Processes facilitated by precipitation play an important role on mercury (Hg) accumulation on forest floor and therefore key to Hg cycling in forest ecosystems. Sites along the windward slope of 1,250 to 2,400 m at Mt. Ailao, Southwestern China, have higher precipitation than the leeward slope sites. In this study, measurements of Hg concentration and associated stable isotope composition for soil, fresh, and degraded litterfall samples were made at sites along two slopes of Mt. Ailao to quantify the direct and indirect effects of precipitation on Hg accumulation on forest floor. Higher soil Hg concentrations, larger litterfall Hg depositions, and faster litter decomposition rates were observed on the windward slope (1,250-2,400 m). Data of Hg isotopic signatures suggest that Hg in surface soils is mainly derived from litterfall Hg input. Precipitation enhances litterfall Hg deposition by increasing litter biomass production, reduces litter decomposition rate, facilitates short-term Hg uptake to decomposing litter, and potentially increases microbial activity that increases Hg loss via microbial reduction or runoff. Structural equation modeling results support that the indirect effect of precipitation on increased biomass production merge as the most important factor controlling soil Hg variation. Given the climate forcing on global precipitation pattern and vegetation growth cycle, Hg biogeochemical cycling is likely to continue to evolve under the changing climate.

Plain Language Summary Precipitation has direct and indirect effects on Hg accumulation and transformation on forest floor. The direct effect is that atmospheric Hg enters into forest floor via precipitation, throughfall, and cloud water deposition. In addition, precipitation likely has an indirect effect on Hg accumulation by influencing litter biomass production and postdepositional processes of Hg, such as litter decomposition, Hg reemission, and surface runoff. The objective of this study is to better quantify the direct and indirect effects of precipitation on Hg accumulation on forest floor. The study sites are located along the leeward and windward slopes of Mt. Ailao in Southwest China. Precipitation intensity is the main variable at the same altitude between two slopes. Our results showed that influences from precipitation on soil Hg accumulation are largely through the indirect effects caused by influencing litterfall Hg deposition. We also suggest that the changed precipitation pattern can force variations of litterfall Hg deposition and soil Hg accumulation globally.