A-O-CLAY: USING MINERALOGY TO DISTINGUISH SOIL ZONES ON SAN CRISTóBAL ISLAND, GALáPAGOS

Soil zones on the Hawaiian Islands have previously been shown to vary in mineralogy and chemical properties based on the annual average amount of precipitation, which in turn influences the soil’s hydraulic properties, nutrient availability, and structure. In Galápagos, where strong dry-to-wet precipitation gradients occur over very small areas, the variation in mineralogy and chemical properties is thought to be even more dramatic; however, previous work has largely relied on changes in vegetation to constrain changes in soil mineralogy, or has focused on clay mineralogy to the exclusion of all other grain size fractions. This study uses soil collected from six different sites across an elevation-dependent climosequence across San Cristóbal Island, Galápagos to understand how mineralogy and grain size vary, and how the changes in mineralogy and grain size affect the hydraulic properties of soils. Field work (summer 2015) included measurement of soil moisture in the top 12 cm (Campbell Hydrosense II probe) and collection of soil samples from soil pits, road cuts, and an archaeological excavation. Grain size was determined on dried samples by dry sieving and pipette analysis. Mineralogy was determined on each grain size fraction using a Rigaku Miniflex II X-ray diffractometer. Preliminary results suggest that higher elevation sites have less clay in the root zone than do the lower elevation sites; lower soil moisture values were measured at the high elevation sites. High elevation sites were also observed to have high carbon contents relative to low-elevation sites based on field observations. Despite high volumes of amorphous material in all grain size fractions, the XRD data show that soil mineralogy changes across the climosequence, supporting the idea that soil zones across different elevations are driven by climate. Mineralogy and grain size have an important impact on the soil hydraulic properties across the San Cristóbal climosequence, which will be further explored through spatial and temporal soil moisture data and hydraulic conductivity measurements.