Organic matter control of mercury and lead toxicity in mor layers

This study evaluated the effect of organic matter composition on Hg and Pb toxicity for heterotrophic respiration in mor layers in long-term in vitro dose-response experiments. Pb proved to be a more potent toxin than Hg at comparable metal additions. The degree of litter decomposition and fragmentation and background Hg concentration levels were key factors determining metal toxicity. Higher sensitivity to metal additions in the fermentation layer than in litter and humification layers was corroborated. The role of organic matter composition was further examined in litter after structural disintegration by milling, which significantly increased the sensitivity of heterotrophic respiration to metal additions. A threshold value causing 5% reduction in heterotrophic respiration was estimated at 800 μg Hg kg−1. In boreal forests in the northern hemisphere, the mean regional Hg concentrations in mor layers amount to approximately half this threshold value.