Soil formation and mineralogical changes on basaltic lava vs scoria along a hydroclimatic gradient on Santa Cruz Island, Galápagos

Abstract

Climate and parent material type are important soil forming factors. On Santa Cruz Island, Galápagos, the climate shows a pronounced altitudinal zonation from arid lowlands to humid highlands, and the volcanic parent materials, although chemically rather uniform (basaltic), show pronounced differences in porosity, including dense lava flows and very vesicular scoria deposits. In this study, the effect of these two driving factors on pedogenesis was investigated. Our results show that pH and effective cation exchange capacity (CECeff) decreased, but potential cation exchange capacity (CECpot), phosphate retention, and Alo + ½Feo as well as Alp/Alo increased with increasing elevation regardless of parent material type. Conversely, topsoil bulk density (BD) and organic carbon (OC) content did not show a clear altitudinal pattern on lava but decreased (BD) or increased (OC) with increasing elevation on scoria. Clay mineralogy changed from minor (2:1-type) mixed layer phases in the low-elevation arid zone to vermiculitic (lava) or kaolinitic mineralogy (scoria) at mid-elevation (humid), whereas short-range-order (SRO) minerals (allophane and ferrihydrite) dominated in the colloidal fraction of both lava- and scoria-derived soils under very humid conditions at high elevation. The sequence of soil types developed on lava was, with increasing elevation, Cambisol → Phaeozem → Andosol, while on scoria it was Regosol → Umbrisol → Andosol. Our results show that climate is the overriding factor controlling the major mineralogical and pedogenic changes in the studied environment; however, the higher porosity of the scoria parent materials considerably accelerated weathering and pedogenic processes compared to the denser lavas. Our study highlights the importance of the parent material’s physical nature in modulating rates of chemical weathering and pedogenesis.