Analysis of Dominant Control Factors of Cadmium Phytoavailability in Greenhouse Soils with a High Cadmium Geological Background

Using a greenhouse vegetable base in Zhijin County, Guizhou Province as an example, simple correlation, principal component and path analysis were applied to study the characteristics of Cd accumulation in vegetables of a greenhouse vegetable base in a high Cd geological background area. The main sources of soil Cd, its phytoavailability and its correlation with soil pH, soil organic matter (SOM), alkaline nitrogen (NA) and other basic soil physicochemical properties were also studied, and the main controlling factors of soil Cd bioeffectiveness were analysed. The results show that the Cd content in the topsoil of 75.32% of the sites of the base was greater than the background value of soil Cd in the topsoil of Guizhou Province (0.659 mg·kg^-1), and the bioaccumulation of Cd in greenhouse vegetables were in the order of cabbage > tomato > cowpea, with none of the three vegetables exceeding National Food Safety Standard (GB 2762-2017). Correlation coefficient (R=0.785) between Cd contents in vegetable fruits (Cd_p) and the content of available Cd in soils (Cd_A) was higher than that (R=0.508) between Cd_p and total Cd (Cd_T), implying that Cd_A is better than soil Cd_T for assessing Cd contamination in vegetable fruits and for predicting the safety risk of vegetable production. The content of Cd between topsoil and subsoil using independent-samples t test showed that the main source of Cd in greenhouse soils was weathering of the parent material in high Cd geological background area, and its high Cd content and high nitrogen application are direct and indirect factors for the increasing of the biological effectiveness of soil Cd. Cd_A was correlated significantly with Cd_T, pH and available nitrogen (P < 0.05), but not with soil organic matter (SOM), available phosphorus and available potassium. Therefore, cowpea can be considered for the development of facility vegetable cultivation in areas with high Cd geological background, and the amount of N fertilizer application should be strictly controlled to ensure the safety of the "vegetable basket".