TP) and Cd in PSS cultivation systems. However, soil organic matter (SOM), total nitrogen (TN), total phosphorus (TP), and total potassium (TK) had different interactions with other heavy metals in each region under both cultivation systems due to the variations in crop management techniques such as planting intensity and fertilizer input. Partial least square path model (PLS-PM) analysis showed that the different cultivation regions had a greater impact than the type of cultivation system, which should be carefully taken into account in mitigating pollution risk. Additionally, research on heavy metals species and fractions in connection to their bioavailability under various agricultural production systems is advised for improved food safety.