Increased mercury in forest soils under elevated carbon dioxide
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Fossil fuel combustion is the primary anthropogenic source of both CO2 and Hg to the atmosphere. On a global scale, most Hg that enters ecosystems is derived from atmospheric Hg that deposits onto the land surface. Increasing concentrations of atmospheric CO2 may affect Hg deposition to terrestrial systems and storage in soils through CO2-mediated changes in plant and soil properties. We show, using free-air CO2 enrichment (FACE) experiments, that soil Hg concentrations are almost 30% greater under elevated atmospheric CO2 in two temperate forests. There were no direct CO2 effects, however, on litterfall, throughfall or stemflow Hg inputs. Soil Hg was positively correlated with percent soil organic matter (SOM), suggesting that CO2-mediated changes in SOM have influenced soil Hg concentrations. Through its impacts on SOM, elevated atmospheric CO2 may increase the Hg storage capacity of soils and modulate the movement of Hg through the biosphere. Such effects of rising CO2, ones that transcend the typically studied effects on C and nutrient cycling, are an important next phase for research on global environmental change.
KeywordsGlobal change Soil organic matter Hg deposition Throughfall Free-air carbon dioxide enrichment
We thank D. Richter for soil samples, E. A. Leger and F. J. Rohlf for statistical advice, C. Iversen, R. Oren and the FACE staff for field support, J. Lichter for conversations and data on pre-treatment soils, R. K. Kolka for advice on stemflow collectors, S. Lindberg and N. Bloom for throughfall sampling advice, J. Varekamp for use of his DMA-80, W. Schlesinger for advice during manuscript preparation, and H. Heilmeier and two anonymous reviewers for comments on this manuscript. This work was supported by the US Department of Energy, Office of Science-Biological and Environmental Research, and fellowships from the National Science Foundation (S. M. N.) and Department of Energy (S. M. N.).
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