Weathering and soil formation in rhyolitic tephra along a moisture gradient on Alcedo Volcano, Galápagos

• DOI: 10.1016/j.geoderma.2019.01.051
• Publication Date: 2019-03-04T18:48:26Z
• Authors: I Nyoman Candra; Martin H. Gerzabek; Franz Ottner; Johannes Tintner; Karin Wriessnig; Franz Zehetner
• Keywords: 0401 agriculture, forestry, and fisheries; 04 agricultural and veterinary sciences; 15. Life on land

Abstract The Galapagos archipelago has extensively been studied because of its geological and climatic setting and especially for its peculiar flora and fauna. However, there is still little information about its soils. The objective of this study was to investigate weathering and soil formation on Alcedo Volcano, which is the only volcano of Galapagos that has erupted abundant rhyolitic materials. Soils were sampled from three sites along an elevation/moisture gradient, site 1 at 872 m above sea level (a.s.l.) (humid zone), site 2 at 621 m a.s.l. (dry zone) and site 3 at 377 m a.s.l. (very dry zone). Soil pH (in H2O) ranged from 6.0 to 6.6 but did not vary significantly among the three sites. Water retention (at 1500 kPa) and phosphate retention increased with elevation/moisture. Similarly, soil organic carbon (SOC) stocks increased from approx. 1.1 kg m−2 in the very dry zone to approx. 7.5 kg m−2 in the humid zone. Allophane and ferrihydrite were the dominant colloidal constituents in the humid zone (site 1), whereas halloysite was found in the dry zone (site 2) and only traces of halloysite in the very dry zone (site 3). At site 1, the soil was borderline between andic and vitric properties and was classified as Silandic Tephric Andosol or Vitric Tephric Andosol for andic and vitric character, respectively; at site 2, the soil had vitric properties and was classified as Vitric Tephric Skeletic Andosol; and at site 3, the soil was borderline with respect to the expression of vitric properties and was classified as Vitric Tephric Skeletic Andosol in the presence and as Tephric Skeletic Regosol in the absence of vitric properties. Our results show that different moisture availability strongly affected SOC accumulation and favored the formation of different types of colloidal constituents (amorphous vs. crystalline) in the rhyolitic tephra of Alcedo Volcano. This has important bearings on the expression of andic and vitric soil properties and, hence, on the distribution of Andosols in the studied environment.