Effects of sewage sludge application on root growth and heavy metal uptake of Schefflera arboricola

Objective This study aimed to analyze the effect of sewage sludge (SS) surface and mixed application on Schefflera arboricola root growth and heavy metals uptake, to deeply understand the effect of SS application on landscape plant root growth, and to provide a reference for the safe use of SS in the landscape.

Method We conducted a root box experiment without SS (no SS) and with the surface application (surface SS) or the mixed application (mixed SS) of 10% SS, to analyze the dynamic changes of S. arboricola root morphology, soil pH, and conductivity in different soil layers. Root tissue density, root density, and heavy metal contents of plant were also analyzed. The relationships between soil pH, soil conductivity, root heavy metal contents, and root length were further established by the linear regression analysis.

Result Compared with no SS, mixed SS treatment obviously inhibited the increment of root length, root surface area, and root density. Surface SS treatment significantly increased total root length, root volume, and root density in the 0−20 cm soil layer. After 240 days of treatment, total root length in 0−20 cm and 20−40 cm layers in mixed SS treatment were 66.37% and 51.51% of no SS, respectively. Those for surface SS treatment were 115.43% and 98.66% of no SS, respectively. The maximum root total length, root volume, and root density occurred in the 0−20 cm soil layer in surface SS treatment, while the maximum root dry weight and tissue density occurred in the SS layer in surface SS treatment. Mixed SS treatment significantly increased soil pH, soil conductivity, and heavy metal contents of S. arboricola. The contents of Cd, Zn, Cu, and Ni in roots of different soil layers in mixed SS treatment were 2.32 to 11.70 times higher than those in no SS treatment. The linear regression analysis showed that the in-situ scanning total root length of S. arboricola in 0−20 cm soil layers was significantly positively correlated with in-situ measured soil pH in no SS and surface SS treatment (P<0.001). Total root length of S. arboricola was significantly negatively correlated with the contents of Cd, Zn, Cu, and Ni in roots (P<0.001).

Conclusion Surface application of 10% SS increases soil pH and promotes S. arboricola root growth. Mixed application of 10% SS significantly increases root heavy metal uptake in different soil layers and inhibits root growth of S. arboricola.