Does returning Sphagnum moss to toxic metal polluted peatlands increase aqueous metal mobility?

Sphagnum moss plays an important role in regulating toxic metal and metalloid mobility by influencing peatland pH, dissolved organic matter composition, and ecohydrology. However, historical toxic metal and metalloid pollution has led to the absence of Sphagnum moss peatlands in many landscapes globally. Other often co-occurring pollutants, like sulphate, alter peatland biogeochemistry, leading to enhanced peat decomposition and altering peatland pH, dissolved organic matter composition, and ecohydrology. Furthermore, in these polluted landscapes, toxic metals and metalloids are preferentially stored in organic soils relative to mineral soil ecosystems and peatlands are thought of as landscape sinks for these pollutants. As Sphagnum moss returns to these polluted peatlands, whether naturally or from peatland restoration activities, it is unknown whether these landscape stores of toxic metals and metalloids is at risk of mobilizing to sensitive downstream ecosystems.

During historical smelting operations in Sudbury, Ontario, Canada an estimated 12,000 t of copper and nickel were released to the atmosphere, most of which was deposited within 100 km of the smelters where peat concentrations can exceed 1000 mg kg^-1. Here, we used a spatial gradient of peatlands at varying levels of impact (high, moderate, low, none) in the region surrounding Sudbury as a model for recovery over time to understand the potential mobilization of toxic metals and metalloids due to the return of Sphagnum moss. In peatlands with no Sphagnum recolonization, both copper and nickel (along with other toxic metals and metalloids like methylmercury and arsenic) pore water concentrations were elevated (> 10 µg L^-1) relative to peatlands with higher Sphagnum moss cover and lower initial impacts. These conditions coincided with higher dissolved organic matter (DOM) concentrations and humification levels but divergent relationships between DOM humification and copper/nickel concentrations were observed. There was no clear trend in apparent partitioning coefficient with Sphagnum recovery, while pH was the highest in the most impacted peatlands (no Sphagnum recovery, pH ~4 - 5). In the surficial peat (i.e., the surface of moss recolonization, 0-10 cm), a decrease in pH was not correlated (p > 0.1) with either water extractable copper or nickel and the apparent partitioning coefficients of either metal. While, in deeper, lower hydraulic conductivity peat, (10-20 cm) only the copper apparent partitioning coefficient significantly (p < 0.0001) declined with decreasing pH, suggesting increased geochemical mobility but decreased ecohydrological mobility.

The combined results suggest that the return of Sphagnum moss does not necessarily increase the risk of historical toxic metal release due to the numerous hydrobiogeochemical feedbacks that operate in peat and peatlands. As such, promptly returning Sphagnum moss to these polluted peatlands is critical to mitigating the potential for catastrophic metal release due to wildfires and droughts.