Exploring the role of soil geochemistry on Mn and Ca uptake on 75-year-old mine spoils in western Massachusetts, USA.

Manganese pollution to plants, soils, and streams from Mn-rich mine spoils is a global and persistent issue. Some former mining sites can be revegetated readily while others struggle to support plants. We explored Mn in plants and soils following 75 years of soil development and reforestation of a pine-northern hardwood forest at the former Betts Mine in western Massachusetts, USA. We studied soils on four Mn-rich mine spoils and at two control sites: an undisturbed location adjacent to the mine and on a non-Mn mineral bearing rock formation to determine if soil conditions have influenced the uptake of Mn and Ca by vegetation. We collected mid-season foliage from five dominant canopy trees and four common understory plants and excavated three soil pits at each site during July 2018. We found that control sites had lower total Mn (980 ± 140 µg g-1) in their soils than on the mine spoil sites (5580 ± 2050 µg g-1). Our soil data indicated that < 1% of total Mn was strong acid extractable at mine spoil soils and control sites. Surprisingly, the canopy trees established on mine spoils at the Betts Mine had equal to or lower foliar Mn concentrations (840 ± 149 µg g-1) and lower Mn/Ca ratios (0.3 ± 0.1 mol mol-1) than at control sites (1667 ± 270 µg g-1; 1.1 ± 0.2 mol mol-1), refuting our hypothesis of mine spoils driving highest Mn uptake. Soil pH and physicochemical properties suggest Mn primarily exists within primary minerals or form insoluble oxides at the mine spoil sites. Our results suggest higher Ca availability and pH in soils likely reduced Mn uptake and promoted reforestation of the mine spoils.