Silicon-mediated alleviation of aluminum toxicity in Eucalyptus species: Distinct Al uptake, microbial and metabolic mechanisms in sensitive and tolerant species

Aluminum (Al) toxicity in acidic soils severely restricts plant growth, particularly reducing the productivity of fast-growing timber species—Eucalyptus. Silicon (Si), a beneficial element, can alleviate Al toxicity, but its underlying mechanisms in tree species with contrasting Al tolerance remain unclear. Here, we investigated the effects of Si addition on plant growth, Al uptake, and rhizosphere microbial community and metabolic composition in an Al-sensitive species (Eucalyptus tereticornis) and an Al-tolerant species (Eucalyptus urophylla). Results showed that Al stress significantly inhibited the relative growth rates of roots, stems and leaves, and decreased photosynthetic efficiency. However, Si addition effectively reduced these adverse effects by increasing photosynthetic rate, pigment content, water use efficiency, electron transport rate, and maximum photochemical efficiency of PSII. Aluminum stress also increased soil water-soluble, inorganically bonded and exchangeable Al, promoting root Al accumulation, which was higher in E. urophylla than in E. tereticornis. Microbiome and metabolome analyses revealed species-specific mechanisms by which Si alleviates Al toxicity. In E. tereticornis, Si directly increased bacterial, fungal and metabolite diversity, which facilitated the immobilization of water-soluble and exchangeable forms into less bioavailable forms, including organically bonded, hydrates, and hydrous oxides, thereby reducing Al uptake and upward translocation. In E. urophylla, Si indirectly increased bacterial and metabolite diversity, and recruited specific microbial taxa, including Candidatus Solibacter, Acidothermus, and Occallatibacter, thereby promoting Al uptake, upward translocation and internal detoxification. Our findings reveal distinct Si-mediated mechanisms for alleviating Al toxicity in Eucalyptus, offering practical guidance for the management of Eucalyptus forests in acidic soils.