Long-term effects of sewage sludge applications on soil properties, cadmium availability and distribution in arable soil

Long-term migration and crop uptake of cadmium (Cd) were investigated in a cultivated field experiment, that had been started 41 years earlier on a structured clay loam. One treatment was supplied biennially with digested sewage sludge and PK-fertilizer, while the control received calcium nitrate and PK-fertilizer. The estimated total Cd supply was ca 153 and 13 mg Cd m^-2 in the sludge and control treatments, respectively. In the sludge treatment, 92% of applied Cd was recovered in the topsoil and 7% in the upper 17 cm of the subsoil. The total loss of Cd out of the profile was negligible after 41 years, even though increases in dissolved organic carbon (DOC) concentrations were observed throughout the profile. Thus, macropore transport of Cd, either as Cd2+ or complexed by DOC was apparently not significant. This was attributed to the strong sorption capacity of the arable soil and incorporation of the sludge into the soil matrix, thereby increasing the contact of sludge-borne Cd to soil particles and protecting the Cd from transport in macropores. Cd solubility in equilibrium extracts of Ca(NO3)2 increased by a factor of 20 in the sludge treatment compared to the control. This was reflected in the Cd concentration of the straw fraction in barley, which was almost doubled in the sewage sludge treatment. The grain fraction showed, however, no significant increase in Cd concentration. As crop biomass increased as a consequence of sewage sludge supply, the Cd offtake in harvest was more than twice as high compared to the control. The sewage sludge applications did not pose a high risk for leaching losses of Cd in this soil, but resulted in increased Cd concentrations in the straw fraction.