MANGANESE UPTAKE BY NORTHERN HARDWOODS ON MN-CARBONATE RICH MINE-TAILINGS – A GEOCHEMICAL CONUNDRUM IN WESTERN MASSACHUSETTS

Manganese toxicity to plants in natural settings is unclear as many factors can affect tolerable soil concentrations. Seemingly normal forest ecosystems can develop and function on soils with Mn concentrations that could be considered toxic. One such case exists in western Massachusetts, with a secondary northern hardwood stand with underlying soils derived from Mn-carbonate rich mine tailings. Our objective was to determine if foliar Mn concentrations were at toxic levels and if any biogeochemical process was allowing for their sustained growth. We explored Mn concentrations in soils and foliage at four sites with soils derived from mine tailings and at two control sites with granite/schist glacial-till parent material. We collected mid-season foliage samples from five dominant canopy trees and four common understory plants, and excavated three soil pits at each site during July 2018. Our results were counter-intuitive: the northern hardwood forests established on mine tailings had lower foliar Mn concentrations and lower Mn/Ca ratios than at our two control sites even though control sites had lower total Mn in their soils (980 µg g^-1) than on the mine tailings (5580 µg g^-1). Our soil data indicated that < 1% of Mn was potentially bioavailable on the mine tailings and control sites. However, soil physicochemical properties suggests Mn primarily exists as insoluble oxides on the mine tailing but the greater soil acidity at the control sites caused greater Mn uptake by the forest vegetation.