Using Mercury Stable Isotopes to Quantify Directional Soil-Atmosphere Hg(0) Exchanges in Rice Paddy Ecosystems:...

Area of Science:

• Environmental Chemistry
• Geochemistry
• Biogeochemistry

Background:

• Gaseous elemental mercury (Hg(0)) emissions from soils represent a substantial portion of global Hg(0) emissions.
• Previous research often overlooks distinct biotic and abiotic reduction pathways, potentially underestimating soil Hg(0) emissions, particularly from paddy soils.
• Understanding these diverse emission sources is crucial for accurate atmospheric mercury budget assessments.

Purpose of the Study:

• To differentiate and quantify mercury (Hg) reduction pathways (photoreduction, microbial reduction, abiotic dark reduction) in paddy soils using stable Hg isotopes.
• To determine the relative contributions of these pathways to gaseous elemental mercury (Hg(0)) emissions from paddy soils.
• To improve the estimation of Hg(0) emissions from rice paddy ecosystems by accounting for multiple reduction processes.

Main Methods:

• Utilized stable mercury (Hg) isotopes to analyze the isotopic composition (δ²⁰²Hg and Δ¹⁹⁹Hg) of Hg(0) produced via different reduction pathways.
• Employed an isotopic signature-based ternary mixing model to apportion Hg(0) emissions among photoreduction, microbial reduction, and abiotic dark reduction.
• Measured Hg(0) exchange fluxes between the atmosphere and paddy soils under various conditions.

Main Results:

• Significantly distinct isotopic signatures were observed for Hg(0) originating from photoreduction, microbial reduction, and abiotic dark reduction.
• Hg(0) emissions from paddy soils were predominantly driven by release into the atmosphere, with average fluxes ranging from 2.2 to 16.8 ng m^-2 h^-1.
• Photoreduction was identified as the primary contributor to Hg(0) emissions, while microbial and abiotic dark reduction accounted for up to 36% and 25% respectively on day 110.

Conclusions:

• Mercury isotope analysis provides a powerful tool for distinguishing Hg(0) reduction pathways in soils.
• Paddy soils are significant sources of atmospheric Hg(0), with contributions from multiple biotic and abiotic processes.
• Accurate estimation of soil Hg(0) emissions requires consideration of photoreduction, microbial reduction, and abiotic dark reduction pathways.