Effect of heavy metals and storage time on two types of forest litter: Basal respiration rate and exchangeable metals

Two types of forest litter, MOR and MULL, were treated with 0 (control), 25, 100, 400, 1600, and 6400 mg kg(-1) Cd, Cu, Pb, or Zn after different storage times (35, 75, and 125 days at approx 5 degrees C). Highly significant effects on respiration rate were observed for dose of heavy metals, type of litter, type of metal, and storage time. The respiration rate of untreated MULL litter was lower than that of untreated MOR in all incubations, and the slope of the relation to the dose of metals was steeper for MOR. Respiration rates after storage were lower than in fresh litter, and the slope of the relation between respiration rate and metal dose,vas less steep after storage. In the first incubation, MULL litter was more sensitive to Cd, Cu, and Ph and less sensitive to Zn than MOR litter. After 125 days of storage, no single significant effect was found in MULL litter, whereas in MOR litter all metals still inhibited respiration rate significantly. The relative toxicity of metals was similar for both litter types, and the average EC50respiration values were (mg kg(-1)) Cu = 3880, Zn = 5610, Cd = 6320, and Pb = 24800. The percentages of exchangeable metals (1 M NH4OAc, pH = 7) in MULL litter were lower on average than in MOR litter, and the order of solubility of the metals was Cd > Zn > Ph > Cu. Storage caused no significant difference in the average percentage of exchangeable metal. The highest doses of heavy metals increased the amounts of Ca, K, Mg, and Na extracted. (C) 1998 Academic Press