Effects of coated manganese and zinc fertilizers on cadmium accumulation in spinach and soil cadmium bioavailability

Cadmium (Cd) contamination has severely threatened the safe production of vegetables in plots. This study aims to clarify the effects of coated manganese and zinc fertilizers on cadmium accumulation in spinach and soil cadmium bioavailability. Polyurethane-coated manganese sulfate (MnSO₄) and zinc sulfate (ZnSO₄) fertilizers were prepared for the safe utilization of Cd-polluted vegetable plots. A pot experiment was conducted on alkaline Cd-contaminated soil. The coated and granular Mn and Zn fertilizers were applied at two dosages (Mn and Zn: 45 and 90 mg/kg). A systematic investigation was made to explore the impacts of Mn and Zn fertilizers on uptake and translocation of Cd, Mn, and Zn in spinach and Cd speciation in soils. An analysis was also made to examine the dynamic variations in pH values and the bioavailability of Cd, Mn, and Zn in soils from different growth stages of spinach. The results showed that the application of Mn and Zn fertilizers decreased the Cd content in the shoots of spinach (P<0.05), with the maximum reductions of 23.09% and 33.47%, respectively. No significant differences were observed between the treatments of coated and granular fertilizers. The treatments of Mn/Zn fertilizers increased the Cd content in the roots of spinach and significantly inhibited the translocation of Cd from the roots to the shoots. The Mn content in the shoots and roots of spinach treated with coated Mn fertilizer as well as the Zn content in the roots of spinach treated with coated Zn fertilizer were significantly higher than those under the granular fertilizers (P<0.05). Low-dose granular Mn and Zn fertilizers significantly reduced soil pH values during the early or mid-growth stages of spinach (P<0.05), whereas the low-dose coated Mn and Zn fertilizers significantly decreased soil pH values during the mid- or late-growth stages (P<0.05). At the early growth stage of spinach, the low-dose Mn and Zn fertilizers significantly increased soil available Cd content (P<0.05), with stronger effects in granular fertilizers, compared with coated fertilizers. In contrast, the coated Mn and Zn fertilizers facilitated the transformation of soil Cd from exchangeable or carbonate-bound forms to iron-manganese oxide-bound or residual forms during the late growth stage, thereby significantly decreasing the content of available Cd (P<0.05), with the maximum reductions of 11.78% and 11.18% for Mn and Zn fertilizers, respectively. At the early growth stage of spinach, the content of soil available Mn/Zn under the treatments of granular Mn/Zn fertilizers was higher than that under the coated fertilizers, whereas the opposite trends were observed at the middle or the late growth stages. The correlation analysis indicated significant negative relationships between the total Cd accumulation amount in spinach and the content of available Mn and Zn in soils (P<0.05). The Cd translocation factor in spinach was significantly negatively correlated with the content of Mn and Zn in the shoots and roots of spinach (P<0.05). Overall, the coated Mn or Zn fertilizers with a dose of 45 mg/kg were the most effective among treatments. The content of Mn and Zn in spinach was enhanced to reduce Cd availability in soils. The finding can provide a promising potential for the safe utilization of Cd-contaminated vegetable plots.