Manganese accumulation characteristics and rhizosphere decontamination mechanisms of Phytolacca icosandra L.

The role of microbial metagenomics in understanding ecological changes associated with rhizosphere heavy metal decontamination by plants has often been overlooked. The aim of this study was to scrutinize the structural, enzymological and metagenomic mechanisms leading to manganese (Mn) decontamination in the rhizosphere by Phytolacca icosandra. Seedlings of P. icosandra were planted for three months under six sets of Mn treatment (addition of 0, 250, 500, 100, 2500, and 5000 µg/g Mn in original soils) using a complete randomized block design. When Mn addition increased to 2500 and 5000 µg/g, the shoot biomass of P. icosandra was reduced by 3.14% and 51.85% respectively, while root biomass was reduced by 5.91% and 47.29% respectively. When Mn uptake increased in the roots/shoots and leaves after the addition of 5000 µg/g Mn, plant chlorophyll a, and malondialdehyde (MDA) concentrations decreased by 4% and 17% respectively. As a result, lipids in the cell membranes of the leaves were no longer destroyed and cell membrane degradation was inhibited by Mn gradients. With addition of 2500-5000 µg/g Mn, the overall weight and height biomasses of P. icosandra was reduced in size to absorb excess Mn. Soil N bioavailability decreased the most at the highest Mn concentration. Above 2500 µg/g Mn, rhizosphere acidification increased (pH < 7). The abundances of rhizosphere bacteria Ramlibacter sp. and Planctomycetaceae bacterium LX124 decreased when soil Mn concentration increased, while those of Tumebacillus avium, Devosia riboflavina, and Flavobacterium fevense increased, paving the way towards microbial biomarkers of soil Mn decontamination. These findings provide valuable knowledge on remediating heavy metal-contaminated agricultural soils.