Quantifying Mercury Distribution and Source Contribution in Surface Soil of Qinghai-Tibetan Plateau Using Mercury Isotopes

Long-range transport and atmospheric deposition of gaseous mercury (Hg0) result in significant accumulation of Hg in the Qinghai-Tibetan Plateau (QTP). However, there are significant knowledge gaps in understanding the spatial distribution and source contribution of Hg in the surface soil of the QTP and factors influencing Hg accumulation. In this study, we comprehensively investigated Hg concentrations and isotopic signatures in the QTP to address these knowledge gaps. Results show that the average Hg concentration in the surface soil ranks as follows: forest (53.9 ± 36.9 ng g-1) > meadow (30.7 ± 14.3 ng g-1) > steppe (24.5 ± 16.1 ng g-1) > shrub (21.0 ± 11.6 ng g-1). Hg isotopic mass mixing and structural equation models demonstrate that vegetation-mediated atmospheric Hg0 deposition dominates the Hg source in the surface soil, with an average contribution of 62 ± 12% in forests, followed by 51 ± 10% in shrub, 50 ± 13% in steppe, and 45 ± 11% in meadow. Additionally, geogenic sources contribute 28-37% of surface soil Hg accumulation, and atmospheric Hg2+ inputs contribute 10-18% among the four types of biomes. The Hg pool in 0-10 cm surface soil over the QTP is estimated as 8200 ± 3292 Mg. Global warming, permafrost degradation, and anthropogenic influences have likely perturbed Hg accumulation in the soil of QTP.