Manganese cycling in central Ontario forests: Response to soil acidification

Soil acidification results in enhanced mobilization of many potentially phytotoxic metals, including Mn. In Canada, there is also concern that emissions of Mn from the gasoline additive methylcyclopentadienyl manganese tricarbonyl (MMT) may contribute to the environmental risk posed by Mn. As a first step toward understanding the risk posed by Mn in acidifying forest systems in Ontario, the authors sampled foliage and soil from 46 forest plots in central Ontario that encompassed a soil pH range of 3.9–8.1. Increased availability of Mn at lower soil pH was clearly demonstrated, as water-extractable Mn increased and molar ratios of Ca:Mn in foliage decreased at sites with lower soil pH, although Mn availability appears to be determined by both soil pH and Mn content in soil. The cycling of Mn was studied in detail at a representative acidified coniferous forest catchment (Plastic Lake) in central Ontario. Between 1980 and 2002, Mn bulk deposition at PC ranged from 3 to 7 mg m−2 a−1 and there was no evidence that Mn deposition increased over this time period despite increased usage of MMT. Temporal patterns of Mn concentrations in stream water draining the Plastic Lake catchment were highly correlated to SO42- levels in stream water (r2 = 0.90), indicating that acid leaching is currently driving Mn losses. Furthermore, Mn outputs in stream water draining the terrestrial catchment exceeded bulk deposition inputs, whereas the lake was a net sink for Mn, due to retention in lake sediment. Because Mn availability is determined by soil pH and the Mn content in soil, net losses of Mn from the terrestrial catchment indicate that Mn levels in soil and stream water will ultimately decline over the long-term, and therefore Mn is unlikely to have a negative impact on forests in this region.