Industrial pollution from atmospheric deposition has contaminated surface soils with metals such as Pb and Hg across broad geographic regions. Other metals have also been emitted and deposited to the land surface but these are less well known. To evaluate impacts of heavy metals on the health of the terrestrial and aquatic ecosystems, it is important that we understand their residence time, transport, and behavior in soils. Vegetation can increase metal solubility in the soil through rhizosphere acidification and exudation of organic chelators. Soluble metals can then be more quickly transported into stream waters. Conversely, vegetation may increase the residence time of industrial pollution through biotic uptake. Often, metals taken up from pore fluids are eventually returned to the soil as leaves or woody material. During this biotic cycling, vegetation acts as a capacitor, storing contaminant metals and then releasing it slowly to the environment over time. For example, interdisciplinary studies at the Susquehanna Shale Hills Observatory, a Critical Zone Observatory in central Pennsylvania U.S.A., have enabled us to identify that significant Mn has accumulated in the watershed from atmospheric deposition. Using available geochemical databases, we have shown that Mn addition to surface soils is common in industrialized regions. We demonstrate with soil and leaf chemistry that vegetation actively recycles the Mn, potentially increasing its residence time in the soil. Soluble Mn is transferred from soil to vegetation through soil pore fluids, and we use soil pore fluid chemistry to evaluate these fluxes. Eventually, Mn is released from vegetation and transported into stream waters, and we show that dissolved metal loads in rivers have decreased over time, reflecting changes in atmospheric inputs over the past century. To elucidate the role of trees in release of nutrients and toxic Mn, we carried out controlled greenhouse experiments. The effluent solutions were monitored from two types of pots: soil + trees and soil-only (control). From these experiments, we can better quantify the effects of vegetation on pollution and nutrient retention in soils and the rate of their release to groundwater.
Brantley, S.L., Herndon, E.M., Jin, L., Eissenstat, D., Raymond, P. (2010): Vegetation: A natural capacitor for contaminant metals input into the Critical Zone (Invited). AGU Annual Fall Conference Proceedings.
This Paper/Book acknowledges NSF CZO grant support.