Trichoderma harzianum DAA8 and Trichoderma asperellum LDA4 synergistically enhance cadmium and chromium phytoextraction by king grass and Sedum alfredii in co-contaminated soils.

Bioremediation represents a sustainable, environmentally friendly, and economical approach to mitigating heavy metal contamination in soils. This study investigated the synergistic effects and mechanisms of Trichoderma harzianum DAA8 and Trichoderma asperellum LDA4 strains inoculation in king grass (KG) and Sedum alfredii (S. alfredii) for phytoremediation of Cd-Cr co-contaminated farmland soils. The inoculation of DAA8+LDA4 effectively enhanced plant biomass by 15.37-23.28 % in king KG and 10.91-23.75 % in S. alfredii, while increasing Cd/Cr extraction by 25.54-37.20/14.19-17.30 % in KG and 15.42-22.45/8.86-9.92 % in S. alfredii under low-high pollution level compared to uninoculated plants. The maximum removal efficiency was 32.56 % for Cd in S. alfredii and 22.56 % for Cr in KG inoculated with DAA8+LDA4. Root secretions of jasmonic acid, palmitic acid, oleic acid, and isobavachalcone were closely correlated with Cd-Cr phytoextraction under low and high pollution levels. DAA8+LDA4 colonization induced favorable rhizosphere conditions by reducing soil pH (1.57-2.12 and 2.17-2.25 units), increasing soil organic matter (2.92-4.64 and 1.03-2.48 %), enhancing electrical conductivity (17.26-22.91 and 8.20-16.09 %), and improving cation exchange capacity (22.91-23.12 and 20.72-21.71 %) under low-high pollution levels in inoculated KG and S. alfredii respectively, as compared uninoculated KG and S. alfredii. The inoculation also improved soil enzyme activities and significantly increased the relative abundance of rhizosphere fungal (KG; Ascomycota; 3-6 %, and S. alfredii; 3-7 %) and bacterial (KG; Actinobacteriota; 2-7 %, and S. alfredii; 7-11 %) communities as compared to unlocated KG and S. alfredii. Nitrogen fixation and nitrification were more prominent in the KG rhizosphere with DAA8+LDA4 inoculation compared to S. alfredii under both pollution levels. The findings demonstrate that phytoremediation efficiency of KG and S. alfredii was improved by T. harzianum DAA8 and T. asperellum LDA4 through regulation of plant growth, root exudation, and microbial functioning in Cd/Cr-polluted soils. The findings of the present work highlight the potential of Trichoderma inoculation as a sustainable, effective strategy to promote phytoremediation, providing an effective solution to the remediation of HM-polluted lands and reduction of the environmental cost of cadmium and chromium pollution.