Dry deposition of gaseous elemental mercury to plants and soils using mercury stable isotopes in a controlled environment

Uptake of gaseous elemental mercury (Hg0(g)) by three plant species and two soil types was measured using mercury vapor enriched in the 198 isotope (198Hg0(g)). The plant species and soil types were: White Ash (Fraxinus Americana; WA); White Spruce (Picea Glauca; WS); Kentucky Bluegrass (Poa Partensis; KYBG); Plano Silt Loam (4% organic matter; PSL); and Plainfield Sand/Sparta Loamy Sand (1.25–1.5% organic matter: PS). The plants and soils were exposed to isotopically enriched Hg0(g) in a 19 m3 controlled environment room for 7 days under optimal plant growth conditions (20 °C, 140 Wm−2 between 300 nm and 700 nm; 70% RH) and atmospherically relevant Hg0(g) concentrations. Mercury was recovered from the samples using acidic digestions and surface leaches, and then analyzed for enrichments in 198Hg by ICPMS. The method was sensitivity enough that statistically significant enrichments in 198Hg were measured in the plant foliage at the end of Day 1. Whole leaf digestions and surface-selective leaches revealed that accumulative uptake was predominantly to the interior of the leaf under the conditions studied.Uptake fluxes for WA increased between the first and third days and remained constant thereafter (WA; Day 1 = 7 ± 2 × 10−5 ng m−2 s−1; Days 3–7 = 1.3 ± 0.1 × 10−4 ng m−2 s−1; where m2 refers to one sided leaf area). KYBG demonstrated similar behavior although no Day 3 measurement was available (Day 1 = 7.5 ± 0.5 × 10−5 ng m−2 s−1; Day 7 = 1.2 ± 0.1 × 10−4 ng m−2 s−1). Fluxes to White Spruce were lower, with little difference between Days 1 and 3 followed by a decrease at Day 7 (WS; Days 1–3 = 5 ± 2 × 10−5 ng m−2 s−1; Day 7 = 2.4 ± 0.2 × 10−5 ng m−2 s−1). Uptake of Hg to soils was below the method detection limit for those media (PSL = 3 × 10−2 ng m−2 s−1; PS = 3 × 10−3 ng m−2 s−1) over the 7 day study period. Foliar resistances calculated for each species compared well to previous studies.

Research highlights
► Plants and soils were exposed to Hg0(g) enriched in the 198 stable isotope to measure foliar accumulation in a controlled environment growth chamber.
► Plant foliage exhibited statistically significant enrichments in the Added 198 stable isotope after 1 day, and accumulation continued for the 7 day duration of the experiment.
► Accumulation occurred primarily through the leaf stomata-interior pathway, rather than to the cuticle.
► Resistances to uptake were calculated and predicted to be dominated by the interior resistance rather than the stomata at optimal growth conditions.