Abstract: In a pot experiment, the sewage sludge from Xiamen Yundang Wastewater Treatment Plant was applied as organic fertilizer to study its effects on the growth and yield of pakchois and the transfer and accumulation of heavy metals in pakchois-soil system. The sludge was rich in nitrogen, phosphorus, potassium and organic matter, and its heavy metals contents were lower than the limits of national control standard for the pollutants in sludge for agricultural use. Appropriate application of the sludge could promote the growth and yield of pakchois. When the application rate was 5%, the pakchois had the maximum growth and biomass; but when the application rate was higher than 15%, the pakchois growth was remarkably inhibited. The root exudates of pakchois could fix and enrich the heavy metals in rhizosphere soil, and restrict their transfer into pakchois. The enrichment coefficients of test nine heavy metals in pakchois were all lower than 1.0. When the application rate of the sludge was 5%, the Cd, As, Ni, Pb, Cr, Cu and Zn contents in pakchois were all lower than the limits of national food hygiene standard of China. After planting pakchois, the chemical speciation of the heavy metals in rhizosphere soil transformed to some extent, e.g., the Cd and Cr transformed from residual form to bio-available, Cu and Zn transformed from oxidizable to acid-soluble, Mn and Pb transformed from reducible to oxidizable, while Ni appreciably transformed from bioavailable to residual.

Key words: Red tide, Photosynthesis, Growth, Prorocentrum minimum, Zinc limitation