IMPORTANCE OF VEGETATION FOR MANGANESE CYCLING IN FORESTED WATERSHEDS

Ecosystems throughout the world are experiencing significant biogeochemical perturbation from anthropogenic activities. As one example, manganese (Mn) is enriched in surface soils at the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) in Pennsylvania due to past atmospheric inputs from industrial sources. At the SSHCZO, large quantities of Mn that are leached from soil components into solution are taken up by vegetation each year; as a result, only relatively small quantities of Mn are transferred from hillslopes into the stream. Here, we combined mass balance models at the containerized seedling (mesocosm) and pedon-scales with synchrotron-source spectroscopies (X-ray fluorescence and X-ray absorption near edge structure) to examine Mn geochemistry in the soil-plant system and the impact of vegetation on Mn transport in forested catchments.

Manganese uptake into vegetation exceeded Mn losses in soil leachate, and net Mn loss from soils decreased in the presence of vegetation due to uptake into plant tissues. Since Mn is a micronutrient for plants, it is not expected to be taken up in large quantities; yet the ratio of uptake to leaching was higher for Mn (~20-200X) than for major plant nutrients (Ca, Mg, and K) and other metal cations (Fe and Al) (<5X). Given these observations, we conclude that vegetation exerts a stronger influence on the cycling of Mn than other major cations in this ecosystem. In contrast, Na did not accumulate in plant tissue and was preferentially lost in soil leachate. Tree roots and leaves were dominated by soluble Mn(II) that was rapidly oxidized and immobilized as Mn(III/IV)-oxides during decomposition of roots and leaves. Therefore, biological cycling slows the loss of Mn from watersheds because trees take up dissolved Mn from the soil and store it in biomass for years to decades. Mn that is stored in biomass is oxidized and immobilized as Mn-oxides in the soil during decomposition. This study demonstrates that vegetation plays an important role in controlling the Mn retention in watersheds, and that certain types of vegetation, such as the temperate forest studied here, have the ability to slow the rate that Mn is transferred from soils into water systems.