Aggregate Stability and Aggregate-Associated Organic Matter along a Soil Chronosequence on the Galápagos Archipelago

Purpose: Soil aggregate stability is a crucial property affecting soil erodibility, water infiltration and carbon sequestration. This study aimed to determine ultrasonic aggregate stability (USAS) as well as solid and dissolved organic carbon (OC and DOC) associated with aggregate fractions of different aggregation strength and size in volcanic soils along an Andosol-Luvisol-Cambisol chronosequence on the Gal & aacute;pagos Islands. Methods: Aggregate stability was determined by ultrasonication at different energy levels, i.e. 20, 100, and 500 J mL(- 1). OC was measured in different aggregate size fractions, i.e. macroaggregates (250-2000 mu m), microaggregates (63-250 mu m), and the fraction < 63 mu m, and released DOC was determined. Results: Aggregate breakdown increased with ultrasonic energy input. The Andosol (short-range order minerals, high OC) had the highest aggregate stability among the studied soils. The OC contents in the stable macro- and microaggregates (at 20 J mL(- 1)) were highest in the Andosol (20.4 and 20.1%, respectively), followed by the Luvisol (11.6 and 10.8%, respectively) and the Cambisol (6.5 and 6.7%, respectively). The decreasing aggregate-associated OC stabilization with increasing soil age coincided with mineralogical changes from short-range order phases to high-activity clays to low-activity clays. The release of DOC during sonication was highest for the intermediate-aged soil (Luvisol) and mainly occurred at low and intermediate energy levels, while for the young soil (Andosol), released DOC steadily increased until the highest energy level. Conclusions: Our results imply that mineralogical/pedogenic changes over millennial time scales can make volcanic soils more susceptible to losses of OC by leaching and water erosion.