Divergent mercury dynamics linked to POM-to-MAOM shifts in Tibetan Plateau thermokarst soils

Thermokarst development restructures soil profiles and hydrological pathways, but whether contrasting landforms lead to different mercury (Hg) trajectories remains unclear. Here, we compared an open-slope thaw slump (Eboling) and a closed-depression thaw subsidence (Amdo) on the Tibetan Plateau using soil physical fractionation into particulate organic matter (POM) and mineral-associated organic matter (MAOM). At both sites, permafrost degradation drove a common redistribution of Hg from POM to MAOM, as evidenced by increasing MAOM-Hg proportion in total Hg. Despite this shared fraction-level response, net Hg outcomes diverged fundamentally between landforms. At Eboling (control to exposed), total Hg concentration increased from 42.94 ± 13.73-57.35 ± 17.50 ng g⁻¹ (+33.6%), MAOM-Hg concentration increased 93.9%, and Hg stock rose by 234.3%, with the bulk Hg/SOC ratio increasing-indicating progressive Hg enrichment. In contrast, at Amdo (control to subsided), total Hg concentration decreased from 21.76 ± 6.20-10.08 ± 2.72 ng g⁻¹ (-53.7%), with POM-Hg and MAOM-Hg concentrations declining by 72.7% and 45.2%, respectively. Hg stock decreased by 12.2%, with the bulk Hg/SOC ratio remaining constant-indicating coupled Hg-organic matter loss. Principal component analysis further linked degraded Eboling soils with Hg stock and MAOM-Hg, while subsided Amdo soils showed weaker association with mineral-related Hg retention. This divergence is explained by landform-specific controls: at the open-slope thaw slump, strong lateral redistribution and higher iron/aluminum oxide contents favored Hg retention; at the closed-depression thaw subsidence, localized subsidence and reduced Hg retention in the mineral-associated fraction led to coupled Hg-organic matter loss. Our findings demonstrate that permafrost degradation drives a common Hg redistribution from POM to MAOM, but that net source-sink outcomes are landform-dependent-a key insight for regional Hg cycling assessments under climate warming.