Uptake of silver by brown rice and wheat in soils repeatedly amended with biosolids

There have been numerous studies simulating the behaviour and fate of silver (Ag) applied to soils in biosolids in recent decades but the results remain to be verified under actual farming conditions. Here, we report a study of the transfer of Ag along the biosolids-soil-crop pathway with repeated biosolid applications over a four-year period to three contrasting soil types under rice-wheat rotation cultivation. A systematic investigation of Ag concentrations in biosolids throughout China shows Ag concentrations ranging from 0.64 to 7.47mgkg^-1 with a mean value of 3.58mgkg^-1 and a median of 3.02mgkg^-1 on a dry weight basis. Silver concentrations were significantly higher in industrial biosolids than in domestic or mixed flow biosolids. Biosolids application enhanced Ag accumulation in whole wheat. Silver concentrations in whole wheat increased to 20.8, 20.5 and 4.87μgkg^-1 after four years of high-metal industrial biosolids application to an acid Typic Ali-Perudic Aragsol, a neutral Typic Hapli-Stagnic Anthrosol, and a calcareous Typic Carbonati-Perudic Ferrosol, respectively. Moreover, the Ag translocation factor also increased in wheat following biosolids application with values of 5.6, 3.1 and 1.4, respectively. However, Ag accumulation in rice was found only in the acid soil with no discernible increase (p>0.05) in the translocation factor. The seasonal redox cycle may contribute to this phenomenon. A seedling incubation experiment confirms the influence of soil water regime on Ag bioavailability with a higher Ag translocation factor during the wheat growing season than the rice growth period. Incorporating a fallow period during the wheat (winter crop) season might be a suitable strategy for repeated biosolids application.