Crab Shell Biochar and Compost Synergistically Mitigate Heavy Metal Toxicity in Soil–Plant System

Addressing the threat of heavy metal contamination in agriculture, this study evaluated the efficacy of crab shell biochar (CB) and compost (CO) in immobilizing copper (Cu), zinc (Zn), and lead (Pb). The objective was to determine the impact of solitary and combined applications of CB and CO on soil physicochemical properties, nutrient availability, HMs bioavailability, subsequent growth, and oxidative stress responses in spinach plants. The experiment involved two soil types (clay loam and sandy clay loam) with differing initial properties, which were simultaneously spiked with 300 mg kg−1 Cu, 500 mg kg−1 Zn, and 400 mg kg−1 Pb, aged for 30 days, and then treated with varying doses of CB and CO (e.g., 1% and 1.5% w/w). Key results demonstrated that the combined application of 1.5% CB + 1.5% CO was most effective, significantly (p < 0.05) increasing soil pH and reducing DTPA-extractable Cu (by 53–64%), Zn (42–50%), and Pb (57–59%) in both soil types. This treatment also led to a pronounced decrease in the bioaccumulation factor (BF) of HMs in spinach, coupled with improved plant growth parameters (height, fresh/dry weight, chlorophyll content) and reduced oxidative stress (as indicated by lower levels of MDA and antioxidant enzymes). We conclude that the synergistic interaction between CB and CO creates a multi-mechanistic immobilization system, offering a highly effective strategy for the remediation of heavy metal-contaminated soils and the safe cultivation of crops.