Evidence of increased mercury methylation in boreal soils following vehicular disturbance by forest harvesting

Forest harvesting operations using heavy equipment may result in soil disturbance and hydrologic disruption given necessary stream crossings and work around saturated soils. Wheel track depressions are a noticeable soil disturbance where water may collect and these small pools can become anoxic under certain conditions, creating hotspots for mercury methylation. Boreal forest soils are a known sink for atmospheric mercury and although forest operations guidelines seek to minimize soil disturbances, the aerial extent of disturbance and ponding is sometimes extensive (exceeding 10% surface area), particularly in vulnerable areas of wet organic soils. To better understand the impact of soil disturbances on methylmercury levels, the microbial community involved in methylation, and to support mitigation efforts, we sampled soil from vehicular-damaged areas (i.e., inundated wheel track depressions) paired with unimpacted control soils. Our research focused on three experimental headwater catchments subject to forest management located near the town of Dryden in northwestern Ontario. Coupling of soil methylmercury and microbial community structure analyses demonstrated that methylmercury concentrations in vehicular-damaged saturated soils were up to four times higher than in intact harvested control soils. Genomic analysis further indicated differences in the soil bacterial community within a few months post-harvest, showing an increased prevalence of families and taxa that methylate mercury. This study sheds light on how forestry operations can impact soil mercury concentrations and microbial communities in affected regions, underscoring the importance of refining forest management practices to mitigate the risk of mercury methylation and its potential environmental consequences. • Harvesting wheel tracks can create soil depressions that pond and stagnate. • MeHg in saturated wheel tracks was up to four times higher than paired controls. • Disturbed soils showed microbial shifts, including Hg-methylating taxa. • Increases of, e.g., Geobacter and Clostridium , appeared within months after harvest. • Results highlight need to minimize soil disturbance in wet organic soils.