Microtopography and Seasonal Soil Moisture Govern Methylmercury Dynamics in Upland Soils of a Subtropical Coastal Montane Watershed

Abstract Vegetated landscapes are widely known as important sinks of mercury (Hg) but the controls on the production of neurotoxic methylmercury (MeHg) in terrestrial upland ecosystems remain unresolved, particularly at fine spatial scales. In this study, we investigated the distribution of total Hg (THg) and MeHg in surface soils across an elevational gradient in a subtropical coastal montane watershed separately in dry and wet seasons at Hong Kong of southern China. Mean surface soil THg was the highest in the lower-elevation forest zone and at lower levels in the middle- and upper-elevation shrubland/grassland zones. Topsoil (0–4 cm) MeHg increased from dry to wet season at lower-elevation forest sites and decreased or changed little at most upper-elevation sites, although site-level elevation-zone contrasts were not statistically significant after false-discovery-rate adjustment. At the site level (n = 11), the best-supported model within a 12-model candidate set for seasonal topsoil MeHg change included seasonal moisture change, plan curvature, and inorganic Hg (IHg = THg – MeHg; adjusted R2 = 0.758, p = 0.0043), implying exploratory watershed-scale associations among moisture, microtopography, Hg substrate, and MeHg heterogeneity.