Effect of heavy metals and mineral nutrients on forest litter respiration rate.

Two hypotheses were tested: (1) heavy metals such as Zn, Pb and Cd can suppress the respiration rate of forest litter at low-moderate pollution levels, and (2) mineral nutrients such as K, Ca and Mg can counteract the toxicity of heavy metals when applied onto the polluted litter. In a completely randomised design, three doses of heavy metals were used: Cd-10, 50, 250; Pb-100, 500, 2500; Zn-200, 1000, 5000 microg/g dwt litter, respectively. For Ca, Mg and K, the doses corresponded to 100, 500 and 2500 microg/g. A significant decrease in cumulated CO2 evolution after 4 weeks of incubation was found for the litter samples treated with medium doses of Zn, Cd/Ca, Cd/Mg, Pb/Ca, Pb/Mg, Pb/K, Zn/Mg, Zn/Ca, Zn/K and for all the highest-dose treatments. The largest drop in respiration rate in both the medium and the highest doses was caused by additions of Zn either alone or in combination with K, Ca or Mg. The additions of mineral nutrients were found to decrease the litter respiration rate below the value measured for the respective heavy metal alone in the case of Cd/Ca, Cd/Mg, Pb/Ca, Pb/Mg and Pb/K in the medium-dose treatments, and for Cd/Ca, Cd/Mg, Cd/K, Pb/Ca and Pb/Mg in the highest-dose treatments. In all other cases, additions of the mineral nutrients did not influence the respiration rate significantly when compared to the effect of the respective heavy metal.