Distribution of heavy metals in maize and mudflat saline soil amended by sewage sludge

Sewage sludge has been considered as the economic choice for soil amendment. However, whether sewage sludge can effectively amend mudflat saline soil remains unclear. This paper was one of first studies to fill this knowledge gap by assessing the persistent effects of one-time application of sewage sludge on distribution of heavy metals in maize (Zea mays L.) grown in mudflat saline soil. On the premise of total amount of heavy metals from sewage sludge being in compliance with agricultural standard in China (GB/T 24600-2009), sewage sludge was used through one-time application, with the rates of 0, 30, 75, 150, and 300 t ha(-1), to amend infertile mudflat soils. Ryegrass, Sesbania, and ryegrass were chosen as green manures for three consecutive seasons, successively planted and tilled. Maize was chosen as a test crop and sowed in 10th July 2013 and harvested 72 days after sowing. The results indicated that the sewage sludge amendment (SSA) increased uptake of cadmium (Cd), copper (Cu), manganese (Mn), nickel (Ni), lead (Pb), and zinc (Zn) in maize tissues, whereas it did not inhibit maize growth, and dry matter of maize plant tissues increased with increasing SSA rates. Total Cd, Cu, Pb, and Zn concentrations in mudflat soil increased with increasing SSA rates, but there were no significant changes in total Mn and Ni concentration in response to SSA. The highest Cd, Cu, Mn, Ni, and Pb concentrations were observed in roots of maize, and the root system acted as a barrier for metals, so metal uptake and translocation were lower in aerial parts of maize plant. The labile fraction of Cd, Cu, Mn, Ni, Zn, and immobile fraction of Pb in the sludge-amended soils increased with increasing SSA rates. The metal levels in grains were negligible in all the treatments and below the maximum permitted concentrations as specified in the national control standard in China (GB2715-2005). Sewage sludge can be applied with green manure for mudflat saline soil amendment, which provides an innovative solution for arable land resources and solid waste disposal.