Mechanisms activating trace heavy metals in the rhizosphere microenvironment of Amaranthus hypochondriacus L.

Soil heavy metal activation is closely related to rhizosphere soil pH, O₂ and enzyme activity levels. In this study, we used laser ablation inductively coupled plasmamass spectrometry (ICPMS), planar optode (PO), soil in situ enzyme spectrum analysis, and the diffusive gradients in thin films (DGT) method to investigate heavy metal accumulation in Amaranthus hypochondriacus L. growing in polluted soil (Cd, Pb and Zn) and unpolluted soil, as well as dissolved oxygen (DO), pH, soil enzyme activity, and trace heavy metal dynamics in rhizosphere and nonrhizosphere soils. The results revealed that the growth of A. hypochondriacus was significantly inhibited under combined Cd, Pb, and Zn stress, with the most pronounced effects on root and branch biomass. Radial oxygen loss (ROL) and alkalization occurred in the roots of A. hypochondriacus, which were influenced primarily by root growth and environmental conditions. The enzyme activity patterns indicated that acid phosphatase (ACP), alkaline phosphatase (ALP), and β-glucosidase (BG) activities were primarily associated with the root system and that the activities of these enzymes were relatively low in nonrooted soil. Under heavy metal stress, the ACP, ALP and BG activities in the hotspots of soil increased. DO, pH, and soil enzyme activity were significantly correlated with the presence of heavy metals Cd, Pb, and Zn, influencing the mobilization mechanisms of trace metals in the rhizosphere. These findings lay the groundwork for understanding the impact of root-induced alterations in O₂, pH, and soil enzyme activity on the mobility of trace heavy metals in soil.