Background and aims <p><i>Salvia rosmarinus</i> Spenn. (rosemary) was used in an off-site phytoremediation experiment conducted in contaminated soil from the Sierra Minera of La Unión-Cartagena mining area (Murcia, Spain). This study examined the potential reintegration in the soil of potentially toxic elements (PTEs) coming from plant litter degradation and their environmental impact, aiming to evaluate the efficiency of the remediation process.</p> Methods <p>The sustainability of the phytoremediation approach used was assessed through the analysis of the decomposition dynamics of phytoremediation-derived biomass in the soil, and its role in the biogeochemical cycling of carbon (C) and PTEs. A 201-day incubation experiment was conducted, and both plant litter and soil were analyzed before and after the incubation to evaluate the changes in the DTPA-extractable fraction of the PTEs.</p> Results <p>The results of C mineralization kinetics revealed a spectrum of microbial activity patterns, with CO₂-C emission dynamics ranging from zero–first order in the absence of litter, to first-order or two-step decay models in littered samples, depending on the biomass degradation stage. The decomposition process, which led to the mineralization of 21–50% of litter C, also influenced the cycling of PTEs, significantly decreasing the DTPA-extractable concentrations of copper (Cu, 15%), manganese (Mn, 50%) and lead (Pb, 11%).</p> Conclusions <p>These findings revealed the role of organic matter age and decomposition stage in regulating C mineralization, and support the potential of phytoremediation as a sustainable ecosystem restoration method, enhancing microbial activity, promoting C sequestration, and reducing PTEs bioavailability in the soil.</p> Graphical abstract <p></p>