Migration and risk assessment of heavy metals from swine manure in an organic fertilizer - soil - ryegrass - rex rabbit system: Based on field trials

Organic fertilizers were produced through maggot-composting (MC) and natural composting (NC) using swine manure, and the migration, contamination, and health risks of heavy metals (Zn, Cu, Cd, Cr, Pb) were evaluated within a fertilizer - soil - ryegrass - Rex rabbit system. After 70 days of treatment, heavy metals were concentrated by 43.23 % to 100 % in MC and 52.82 % to 90.91 % in NC, higher concentration rate was observed in Cr and Cu in NC. The residual fractions of Zn, Cu, CD, Cr, Pb increased by 0.4 % to 30.2 % in MC, and 3.4 % to 29.7 % in NC. More stable forms of Zn, Cr, and Cu were observed in NC, whereas Pb showed greater stability in MC. Post-fertilization, statistically higher levels of the five elements were obtained in NC treatment. Even their concentrations decreased in the soil, however, high initial background levels caused Cr and Zn to exceed standard thresholds in both treatments. Despite this, the geo-accumulation index (I_geo) and pollution load index (PLI) below 1, did not indicate a pollution risk. Cr levels exceeded feed limits in both roots and leaves, although the low BCF value in roots suggested limited risk. In rabbits fed with 48 % ryegrass, significantly higher levels of Zn and Cu were detected in NC, with elevated Cr levels posed moderate to severe pollution risks in two treatments. Health risk assessments, including non-carcinogenic (HQ_f) and carcinogenic risks (CR_f), revealed Cr was the most cautionary element for both children and adults, even no significant carcinogenic risk exist. These findings provide key insights into mitigating heavy metal contamination from animal waste to rabbit meat.