Changes in Organic Soil Mercury Concentrations Over 50 years at the Hubbard Brook Experimental Forest, New Hampshire, USA

Over the past half century, air quality management efforts have led to substantial decreases in mercury emission across North America and Europe. Subsequent declines of mercury concentrations in air and precipitation have been well documented, resulting in lower mercury fluxes in wet deposition. The responsiveness of ecosystems to these decreasing inputs is an on-going point of scientific inquiry and for some matrices, considerable uncertainty exists. Organic surface soils are one such example, with relatively little known about how and on what time scale soils react to changes in mercury deposition. Here, we present an analysis of total mercury in organic soils from the Hubbard Brook Experimental Forest (HBEF), New Hampshire USA, spanning over 50 years. Archived soil samples representing the Oie and Oa horizons in the reference watershed (WS6) were oven-dried, milled, and analyzed via direct mercury analyzer. Trends in total mercury concentration varied over time and among organic soil horizons, with overall mercury concentrations decreasing. Changes in Oie mercury concentration were more pronounced that the Oa horizon and matched modeled deposition for the experimental area (GEOS-Chem). Conversely, Oa mercury concentrations showed little agreement with deposition and are likely integrating atmospheric inputs over a much longer period. Overall, results suggest that organic soils at HBEF are dynamic and responsive to changes in atmospheric emissions.