Objective This paper aims to investigate the migration and accumulation of heavy metal mercury (Hg) in the soil-Styphnolobium japonicum system from sludge products in urban life, so as to explore the response mechanism of S. japonicum seedlings to Hg stress, and to clarify the application potential of sludge products in cultivation of S. japonicum seedlings.

Method The study focused on S. japonicum seedlings, employing a pot experiment to establish three gradients of sludge application rates (0, 2, and 4 kg/m²) and four sampling times (40, 80, 120, and 160 d). The treatments were divided into a transplanted S. japonicum group and a pure soil group to explore the dynamic changes in soil Hg content. The study further investigated the enrichment and migration patterns of Hg in the roots and leaves of S. japonicum, as well as the characteristics of subcellular distribution and the forms of existence.

Result (1) With the increase in the duration of sludge product application, the soil Hg content significantly decreased. In the soil planted with S. japonicum, the addition of sludge products resulted in significantly lower Hg content compared with pure soil in short term. (2) The amount of sludge product, sampling time, and their interaction all significantly affected the Hg content in the roots and leaves of S. japonicum. At 40 and 160 d, the application of sludge products significantly promoted the absorption of Hg by roots and leaves of S. japonicum. The use of sludge products in early stage can promote the transfer of Hg between roots and leaves, while in the later stage, it inhibited it. A low application rate of sludge products can significantly increase the biomass of S. japonicum, thereby increasing the accumulation of Hg. (3) In the roots and leaves of S. japonicum, Hg was mainly distributed in cell wall and soluble fractions, with a combined proportion exceeding 90%. (4) The residual form of Hg in roots and leaves of S. japonicum accounted for the largest proportion, with a range of 57.2% to 59.7% and 52.6% to 69.0%, respectively.

Conclusion S. japonicum can cope with the Hg stress brought by low application rates of sludge products, mainly through the pathways of cell wall fixation, vacuolar compartmentalization, and the transformation of Hg into a low-activity form for storage.