Prediction model and threshold of soil cadmium contamination in cucumber greenhouses

The accumulation of heavy metals in soil is difficult to reverse, and this threatens China's agricultural safety. Nevertheless, the current implementation of heavy metal standards involves determining the risk screening value based on the pollutant items and pH value. However, there is a risk of contamination in edible agricultural products, which cannot meet quality and safety standards. In order to guide the selection of suitable crops for different soil types to avoid the risk of cadmium (Cd) pollution and to fully utilize land under the premise of safe production, thresholds for Cd pollution of different crops in different soil types need to be developed urgently. A Cd pollution micro-plot experiment with cucumber as the research object was conducted for two consecutive years to establish a prediction model and threshold of Cd pollution in greenhouse soils. The results showed that the total Cd and available Cd in greenhouse cucumber soil exhibited significant linear, logarithmic, power functions, and exponential relationships with the Cd content in cucumbers. The total Cd content showed the highest correlation coefficient according to the exponential model, while the available Cd content had the highest correlation coefficient according to the linear model. On the basis of soil conditions with pH≥7.5 and loamy texture, the risk thresholds for total Cd and available Cd in facility cucumber soils were proposed to be 2.13 mg·kg^-1 and 0.26 mg·kg^-1, respectively, based on extremely significant correlation models and national food hygiene standards. The proposed threshold was further verified by the soil urease activity; thereby, it was concluded that soil Cd contents less than or equal to 2.13 mg·kg^-1 had no significant effect on soil urease. Hence, the determined thresholds are feasible after soil urease verification. When the Cd content in the soil is greater than 0.8 mg·kg^-1 and less than or equal to 2.13 mg·kg^-1 (soil pH≥7.5), cucumbers can be recommended to replace other risk crops to meet the quality and safety requirements of edible agricultural products. The results of this study provide a scientific and theoretical basis for cucumber planting in the Cd-contaminated soil area of northern China, and thus hold practical significance. In this study, referring to the large body of existing research on the threshold value of vegetable soil Cd, the threshold value of soil Cd obtained by the soil sensitive soil index of urease activity and by constructing a cucumber soil Cd mathematical model are mutually confirmed, and a safe and reasonable threshold value of soil Cd under the experimental conditions is obtained.