Modelling the sensitivity of soil mercury storage to climate-induced changes in soil carbon pools

Abstract

Substantial amounts of mercury (Hg) in the terrestrial environment reside in soils and are associated with soil organic carbon (C) pools, where they accumulated due to increased atmospheric deposition resulting from anthropogenic activities. The purpose of this study was to examine potential sensitivity of surface soil Hg pools to global change variables, particularly affected by predicted changes in soil C pools, in the contiguous US. To investigate, we included a soil Hg component in the Community Land Model based on empirical statistical relationships between soil Hg / C ratios and precipitation, latitude, and clay; and subsequently explored the sensitivity of soil C and soil Hg densities (i.e., areal-mass) to climate scenarios in which we altered annual precipitation, carbon dioxide (CO₂) concentrations and temperature. <br><br> Our model simulations showed that current sequestration of Hg in the contiguous US accounted for 15 230 metric tons of Hg in the top 0–40 cm of soils, or for over 300 000 metric tons when extrapolated globally. In the simulations, US soil Hg pools were most sensitive to changes in precipitation because of strong effects on soil C pools, plus a direct effect of precipitation on soil Hg / C ratios. Soil Hg pools were predicted to increase beyond present-day values following an increase in precipitation amounts and decrease following a reduction in precipitation. We found pronounced regional differences in sensitivity of soil Hg to precipitation, which were particularly high along high-precipitation areas along the West and East Coasts. Modelled increases in CO₂ concentrations to 700 ppm stimulated soil C and Hg accrual, while increased air temperatures had small negative effects on soil C and Hg densities. The combined effects of increased CO₂, increased temperature and increased or decreased precipitation were strongly governed by precipitation and CO₂ showing pronounced regional patterns. Based on these results, we conclude that the combination of precipitation and CO₂ should be emphasised when assessing how climate-induced changes in soil C may affect sequestration of Hg in soils

Mercury in the terrestrial environment: a review

“…Hg occurs naturally in soils from geologic sources [12] or as the result of natural events such as forest fires and volcanic eruptions [49]. The total amount worldwide of Hg accumulated in the soils of terrestrial environments is estimated at 200-300 Gg [112][113][114]. Smith-Downey et al [60] suggested that organically bound Hg in preindustrial soils is 200 Gg and that a 20% increase in organically bound soil Hg (to 240 Gg) has occurred from preindustrial steady-state conditions to the present day.…”

The Millennial model: in search of measurable pools and transformations for modeling soil carbon in the new century

“…A growing number of soil models consider an explicit microbial biomass pool that affects the decomposition rate of SOC (Schimel and Weintraub 2003;Allison et al 2010;German et al 2012). These models often require greater numbers of parameters and equations, but may have an improved ability to predict responses to novel environmental conditions, e.g., global change scenarios Hararuk et al 2013). Questions remain about the feasibility of applying microbial models to global SOC predictions (Bradford et al 2016).…”

Tundra uptake of atmospheric elemental mercury drives Arctic mercury pollution

“…(Table S6). If a soil Hg pool of 27 mg m -2 at Toolik Field station (Table S5; top 40 cm) is representative of the global tundra belt, Arctic tundra soils would contain ~143 Gg of Hg, which would account for almost half the total estimated global soil Hg pool size of 300 Gg based on temperate studies for this soil depth 20,29 . Further, our study provides the first independent experimental verification of source attribution by Hg isotope signatures, which at this tundra site show that Hg stored in vegetation and soils is predominantly derived from atmospheric Hg 0 consistent with direct deposition measurements.…”