Fractions and Availability of Nickel in Loessial Soil Amended with Sewage or Sewage Sludge

Abstract

Application of sewage effluent or sewage sludge to cropland may increase Ni content in soil and plants, and affect food quality. In this study, soil and winter wheat (Triticum aestivum L.) samples from 14 sites of an area of 12 000 km2 were collected. One selected soil was treated with sewage sludge at the rates of 0, 7.5, 15, 30, 60, 120, and 210 Mg ha−1 (0, 2.5, 5, 10, 20, 40, and 70 kg Ni ha−1). The soil was incubated, potted, and planted to winter wheat that was grown until harvest. All soil samples were then extracted by the DTPA method for Ni, and the soil samples were also fracfionated into exchangeable (EXC), carbonate (CAB), oxide (OXD), organic (ORG), and residual (RES) fractions. Wheat parts from both field and pot studies were analyzed for Ni. With application of sewage effluent, Ni was retained in the surface soil, and was mainly present in the OXD and CAB fractions. The amendment of sewage sludge increased Ni in the OXD, CAB, and EXC fractions. Principal component analysis indicated that Ni content in the EXC fraction was an intensity factor, while Ni contents in the OXD, CAB, and ORG fractions were capacity factors, which represented a potential source of available Ni in the soils. In the field study, wheat leaves showed higher Ni concentrations than the seeds and stem. In the pot experiments, sludge application rates higher than 60 Mg ha−1 caused Ni accumulation in the wheat seeds.