Patterns in Soil Chemical Weathering Related to Topographic Gradients and Vegetation Structure in a High Andean Tropical Ecosystem

Although climate exerts a major control on mineral weathering and soil formation processes, the combined effect of vegetation and topography can influence the rate and extent of chemical weathering at the hillslope scale. In this paper, we examined spatial patterns in volumetric strain and soil weathering extent associated with topographic gradients and vegetation patterns. In a high Andean catchment, we selected 10 soil toposequences on andesitic flows: 5 under tussock grasses, 3 under cushion forming plants, and 2 under native forest. Along each toposequence, one pit was excavated at the shoulder, backslope, and toeslope resulting in 30 soil profiles. Depth-weighted total soil porosity of the 30 soil profiles averaged 646%. The association between volumetric strain and soil organic C indicates that biotic agents can be effective in dilating the regolith during weathering. The young, postglacial volcanic soils were depleted in mono-divalent and divalent cations, with total mass losses ranging between 793 and 1610kg/m(2). The accumulation of Al-humus complexes in the soil matrix plays an essential role in chemical transformation of the nonallophanic soils. Beyond the marginally significant topographic control on chemical weathering extent, our data show highly significant differences in chemical weathering extent between vegetation communities with total mass losses in forest soils being, respectively, 19% and 22% higher than in grasslands and cushion-forming plants. The vegetation mosaic in alpine ecosystems might therefore provide essential clues to understand soil chemical weathering patterns caused by spatially varying soil particle and water residence times. Plain Language Summary In a typical cloudy day in the high Tropical Andes, the patchwork mosaic of tussock grasses, cushion plants, and Polylepis forests provides essential clues on soil chemical weathering patterns. Physicochemical data from postglacial volcanic soils sampled at 3,800m demonstrate that the Andosols experienced intense chemical weathering with accumulation of metal-humus complexes. Beyond the topographic control on soil development, significant differences in soil chemical weathering degree are associated with vegetation patterns.