Measurement of the Vertical Distribution of Gaseous Elemental Mercury Concentration in Soil Pore Air of Subtropical and Temperate Forests

Abstract

Solid–gas–water phase partitioning of mercury (Hg) and the processes governing its diffusivity within soils are poorly studied. In this study, landscape and forest species dependences of gaseous elemental Hg (Hg(0)) in soil profiles (0–50 cm) were investigated over four seasons in eight subtropical (130 days) and temperate (96 days) forest plots. The vertical soil pore Hg(0) concentrations differed between subtropical (Masson pine, broad-leaved forest, and open field) and temperate (Chinese pine, larch, mixed broad-leaf forests, and open field) catchments, with annual averages ranging from 6.73 to 15.8 and 0.95 to 2.08 ng m–3, respectively. The highest Hg(0) concentrations in soil gas consistently occurred in the upper mineral or organic horizons, indicating immobilization of Hg(0) in mineral soils. A strongly positive relationship between pore Hg(0) concentrations and ratios of Hg to organic matter (SOM) in soils suggests that the vertical distribution of Hg(0) is related to soil Hg(0) formation by Hg­(II) reduction and sorption to SOM. Temperature was also an important driver of Hg(0) production in soil pores. Based on measurements of soil–air Hg(0) exchange, diffusion coefficients (D s) of Hg(0) between soil and atmosphere were calculated for field sites, providing a foundation for future development and validation of terrestrial Hg models.

Deposition and Re-Emission of Atmospheric Elemental Mercury over the Tropical Forest Floor

“…The soil pore Hg 0 gas concentration in this tropical rainforest is lower than those found at other forest sites. Soil pore Hg 0 concentrations of 10–30 ng m –3 had been reported at subtropical sites (Figure S3), which were attributed to the elevated soil Hg concentrations and organic matter on the forest floor that promote Hg II reduction and Hg 0 formation in soil. − Interestingly, the observed soil pore Hg 0 concentrations in the summer season were significantly higher than those found in the winter season in tropical, subtropical, and temperate forests. This is a strong indication that stronger Hg II reduction occurs in summertime forests across the globe.…”

“…The Hg MDF and odd-MIF signatures in near-surface air and 2 m air above ground are comparable, suggesting that the direct deposition did not cause an observable isotopic shift because of the relatively fast vertical transport. Compared to the rainy season, the limited rate of microbial Hg reduction in the dry season 13,57,58 largely constrains the soil pore Hg 0 gas concentrations regardless of the higher porosity of soil during this period. 44,57,58 We compared the vertical distribution of soil pore Hg 0 concentrations to the reported values in other forest sites (Figure S3).…”

Quantification of Atmospheric Mercury Deposition to and Legacy Re-emission from a Subtropical Forest Floor by Mercury Isotopes

“…Air–soil Hg 0 exchange in forest ecosystems is a dynamic bi-directional process, including atmospheric Hg 0 gross deposition and Hg 0 evasion, − and the soil pore Hg 0 gas plays a dominant role in linking the atmospheric Hg 0 to soil Hg II pool. − The atmospheric gross deposition refers to the process that Hg 0 vapor is passively absorbed or actively retained through oxidative complexation on the forest floor from ambient air. Separating the contribution of gross atmospheric Hg 0 deposition from other processes that makes up Hg 0 fluxes represents a challenge because of the limitation of methodology in determining the individual exchange components .…”

Global Mercury Assimilation by Vegetation

“…The terrestrial environment is an important component of the global Hg cycle and is estimated to receive 3600 Mg of atmospheric Hg deposition annually, equivalent to about half of total annual Hg emissions to the atmosphere from anthropogenic and natural sources combined (with the remainder depositing to global oceans) . Atmospheric Hg deposition includes wet deposition via precipitation and snow and dry deposition, which includes the removal of particulate Hg and sorption and uptake of gaseous Hg to Earth’s surfaces. , Multiple lines of evidence show that dry deposition of gaseous elemental Hg (Hg(0)) via vegetation uptake, however, is the dominant Hg source to terrestrial environments whereby atmospheric Hg(0) is taken up by vegetation tissues and subsequently transferred and deposited to soils when tissues are shed (litterfall), plants die off (biomass turnover), and leaf surfaces are washed off (i.e., throughfall). , Global data analyses estimate global Hg deposition by litterfall in the range of 1020–1230 Mg yr –1 , and global throughfall deposition is considered in a similar range. ,− Global model simulations estimate total atmospheric deposition via vegetation in the range of 1310–1570 Mg yr –1 and 1400 Mg yr –1 , constituting one of the largest global sinks of atmospheric Hg, e.g., exceeding global terrestrial wet deposition of 730–1070 Mg yr –1…”

“…2 Atmospheric Hg deposition includes wet deposition via precipitation and snow and dry deposition, which includes the removal of particulate Hg and sorption and uptake of gaseous Hg to Earth's surfaces. 3,4 Multiple lines of evidence show that dry deposition of gaseous elemental Hg (Hg(0)) via vegetation uptake, however, is the dominant Hg source to terrestrial environments whereby atmospheric Hg(0) is taken up by vegetation tissues and subsequently transferred and deposited to soils when tissues are shed (litterfall), plants die off (biomass turnover), and leaf surfaces are washed off (i.e., throughfall). 5,6 Global data analyses estimate global Hg deposition by litterfall in the range of 1020−1230 Mg yr −1 , and global throughfall deposition is considered in a similar range.…”