Developing a sustainable nature-based agricultural vertical system in cadmium polluted urban environments

Sustainable urban development faces challenges in efficiently using spaces, reducing pollutants, and greening cities. We examined how cadmium accumulation in soils from polluted irrigation water affects marigold (Calendula officinalis L.) safe and healthy products and how plant growth-promoting rhizobacteria (PGPRs) can remediate cadmium on marigold for multifunctional urban vertical system development. The study was a factorial arrangement with a randomized complete block design. The first factor was cadmium (0, 1, 2 and, 3 mg/kg soil through irrigation with Cadmium-polluted water), and the second factor was PGPR treatments (P.flu, P.putida, T.SP, Mix1 (P.flu + A.lipo), Mix2 (T.SP+ A.choro), Mix 3 (P.flu + A.lipo + T.SP+ A.choro)), and the control. In all the PGPR treatments, total chlorophyll, flowers' fresh and dry weight and antioxidant enzymes' activity significantly increased (p ≤ 0.05). There was no obvious risk to human health for oral and dermal use of marigold products in bacterial inoculated treatments (1 > Health Risk Index). Using flower of the plants treated with Thiobacillus thioparus strain 300, Mix1 (P.flu + A.lipo), Mix2 (T.SP+ A.choro), Mix3 (P.flu + A.lipo + T.SP + A.choro), if irrigated with up to 2 mg/kg soil Cd were safe and healthy ($\text{Estimated Daily Intake}<0.001$, Carcinogenetic Risk <1E-6, Target Hazard Quotient <1). Therefore, this research, for the first time, showed that using nature-based combined soil bacteria strains, we can create vertical green systems with double functionality: pollution remediation and healthy marigold flower production in cadmium-contaminated soils of up to 2 mg/kg.