Nitrogenous fertilizers affect Cd accumulation in the soil-mulberry-silkworm system: implications for safe utilization of contaminated farmland

Developing the sericulture industry is an important way to safely utilize cadmium (Cd) heavily contaminated farmland, and nitrogenous fertilizer (N fertilizer) is crucial for the quality of mulberry leaves and the yield of silkworm cocoons. This study evaluated the influences of N fertilizers (NH₄Cl, NH₄HCO₃, CO(NH₂)₂, (NH₄)₂SO₄) on the accumulation of Cd in the soil-mulberry-silkworm system, the physiological responses of Bombyx mori L. (silkworm), and the yield and quality of cocoons. The results showed that the four N fertilizers first decreased the ingestion rate (IR) and digestion rate (DR) and then increased them slowly. The maximum concentration of Cd in silkworm excrement (SE) was 0.77 mg/kg And the concentration of Cd in SE is 4.1 ~ 6.5 times as high as in the silkworm body. All the bioconcentration factors (BCFs) of mulberry organs for Cd were lower than 1, but the translocation factors (TFs) from larva to SE were above 1. The NH₄Cl and (NH₄)₂SO₄ increased the BCFs of Cd in root, but CO(NH₂)₂ and NH₄HCO₃ decreased them. CO(NH₂)₂ and NH₄HCO₃ increased the survival rates of larva pupa, but NH₄Cl and (NH₄)₂SO₄ significantly reduced them. The four N fertilizers all increased the ratios of cocoon shells, and NH₄HCO₃ was the highest, followed by CO(NH₂)₂. The whole cocoon weight of CO(NH₂)₂ and NH₄HCO₃ treatments were 1.34 And 1.42 g, respectively, significantly greater than that of the control treatment. NH₄HCO₃ decreased the percentage content of Cd accumulation in silkworms and enhanced it in SE. These results indicated that planting mulberry and raising silkworm with reasonable use of NH₄HCO₃ and CO(NH₂)₂ contributes to the safe utilization of acidic paddy fields heavily polluted by Cd.