Many surface soils are enriched in metals due to anthropogenic atmospheric inputs. To predict the persistence of these contaminants in soils, factors that impact rates of metal removal from soils into streams must be understood. Experiments at containerized seedling (mesocosm), pedon, and catchment scales were used to investigate the influence of vegetation on manganese (Mn) transport at the Susquehanna/Shale Hills Critical Zone Observatory (SSHCZO) in Pennsylvania, USA, where past atmospheric inputs from industrial sources have enriched Mn in surface soils. Large quantities of Mn that were leached from soil components into solution were taken up by vegetation; as a result, only relatively small quantities of Mn were removed from soil into effluent and streams. Manganese uptake into vegetation exceeded Mn losses in soil leachate by 20–200X at all scales, and net Mn loss from soils decreased in the presence of vegetation due to uptake into plant tissues. The majority of Mn taken up by forest vegetation at SSHCZO each year was returned to the soil in leaf litter and consequently immobilized as Mn oxides that formed during litter decomposition. Thus, plant uptake of Mn combined with rapid oxidation of Mn during litter decomposition contribute to long-term retention. Current release rates of soluble Mn from SSHCZO soils were similar to release rates from the larger Susquehanna River Basin, indicating that the processes observed at SSHCZO may be widespread across the region. Indeed, although atmospheric deposition of Mn has declined, surface soils at SSHCZO and throughout the eastern United States remain enriched in Mn. If recycling through vegetation can attenuate the removal of Mn from soils, as observed in this study, then Mn concentrations in soils and river waters will likely decrease slowly over time following watershed contamination. Understanding the role of vegetation in regulating metal transport is important for evaluating the long-term effects of historical and ongoing metal loading to soils.
Spectroscopic (XANES/XRF) characterization of contaminant manganese cycling in a temperate watershed12/01/2014
Many soils around the globe are contaminated with metals due to inputs from anthropogenic activities; however, the long-term processes that retain these metals in soils or flush them into river systems remain unclear. Soils at the Susquehanna/Shale Hills Critical Zone Observatory, a headwater catchment in central Pennsylvania, USA, are enriched in manganese due to past atmospheric deposition from industrial sources. To investigate how Mn is retained in the catchment, we evaluated the spatial distribution and speciation of Mn in the soil–plant system using X-ray fluorescence and X-ray Absorption Near Edge Structure spectroscopies. Weathered soils near the land surface were enriched in both amorphous and crystalline Mn(III/IV)-oxides, presumably derived from biogenic precipitation and atmospheric deposition, respectively. In contrast, mineral soils near the soil–bedrock interface contained Mn(II) in clays and crystalline Mn(III/IV)-oxides that formed as Mn(II) was leached from the parent shale and oxidized. Roots, stems, and foliar tissue were dominated by organic-bound and aqueous Mn(II); however, a small portion of foliar Mn was concentrated as organic-bound Mn(III) in dark spots that denote Mn toxicity. During decomposition of leaves and roots, soluble Mn(II) stored in vegetation was rapidly oxidized and immobilized as mixed-valence Mn-oxides. We propose that considerable uptake of Mn by certain plant species combined with rapid oxidation of Mn during organic matter decomposition contributes to long-term retention in soils and may slow removal of Mn contamination from watersheds.