Effects of litter biochar addition on sludge adaptability and heavy metal uptake of Ruellia simplex

Objective To explore the effects of different addition amounts of litter biochar on growth, heavy metal absorption and accumulation of Ruellia simplex planted in the sludge + soil mixed matrix, and provide a reference for the resource utilization of municipal sludge and garden waste.

Method A pot experiment was conducted to analyze the effects of adding 0 (CK), 1.5% (F_1.5), 3.0% (F_3.0) and 4.5% (F_4.5) of litter biochar on growth, root morphology, physiology, nutrient and heavy metal uptake and accumulation of Ruellia simplex.

Result Compared with CK, F_1.5 significantly increased plant height, root biomass, shoot biomass and whole plant biomass. The plant biomass decreased gradually with the increase of biochar addition. The biomass of F_4.5 was significantly lower than that of CK, showing the characteristic of “low promotion and high inhibition”. Total root length, root surface area, average diameter and root volume of Ruellia simplex reached the maximum in F_1.5 treatment, and all of them gradually decreased with the increase of biochar addition. Among all treatments, the contents of soluble protein and malondialdehyde (MDA) in roots of F_1.5 were the lowest, and the activity of superoxide dismutase (SOD) was the highest. The SOD activity of roots showed a downward trend with the increase of biochar addition, while the change trends of soluble protein and MDA contents were opposite. Compared with CK, all treatments increased the uptakes of N, P and K of R. simplex shoots and roots to different degrees, reduced the contents of Cd and Cu in R. simplex plant to different degrees, and increased the contents of Pb and Ni. The accumulative amounts of N, P, K, Cd, Cu, and Pb in R. simplex plant showed a downward trend with the increase of biochar addition, and the accumulative amounts of N, P, K, Cd, Cu, Pb and Ni of F_1.5 were significantly higher than those of CK.

Conclusion The addition of 1.5% litter biochar significantly promoted R. simplex growth, absorption and accumulation of Pb, Cu, Cd and Ni, but excessive addition would inhibit plant growth and affect the repair effect of substrate. Therefore, the applied amount of biochar should be controlled reasonably in the practical application process.