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.