Free and intra-aggregate organic matter as indicators of soil quality change in volcanic soils under contrasting crop rotations

Soil physical fractionation techniques may provide indicators of changing soil organic carbon (SOC) content; however, they have not been widely tested on volcanic soils (Andisols). In this study, we assessed two fractions as potential indicators in volcanic soils, using two sites in Chile converted from natural grassland to arable and mixed crop rotations, 8 and 16 yr previously. In the 8-yr experiment, SOC had declined under all rotations, with smaller changes where the rotation included 3 or 5 yr of perennial pasture. Whereas the average SOC was only 76% of the level in the preceding natural grassland, the corresponding value after 16 yr for the second site was 98% (and 93% under continuous arable), probably reflecting its high allophane clay content. The fractionation procedure tested proved applicable to both Andisols, but the intra-aggregate light fraction (IA-SOM, isolated in sodium iodide solution at 1.80 g/cm3 after ultrasonic dispersion) accounted for a very small proportion of total SOC (<1%). We suggest that in Andisols, the free light fraction (FR-SOM, isolated in sodium iodide at solution of the same density, but prior to ultrasonic dispersion) is stabilised to a greater extent than in nonvolcanic soils, and the intra-aggregate fraction plays a more minor role as a pool of intermediate turnover. The relative value of each fraction needs to be confirmed through dynamic experiments, using more sites, and including situations where SOC content is initially low.