Soil pH, CEC, and concentration of available Cd
Under similar N dosage, neither CMV application nor the different base-tillering-panicle N fertilizer treatment ratios had an effect on the soil pH, CEC, or CaCl2-Cd concentration (Table 2).
Grain dry weight
Rice grain yield was significantly influenced by various N treatments (Fig. 1). Under the same N dosage, the application of CMV increased rice grain yield. However, compared with the control, the grain yield was not significantly affected by different ratios of base-tillering-panicle N fertilizer treatment (Fig. 1). The grain yield treated with a combination of urea and a high amount of CMV was 29.4% higher than that of the control (p < 0.05) (Fig. 1).
Cd concentrations in rice tissues
The order of Cd concentrations in various parts of the rice plants were as follows: root > straw > brown rice > husk. The Cd concentrations in the rice tissues were significantly affected by the application of base-tillering-panicle urea, and the combined application of urea and CMV (Fig. 2). The Cd concentrations in the roots of N1, N2, N3, and N4 treatments compared with that in control were 36.7%, 43.0%, 22.8%, and 33.2% lower (p < 0.05), respectively. Meanwhile, in N3 and N4 treatments, the Cd concentrations in the straw significantly increased by 60.3% and 53.5% (p < 0.05), respectively (Fig. 2). The Cd concentrations in the husk of N2 and N4 treatments compared with that in control were 51.5% and 40.6% lower (p < 0.05), respectively. In addition, the Cd concentrations in the brown rice in N2 and N4 treatments significantly decreased by 40.7% and 34.1% (p < 0.05), respectively. Thus, the application ratio of base-tillering-panicle inorganic N at 3:4:3 and a basal application of 37500 kg hm-2 CMV could significantly reduce the Cd concentrations in brown rice. The Cd concentration in brown rice in this study ranged from 0.73 mg kg-1 to 1.23 mg kg-1, which was 3.65 to 6.15 times higher than the national limit of Cd in rice grains (0.2 mg kg-1, GB2762-2017).
N concentrations in rice tissues
The application ratio of base-tillering-panicle urea, and the combined application of urea and CMV significantly affected the N concentrations in rice tissues (Fig. 3). The N concentrations in the root, straw, husk, and brown rice were the highest under N2 treatment conditions. Compared with the control, the N concentration in brown rice in N2 treatment was 25.9% higher (p < 0.05). The N concentrations in the straw and brown rice in N3 treatment were 42.2% and 27.7% (p < 0.05) lower than those in N2 treatment, respectively. The N concentrations in the root, straw, and brown rice in the N4 treated were 26.1%, 36.1%, and 26.8% (p < 0.05) lower than those of the N2 treated, respectively.
Concentrations of Zn, Cu, Ca, and Mg in brown rice
The combined application of urea and CMV significantly affected the Zn and Cu concentrations in brown rice, whereas Ca and Mg concentrations displayed no significant effect (Table 3). Compared with the control, the concentrations of Zn and Cu in brown rice decreased with increasing basal CMV application rate. The Zn and Cu concentrations in the brown rice in the N4 treatment was 17.2% and 24.7%, respectively, which was lower than that of the control.
Cd transport coefficients
Cd transport coefficients from root to straw (TFstraw/root) and from straw to brown rice (TFbrown rice/straw) were significantly affected by the application ratio of base-tillering-panicle urea and the combined application of urea and CMV (Fig. 4). Compared with the control, the TFstraw/root of N1, N2, N3, and N4 treatments increased by 94.7%, 91.5%, 107.8%, and 131.9%, respectively, while the TFbrown rice/straw values of N1, N2, N3, and N4 treatments decreased by 42.2%, 46.3%, 50.4%, and 57.5%, respectively.
Correlation between Cd in brown rice and Cd in root and straw, Soil CaCl2-Cd, and TFs of Cd in rice plants
The Cd concentrations in the brown rice were significantly positively correlated with the soil CaCl2-Cd and root Cd concentrations (p < 0.05) (Table 4). The correlation between Cd concentration in brown rice and that in TFS (brown rice/straw) was significantly positive (p < 0.05) (Table 4), while that between in brown rice and TFs (brown rice/root) was significantly negative (p < 0.05) (Table 4).