Effect of humic acid on seedling growth and trace metal accumulation of pak choi (Brassica chinensisL.) cultivated on molybdenum slag-spiked soil

The growth performance and trace metal accumulation of pak choi (Brassica chinensisL.) were investigated to evaluate the ameliorative effect of humic acid on molybdenum (Mo) slag-spiked calcareous soil. Calcareous soil spiked with 5.0% (w/w) slag was amended with humic acid derived from leonardite from 0 to 5.0% (w/w). With increasing application rate, humic acid enhanced the antioxidative capacity of pak choi seedling, as indicated by increases in the activities of antioxidant enzymes (superoxide dismutase, catalase, and peroxidase) and a decrease in malondialdehyde content; humic acid application also increased total chlorophyll content, leaf area, seedling height, and fresh biomass of pak choi. These stimulation effects started to decrease above 2.5-5.0% application of humic acid. The contents of trace metals (Cu, Mn, Zn, As, Cd, and Pb) in the aboveground part of pak choi seedling generally decreased at low rates (0.5% and 1.0%), and then increased with higher rates (2.5% and 5.0%) of humic acid application. Health risk assessment of trace metals based on target hazard quotient (THQ) suggested that consuming pak choi grown on these soils is safe. Low rate (0.5%) of humic acid reduced the potential health risk, while high rates (2.5% and 5.0%) accumulated trace metals and increased health risk. Humic acid could be added to Mo slag-spiked calcareous soil for the yield and food safety of pak choi, but the overuse of humic acid should be avoided