Vegetation transition alters mercury pools and highlights dominant atmospheric deposition source in an alpine timberline soil–plant system

Understanding mercury (Hg) accumulation and distribution patterns in alpine timberline forests is essential for understanding the biogeochemical cycling of Hg in these ecosystems. To this end, we systematically analyzed Hg concentrations and isotopic compositions in the soil–plant system to elucidate the spatial distribution and source contributions of Hg in the timberline ecotone. The transition from coniferous forests to shrubbery resulted in a distinct decrease in Hg pool size—67% in vegetation and 19% in soil, respectively. The Hg isotopic mixing model further demonstrated that vegetation-induced atmospheric Hg0 deposition, with an average contribution of 74 ± 10%, was the dominant source of soil Hg in the timberline ecotone. Hg concentration exhibited an increasing trend from Oi to Oe soil horizons, followed by a decline in the mineral layers. The rising Hg concentrations in organic soils resulted from accelerated organic carbon loss during decomposition, while the decreasing gradient in mineral soils was primarily driven by the combined effects of geological Hg sources and long-term Hg depletion during soil formation. Vegetation transition across the timberline ecotone significantly reduced Hg pool dynamics in the soil–plant system. The upward migration of fir forests driven by climate warming may enhance atmospheric Hg deposition, underscoring the need for a comprehensive assessment of Hg cycling in warming alpine ecosystems.