Effects on mor layer respiration by small experimental additions of mercury and lead

Concentrations of mercury and lead are regionally elevated in mor layers of South Swedish forest soils due to long-range pollution. Observed negative relationships with microbial respiration bring forward suspicion of biological effects. To demonstrate this, experimental additions of Pb and Hg to mor layer samples were performed at 1200 and 5 μg.g−1 dw respectively. These doses were about 10–20 times the current contents in South Swedish mor layers and, in the case of Pb, the level was low compared to lowest reported effect level in literature. Fresh pine needle litter was added to stimulate biological activity. Hg- and Pb-induced reduction of respiration occurred in a slowing-down phase after initial stimulation and later after a second litter input. Effects were most pronounced after 60–80 days and very similar for both metals. The calculated accumulated loss of carbon was reduced by 14% for added Hg and 20% for added Pb during the entire 225 days of the experiment.