Influence of logging residues on MeHg accumulation in soil

Sammanfattning: Forestry has been found to increase the accumulation of methyl mercury (MeHg), a highly neurotoxic compound, in forest soils. However, little is known about how forestry influences catchment processes that governs the mercury (Hg) methylation process. Logging residues are used in harvested catchments in stick roads to reduce soil disturbances caused by forestry machinery. Logging residues left on site after harvest have been suggested to act as a source of high-quality organic matter that stimulates the activity of the microorganisms that carry out the methylation of Hg. In addition, logging residues might influence the activity and abundance of methylating bacteria by reducing the temperature fluctuations in soils below residues and by increasing the soil moisture content. To evaluate the impact of logging residues on the accumulation of MeHg, an experimental field study was carried out in three sites, one in Uppland and two in Västerbotten. The concentration of MeHg was compared between soils covered with residues and soils without residues, and between the lower and upper parts residue piles. Logging residues were not found to influence the levels of MeHg in soils. However, an increased accumulation of MeHg was found in the lower part of residue piles. The accumulation of MeHg in the lower parts was accompanied by a reduced temperature amplitude and an increased water content compared to the upper part of the piles. The increased formation of MeHg might have been mediated by an increased water content in the lower part of the residue piles, possibly by increasing the abundance and activity of Hgmethylating microorganisms due to suboxic/anoxic conditions within biofilms around decomposing needles. The dissolved organic matter composition in soil water differed in soils below residues compared to without residues, but the organic matter signature in soil water under residues was not found to be compliant with an elevated mercury methylation rate. As MeHg accumulated in the lower part of residue piles could become mobilized and transferred to surface water, the suitability of using logging residues in stick roads depends on the location within the catchment. The removal of logging residues could prevent the potential mobilization of MeHg from residue piles. Though, as soil disturbances may cause an increased Hg methylation rate and mediate MeHg export to surface waters, other form of protection, e.g. logging mats, should be used if logging residues are not used to protect soils.