Mitigating cadmium and nickel uptake in lettuce through sulfur-enriched vermicompost: Impacts on soil fertility and sulfur compounds

• Organic amendments reduced Cd and Ni bioavailability in the soil. • Organic amendments improved lettuce growth, and decreased Cd and Ni accumulation. • Sulfur from vermicompost promoted Cd and Ni retention in cell walls and vacuoles. • GSH and PCs played key roles in Cd and Ni tolerance and detoxification of lettuce. Soil amendments can reduce the bioavailability of heavy metals (HMs) in the soil and consequently decrease the uptake of HMs by plants. Sulfur is an essential macronutrient for plant growth and development and plays a crucial role in plant tolerance to HMs. Earthworms were fed cabbage or chicken manure to produce two vermicomposts enriched in sulfur and nutrients, respectively. An incubation and pot experiment were conducted to assess the effects of different organic amendments, including vermicomposts and chicken-manure-processed organic fertilizers, on soil fertility, lettuce growth, cadmium (Cd) and nickel (Ni) uptake, and accumulation as well as sulfur compounds. Experimental results showed that applications of organic amendments not only raised soil pH and electrical conductivity but also increased the concentration of available phosphorus, available sulfur, and exchangeable potassium, calcium, and magnesium in the soil. In terms of the bioavailability of HMs in soil, the application of organic amendments decreased the bioavailability of Cd and Ni in soil. Moreover, organic amendments addition led to promote lettuce growth and decreased the accumulation of Cd and Ni. Furthermore, applications of organic amendments decreased Cd and Ni distribution in the cytoplasm and organelles, but increased Cd retention in the cell wall and vacuoles, especially with vermicomposted cabbage and vermicomposted cabbage combined with chicken-manure-processed organic fertilizers, which contribute to the increase of sulfur and thiol compounds in lettuce.