Efficacy of soil amendments and moss mulch on growth, heavy metal accumulation and secondary metabolites of marigold (Tagetes erecta L.) cultivated in organic soil contaminated with Cd and Zn

This study evaluated the functional performance of two marigold varieties ( Tagetes erecta L.), Thongchalerm (TCL) and Aga072 (AGA), grown in manufactured organic soil under Cd and Zn co-contaminated conditions. The soil was treated with individual or combined applications of Biovis (B), vermicompost (V) and moss (M). Moss was applied as a live plant mulch covering 90%, 45%, and 30% of the soil surface in pots. Biovis and V notably enhanced soil-available phosphorus, potassium and cation exchange capacity (CEC). The highest relative growth rate was observed in BM-treated soil for TCL and in BVM-treated soil for AGA. In TCL, B-treated soil showed the highest accumulation of carotenoids, phenolics, flavonoids and antioxidant activities (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) and 2,2-diphenyl-1-picrylhydrazyl), while the control (organic soil) also exhibited similarly high values, indicating that organic soil alone provides sufficient nutrients. For AGA, the highest levels of secondary metabolites were observed in BVM-treated soil. Both TCL and AGA varieties showed maximum ferric reducing antioxidant power in BM-treated soil and α-glucosidase activity in VM-treated soil, indicating that moss application positively affected the secondary metabolite composition in marigolds. Translocation factor (TF) and bioaccumulation factor (BCF) values exceeded 1 for Cd, demonstrating effective Cd accumulation. For Zn, most treatments showed TF > 1 but BCF < 1, suggesting efficient metal translocation with limited overall accumulation. Overall, the findings highlight the phytoremediation potential of both marigold varieties for Cd and Zn, demonstrating their ability to maintain plant growth and enhance secondary metabolite production under contaminated conditions. • Small-scale manufactured organic soil ensures balanced nutrients for marigold cultivation. • Soil amendments and moss improve marigold growth and yield. • Growth and natural biochemical composition of marigolds depend on soil-improving materials. • Organic soil amendments, applied individually or in combination, enhance marigold phytoextraction capacity. • Moss used as living mulch absorbs heavy metals and contributes to phytoremediation.