Metabolic flexibility of rhizobacteria drives soil nutrient cycling and enhances rare earth elements hyperaccumulation in ferns colonizing degraded mine ecosystems

Rare earth elements (REEs) are crucial for green technologies, but their mining can severely degrade soil. While bacterial succession, often involving photoautotrophic and copiotrophic taxa, aids the colonization of REEs-hyperaccumulators in nutrient-poor soils, its applicability across plant ecotypes and light-limited conditions (e.g., winter) remains unclear. We compared two ferns (Dicranopteris pedata and Blechnum orientale) from core (DpM/BoM) and surrounding (DpS/BoS) areas, analyzing their REEs accumulation and rhizobacterial regulation of soil carbon (C) and nitrogen (N) cycling. Despite lower soil total REEs (5.28-9.37 times lower) and nutrients (TC ≤ 0.67 g kg^-1; TN ≤ 23.33 mg kg^-1), core ecotypes accumulated more REEs (e.g., 2765.03 vs. 131.67 mg kg^-1 Ce in D. pedata; 1486.22 vs. 660.90 mg kg^-1 La in B. orientale). The r/K life-strategy framework (copiotrophs vs. oligotrophs) did not fully explain microbial diversity and functions, as key taxa exhibited metabolic flexibility (e.g., chemoautotrophy). Core soils hosted copiotroph- and chemoautotroph-dominated microbiomes (p_Pseudomonadota, p_Actinomycetota, and p_Bacteroidota), linked to C/N cycling, whereas surrounding soils favored oligotrophs (p_Chloroflexota and p_Acidobacteriota). Both ferns enriched c_Alphaproteobacteria (especially g_Bradyrhizobium) and p_Actinomycetota (especially g_Acidothermus) for C/N fixation, these keystone taxa serve as potential biomarkers for monitoring soil nutrient recovery in REEs mining areas. Our findings indicate that microbial functional traits, beyond taxonomy, drive nutrient cycling in soils and REEs hyperaccumulation in plants, refining and expanding the r/K framework and suggesting seasonally tailored plant-microbe partnerships could optimize C/N cycling and enhance REEs phytoremediation in degraded soils.