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Volcanic ash-derived soils are important globally for their C sequestration potential and because they are at risk of compaction and degradation due to land use change. Poorly or non-crystalline minerals impart enormous capacity for soils to store and stabilize C, but also unusual chemical and physical properties that make quantifying meaningful soil C pools challenging. Here, we optimize a soil physical fractionation method to effectively assess soil organic matter dynamics in volcanic ash soils by first comparing three common methods for Andisols of the same soil series under three land uses. Components of those methods that (1) effectively isolated C pools of different size and turnover and (2) demonstrated potential sensitivity to land use change were then modified for a final, combined method. The isolation of C pools corresponding to fundamental mechanisms of protection within aggregates and organo-mineral control on the stabilization of C, which often function to the extreme in volcanic ash soils, underlie these modifications. Combined application of ultrasonic energy to disrupt aggregates and the removal of light fractions with sequential high density fractionation successfully isolated multiple C pools that ranged in radiocarbon-based turnover time from 7 to 1,011 year in the surface 0–15 cm of mineral soil in an undisturbed, native forest. Soil C accumulates as a result of high, continuous input that cycles through a transitional (century-scale) organo-mineral pool and then either becomes occluded and protected within aggregates (multiple centuries) or enters a continuum of organo-mineral and non-crystalline mineral-dominated pools (from centuries to millennium-scale). Comparison of relative C pool sizes and C isotope signature among soils from native forest, grazed pasture, and managed Eucalyptus plantation revealed the potential for making accurate, direct measurements of soil C change over time with land use and management change or disturbance regime.
