Topographic drainage impedance is associated with organic matter compositions and mercury partitioning in a montane-alpine ecosystem.

We examined associations among topographic position, organic matter compositions, and methylmercury (MeHg) pools and partitioning in montane-alpine soils. Triplicate 60-cm soil cores were collected at 12 sites along a 1,666-4,503 m elevational transect on the eastern slope of Gongga Mountain, spanning an alluvial plain, an intermontane basin, and ridge-shoulder terrain. A drainage impedance index (DI), calculated from plan curvature and slope, represented potential lateral water convergence relative to downslope drainage. DI was negatively associated with the weight-averaged nominal oxidation state of carbon (NOSC) of water-extractable organic matter (adjusted R2 = 0.31, p = 0.034). NOSC was used as a formula-derived molecular descriptor of detected water-extractable organic matter. For exploratory sample-level comparisons, 21 site-depth analytical units were assigned to operational lower-NOSC (NOSC < -0.2; n = 4) or higher-NOSC (NOSC ≥ -0.2; n = 17) groups. The lower-NOSC group contained a larger median contribution of lipid-like formulas and a smaller contribution of phytochemical-like formulas. Basin surface sites contained larger bulk MeHg pools, while ridge sites had a larger operationally water-extractable share of MeHg. These observations identify DI as a candidate screening covariate for spatial variation in MeHg pools and partitioning. Independent transects with direct hydrologic and redox measurements can test the proposed processes in future studies.