Objectives This study aimed to investigate the effects of humate fertilizer application methods on the growth and development of various rapeseed varieties, as well as the absorption, translocation, and accumulation of cadmium (Cd) and chromium (Cr) under combined Cd and Cr pollution. The findings will provide valuable insights into the scientific application of agricultural humate fertilizers.

Methods Hualü 2 (rapeseed variety with low Cd and Cr accumulation) and Hanlü (rapeseed variety with high Cd and Cr accumulation) were selected for pot experiments. The fertilizer used was sodium humate. The contents of Cd and Cr in the treated soil were 1.25 and 227.57 mg/kg, respectively. Three application methods of sodium humate were set up with two (low and high) application levels: basal application (2 and 4 g/kg soil), applying with irrigation (0.5% and 1.5%), and foliar spraying (0.2% and 0.5%). In the basal application, the fertilizer was applied all at once before sowing. In the irrigation application and foliar spraying methods, the fertilizer was applied on the 23rd day after sowing and again after a week. Water was used instead of sodium humate fertilizer in the control. The dry matter biomass and Cd and Cr contents in the stems, leaves, and roots of rapeseed were measured after 40 days of growth. The translocation coefficients of Cd and Cr were calculated.

Results Compared with the control, the dry matter biomass of the stems, leaves, and roots of Hualü 2 treated with low and high dosage basal application and irrigation application was significantly increased. Moreover, the growth-promoting effects were higher on the roots than on the stems and leaves. Two foliar spraying treatments significantly inhibited the growth of Hualü 2. No significant effects on the growth of Hanlü variety were observed under all three methods. However, the inhibitory effects on the root system were greater than those on the leaves and stems. Both the low-dosage basal application and high-dosage irrigation application of sodium humate significantly reduced the Cd and Cr contents and accumulations in the stems and leaves of Hualü 2. The Cd and Cr contents in the stems and leaves of the low-dosage basal application treatment were 0.19 and 0.18 mg/kg, respectively, while those in the high-dosage irrigation application treatment were both 0.17 mg/kg, which were significantly lower than the control values of 0.25 and 0.22 mg/kg. At the same time, the Cd and Cr translocation coefficients of these treatments were significantly lower than the control. The high-dosage basal application, low-dosage irrigation application and low-dosage foliar spraying of sodium humate all significantly reduced the Cd and Cr contents and accumulations in the stems and leaves of Hanlü. The Cd and Cr contents in the stems and leaves of the high-dosage basal application treatment were 0.52 and 0.30 mg/kg, respectively, while those in the low-dosage irrigation application were 0.57 and 0.31 mg/kg, respectively, and those in the low-dosage foliar spraying treatment were 0.50 and 0.20 mg/kg, respectively, which were all significantly lower than the control values of 0.62 and 0.49 mg/kg. At the same time, the Cr translocation coefficients of these treatments were significantly lower than that of the control, while the Cd translocation coefficient showed no significant change. The inhibitory effect of sodium humate on Cd absorption was higher in Hualü 2, whereas that on Cr absorption was higher in Hanlü.

Conclusions Suitable amount of soil basal application and applying with irrigation of sodium humate could effectively promote the growth of rapeseed, inhibit the translocation of Cd and Cr from roots to shoot, and thus produce vegetables with Cd and Cr contents meeting national food safety standards in the Cd and Cr complex contaminant soil. The research suggests that sodium humate fertilization prevents rapeseed plants from Cd and Cr contamination mainly through the dilution effect of larger biomass, and the inhibition of translocation of Cd and Cr.