Soil emissions, soil air dynamics and model simulation of gaseous mercury in subtropical forest

Abstract. Evasion from soil is the largest source of mercury (Hg) to the atmosphere from terrestrial ecosystems. To reduce the uncertainty in estimates of Hg emissions from forest soils, soil-air total gaseous Hg (TGM) fluxes and vertical profiles of soil pore TGM concentrations were measured simultaneously for 130 days to improve parameterization of emission models. The soil-air TGM fluxes, measured using dynamic flux chambers (DFC), showed patterns of both emission and deposition at five study plots, with an area-weighted net emission rate of 3.2 ng m⁻² hr⁻¹. The highest fluxes and net soil Hg emission were observed for an open field, with lesser emission rates in coniferous (pine) and broad-leaved (camphor) forests, and net deposition in a wetland. Fluxes showed strong positive relationships with solar radiation, soil temperature and soil Hg concentrations, and negative correlations with ambient-air TGM concentration and soil moisture. Using experimental field flux observations and quadratic relationships with the five parameters, four empirical models were developed to estimate soil-air TGM fluxes. The highest TGM concentrations in soil gas consistently occurred in the upper mineral horizons in the coniferous (pine) forest and in the organic horizon in the broad-leaved forest. Strong correlations between fluxes and TGM concentrations in upper soil horizons (0–10 cm) suggest that TGM in the pores of surface soil acts as the source for diffusion to the atmosphere. The TGM diffusion coefficients (Ds) between soil and atmosphere was firstly investigated at the field sites, with the range of 0.0042–0.013 m² hr⁻¹. These values should provide a foundation for future model development.