Plant growth and heavy metal bioavailability changes in a loess subsoil amended with municipal sludge compost

The effects of municipal sludge compost (MSC) as a soil amendment are often studied in agricultural soil or topsoil contaminated with heavy metals. However, little is known about the effects of MSC amendments on plant growth and heavy metal bioavailability in subsoil. This study was conducted to investigate the effects of MSC application on plant growth and the mobility and bioavailability of Cd, Cu, and Zn in an amended soil-plant system. A pot experiment was performed to evaluate the translocation of heavy metals to broad bean (Vicia faba L.) grown in loess subsoil previously amended with different application rates of MSC. The subsoil and MSC were homogeneously mixed to achieve six soil-amended treatments (total weight of 8 kg in each pot) in 0, 0.5, 2, 6, 15, and 30% mass ratios (MSC/total). Soil samples amended with MSC were aged for 60 days before sowing. Soil and plant samples were collected after 120 days of growth. Plant height was periodically measured until harvest. The total quantities of heavy metals and their different fractions were analyzed by using graphite furnace atomic absorption spectroscopy (GF-AAS). Compared with the control soil (0% treatment), the average biomass growth rates from the 0.5 to 30% treatments ranged from 14.5 to 170.4% (increasing order), respectively. Cd (0.42-1.85 mg kg(-1)) and Cu (14.95-23.01 mg kg(-1)) mainly concentrated in the plant roots, and Zn (22.06-36.48 mg kg(-1)) mainly concentrated in the plant stems and leaves. Fortunately, the metal concentrations in the edible plant parts (0.03-0.1 mg kg(-1)) remained below the Chinese national standard thresholds (0.2 mg kg(-1)), possibly because of the alkaline soil pH (8.60-7.74), organic matter (7.4-65.9 g kg(-1)) bound to metals, and translocation of less metal to the edible plant parts by biochemical modulation. MSC can enhance subsoil fertility and promote plant development, especially in the 30% treatment. The mobility and bioavailability of heavy metals suggest that Cd is the element needing to be monitored during MSC application. High organic matter content and alkaline pH are the most important factors for controlling Cd levels. More work is required to determine the long-term impacts of sludge amendment on the soil and environment.