Time‐dependent Changes in Soluble Organics, Copper, Nickel, and Zinc from Sludge Amended Soils

An acidic western Washington soil and a near‐neutral eastern Washington soil were each mixed with anaerobically digested Seattle‐METRO sewage sludge and incubated in triplicate at 25 ± 2°C and approximately −33 kPa water potential for 1, 2, and 4 d and 1, 2, 4, 10 and 30 weeks. At the end of each incubation period, saturation pastes were made using 0.01 M CaCl2 and vacuum extracted. Total soluble C, Cu, Ni and Zn, as ‐ell as NH4 + NH3, NO3‐N, ortho‐P, and pH were measured in the extracts. High NH4 + NH3 values produced in the first 2 weeks of incubation caused the pH to increase into the alkaline range. Soluble C values increased during this period, and soluble Cu values increased despite the increasing pH, apparently because of organic complex formation. Soluble Zn declined during the same period. Soluble Ni possessed intermediate chemical behavior, as it appeared to be influenced both by a solid phase with pH dependent solubility and by ability to form organic complexes. Nitrification during incubations longer than 4 weeks led to a pH decline and increased Zn and Ni levels once more. Decreasing Cu levels were also associated with this period, likely due to losses of soluble organic materials via oxidation to CO2.