Predicting Continental-Scale Soil Mercury Concentrations in Australia to Refine Global Frameworks

Current understanding of the global
mercury (Hg) cycle is heavily
skewed toward the Northern Hemisphere. Australia offers a unique context
to address this bias with its vast arid and semiarid regions, highly
weathered, nutrient-poor soils, fire-adapted ecosystems, ancient stable
geology, and distinct biota. We present a continental-scale analysis
of environmental predictors of soil Hg concentrations based on 2,396
samples from 1,141 catchments spanning 5.84 million km2 (∼76% of Australia). Using boosted regression trees and random
forest models, we identified an index of ferrosols as the strongest
predictor of Hg concentrations, particularly in the temperate forests
of southeastern Australia. Other positively associated factors included
loss on ignition (a proxy for soil organic matter), soil nitrogen,
leaf area index, and nickel and lead concentrations, while soil electrical
conductivity and water availability were negatively associated. We
generated a robust, high-resolution (0.005° × 0.005°)
national Hg distribution map using spatially explicit random forest
models. Unlike patterns observed elsewhere, Hg concentrations in Australia
are higher in ferrosols and relatively nitrogen-rich soils than in
podosols and oxisols. Our findings highlight the need to address regional
gaps in Hg research and emphasize the limitations of relying on regionally
specific data sets that do not characterize global patterns.