Thermal regime-dependent behaviour of soil toxic elements and microbiota under mineral mix-amended phytostabilisation

This is the first study to provide insights into the effects of a halloysite-diatomite-based (HD) mineral amendment on Lolium perenne L. grass cultivated with a native microbiome in heavily multimetal-impacted soil, specifically regarding grass growth, microbial activity, soil processes, and the behaviour of potentially toxic elements (PTEs: Cd, Ni, Cu, Pb). Two lab-scale, place-specific thermal regimes were applied: a warmer glasshouse (15–25 °C) and a colder freeze-thaw chamber (FTC; −20–20 °C). Firstly, (i) unenriched vs. HD-enriched soils were separately compared within each thermal regime. Secondly, (ii) for each soil type separately, glasshouse vs. freeze-thaw regimes were compared. These comparisons were made to (i) evaluate the effect of soil enrichment under different thermal conditions and (ii) assess the warm-to-cold (W-to-C) temperature transition for each soil type. The HD-based amendment enhanced metal adsorption, particularly under the freeze-thaw (Ni > Pb > Cu > Cd). In the glasshouse, HD increased mobile Ni (7 % → 15 %) but decreased Cu (9 % → 4 %), with minor Pb/Cd changes. Freeze-thaw stabilised Cu, Pb, and Cd, while Ni remained mobile. The W-to-C temperature transition in HD-enriched soils enhanced Cu/Ni adsorption, unaffected Cd, and slightly decreased Pb. Unenriched soils showed sharp drops in mobile Cd (56 % → 25 %) and Pb (9 % → 4 %), with modest Ni/Cu changes. A similar trend appeared in HD-enriched soils. Freeze-thaw increased copiotrophic taxa, while HD promoted Prochlorococcus and Pseudoxanthomonas, mitigating freeze-thaw stress for certain microbial taxa. These findings establish seasonal shifts as a critical driver in reducing soil pollution and enhancing soil functioning.