Mercury isotope compositions across North American forests. Issue 10 (13th October 2016)

• Record Type: Journal Article
• Authors: Zheng, Wang; Obrist, Daniel; Weis, Dominique; Bergquist, Bridget A.
• Is Part Of: Global biogeochemical cycles. Volume 30:Issue 10(2016:Oct.)
• Page Start: 1475
• Page End: 1492
• Publication Date: 2016-10-13
• Subjects: mercury -- stable isotopes -- forest ecosystems -- biogeochemical cycle -- soils -- atmosphere; Biogeochemical cycles -- Periodicals; Electronic journals; 577.1405
• DOI: 10.1002/2015GB005323
• Languages: English
• ISSNs: 0886-6236

Abstract

Abstract: Forest biomass and soils represent some of the largest reservoirs of actively cycling mercury (Hg) on Earth, but many uncertainties exist regarding the source and fate of Hg in forest ecosystems. We systematically characterized stable isotope compositions of Hg in foliage, litter, and mineral soil horizons across 10 forest sites in the contiguous United States. The mass‐independent isotope signatures in all forest depth profiles are more consistent with those of atmospheric Hg(0) than those of atmospheric Hg(II), indicating that atmospheric Hg(0) is the larger source of Hg to forest ecosystems. Within litter horizons, we observed significant enrichment in Hg concentration and heavier isotopes along the depth, which we hypothesize to result from additional deposition of atmospheric Hg(0) during litter decomposition. Furthermore, Hg isotope signatures in mineral soils closely resemble those of the overlying litter horizons suggesting incorporation of Hg from litter as a key source of soil Hg. The spatial distribution of Hg isotope compositions in mineral soils across all sites is modeled by isotopic mixing assuming atmospheric Hg(II), atmospheric Hg(0), and geogenic Hg as major sources. This model shows that northern sites with higher precipitation tend to have higher atmospheric Hg(0) deposition than other sites, whereas drier sites in the western U.S. tend to have higher atmospheric Hg(II) deposition than the rest. We attribute these differences primarily to the higher litterfall Hg input at northern wetter sites due to increased plant productivity by precipitation. These results allow for a better understanding of Hg cycling across the atmosphere‐forest‐soil interface. Key Points: Atmospheric Hg(0) is the dominant source of Hg to forest ecosystems Direct deposition of atmospheric Hg(0) may be a possible source of Hg to decomposing litter in addition to litterfall and precipitation The spatial distribution of forest Hg isotopes correlates with latitude, precipitation, and elevation