Soil organic carbon and fine particle stocks along a volcanic chrono- and elevation-sequence on the Galápagos archipelago/Ecuador

The Galápagos archipelago consists of a chain of volcanic islands of different ages. We used a chronosequence and altitudinal sequence approach to investigate the influence of soil age (1.5 to 1070 ka) and climate (arid to very humid; 47–866 m a.s.l.) on the stocks of soil organic carbon (SOC) and stocks of silt- and clay-sized particles. We determined SOC stocks of the entire developed profiles. The youngest soil of the chronosequence had the lowest SOC stock (7.7 kg m⁻²), and a first maximum of 21.6 kg m⁻² was observed in an allophanic Andosol (at 4.3 ka age), followed by somewhat lower SOC stocks in older soils and a second maximum in the oldest and particularly deeply developed soil (25.6 kg m⁻²). In the highly weathered soils, the share of subsoil SOC stocks (>20 cm) was 50% or more. Climatic differences induced similar variability in SOC stocks (0.8 to 20.3 kg m⁻²) as did soil age. The SOC stocks along the chronosequence were likely influenced by several factors with partly cancelling effects, i.e. increasing solum thickness with soil age and increasing proportions of fine fractions, but decreasing specific surface area related with changes in the mineralogical composition of the soils from allophanes to increasingly crystalline Fe oxides. Stocks of silt+clay-sized particles varied from 9.6 to 1724 kg m⁻² across all soil profiles studied. Along the chronosequence, the clay particle formation rate decreased logarithmically from >90 g m⁻² a⁻¹ at the youngest to <0.2 g m⁻² a⁻¹ at the oldest sites. Soil age (under humid climate) and changes in moisture regime showed similarly strong effects on the stocks of fine fractions. An important finding of general relevance was the logarithmical correlation of stocks of fine particle fractions and SOC stocks, showing that with increasing stocks of silt- and clay-sized particles in older soils, SOC binding relative to fine particle stocks became less, likely due to altered mineralogical composition and microstructure.