Effect of elevation and slope orientation on pedogenesis of late Holocene volcanic ash on a tropical high mountain in central Mexico

Abstract

The influence of climate on pedogenesis is elevation dependent, but little is known on the influence of slope orientation. This investigation focuses on the development of soils formed on Holocene (1100 and 5600 years old) volcanic ash deposits along an elevation gradient from 3000 to 4300 m a.s.l. on a high mountain in central Mexico. Soil profiles were analyzed on both N- and S-facing slopes at 3000, 3400, 3800, 4000 and 4300 m a.s.l. To determine the effect of climate (dependent on elevation and slope orientation) on pedogenetic processes, other factors such as age and type of parent material, landform position, and slope angle were kept constant. Soil temperature was recorded at 10 cm depth at each site for one year. Significant differences in temperature were found along the elevation gradient and between N-facing and S-facing slopes. Selective extractions of Fe, Al, and Si (using dithionite, oxalate, and pyrophosphate) indicate vitric properties on the 1100-year-old tephra (modern soil) throughout the elevation gradient, while the soil formed on the 5600-year-old tephra (buried by the 1100 year-old tephra) shows andic properties. The orientation of slopes has favored two evolutionary trajectories in the 1100-year-old soil: short-range-order minerals dominate on S-facing slopes, and organo-metallic complexes on N-facing slopes. The lower preservation of ash and pumice layers on S-facing slopes suggests that they have experienced geomorphic instability at centennial scales. The pedogenetic and stratigraphic differences observed between N- and S-facing slopes can be extrapolated to similar valleys and volcanic mountains.